Pascal Tutorial v2.6c

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CORONADO ENTERPRISES

TURBO PASCAL TUTOR - Version 2.6

This documentation and the accompanying software, including all of the example Pascal programs and

text files, are protected under United States copyright law to protect them from unauthorised

commercialisation. This version of the tutorial is distributed under the shareware concept, which means

you are not required to pay for it. You are permitted to copy the disks, and pass copies on to a friend,provided you do not modify any files or omit any files from the complete package. In fact, you are

encouraged to pass complete copies on to friends. You are permitted to charge a small fee to cover the

costs of duplication, but you are not permitted to charge anything for the software itself.

If you find the tutorial helpful, you are encouraged to register with the author and to submit a small fee

to help compensate him for his time and expense in writing it. We will provide you with a beautifully

printed copy of this tutorial if you submit a full registration. See the READ.ME file in the TEXT

directory for additional details.

Whether or not you send a registration fee, feel free to request a copy of the latest list of available

tutorials and a list of the authorised Public Domain libraries that distribute our full line of programming

language tutorials.

Gordon Dodrill - Feb 4, 1991

Copyright (c) 1986, 1988, 1989, 1991, Coronado Enterprises

Coronado Enterprises12501 Coronado Ave NE

Albuquerque, New Mexico 87122

ABOUT THE AUTHOR

The author of this tutorial began programming in 1961 using FORTRAN on an IBM 1620. Since then,

most of his career has been involved with designing digital logic for satellite application. In 1983,

being somewhat burned out with logic design, he began a study of some of the more modern

programming languages and has since made a complete career shift to software development. After

learning Pascal, C was studied, followed by Modula-2 and Ada, and more recently C++. Rather thansimply learning the syntax of each new language, modern methods of software engineering were

studied and applied to effectively utilise the languages. He is currently employed by a large research

and development laboratory where he continues to study, teach, and apply the newer programming

languages.



TURBO PASCAL TUTORIAL - TABLE OF CONTENTS

CORONADO ENTERPRISES ..................................................... .........................................................1

TURBO PASCAL TUTOR - VERSION 2.6.........................................................................................1

INTRODUCTION...................................................................................................................................6

IF YOU KNOW NOTHING ABOUT PASCAL..................................................................................6

LARGER PASCAL PROGRAMS ........................................................ ...............................................6

WHAT IS A COMPILER? ..................................................... ........................................................ ......6

WHICH VERSION OF TURBO PASCAL? ................................................................ ........................7EARLY VERSIONS OF TURBO PASCAL........................................................................................7

WHAT ABOUT TURBO PASCAL VERSION 5.5 & 6.0? ................................................... ..............7

PREPARATION FOR USE OF THIS TUTORIAL ...................................................................... .......8

WHAT ABOUT THE PROGRAMMING EXERCISES?....................................................................8

THE ANSWERS DIRECTORY...........................................................................................................8A SPECIAL NOTE FOR THE SHAREWARE VERSION..................................................................8

CHAPTER 1 : WHAT IS A COMPUTER PROGRAM? ................................................................ ...9

THIS CHAPTER IS FOR NEW PROGRAMMERS............................................................................9

WHAT IS A COMPUTER PROGRAM?.... ................................................................. ........................9

WHAT ARE CONSTANTS? ......................................................... ......................................................9

WHAT ARE VARIABLES? ........................................................... .....................................................9

HOW DO WE DEFINE CONSTANTS OR VARIABLES?................................................................9

WHAT IS SO GOOD ABOUT PASCAL?.........................................................................................10

YOUR FIRST PASCAL PROGRAM .............................................................................. ..................11

WHAT IS AN IDENTIFIER?.............................................................................................................11

NOW FOR THE PROGRAM.............................................................................................................12

A PROGRAM THAT DOES SOMETHING......................................................................................12ANOTHER PROGRAM WITH MORE OUTPUT .................................................. ..........................13

ADDING COMMENTS IN THE PROGRAM...................................................................................13

THE RESULT OF EXECUTION SECTION................................................................. ....................14

GOOD FORMATTING PRACTICE..................................................................................................14

VERY POOR FORMATTING PRACTICE.......................................................................................15

PROGRAMMING EXERCISES........................................................................................................16

WHAT IS A DATA TYPE? .......................................................... .....................................................17

OUR FIRST VARIABLES.................................................................................................................17

OUR FIRST ARITHMETIC...............................................................................................................18

NOW LET'S USE LOTS OF VARIABLES.......................................................................................19

BOOLEAN VARIABLES ....................................................... ....................................................... ....21

WHERE DO WE USE THE BOOLEAN VARIABLES? ................................................................ ..22

SHORT CIRCUIT OR COMPLETE EVALUATION? ................................................................. ....23LET'S LOOK AT THE CHAR TYPE VARIABLE .............................................................. .............23

EXTENDED INTEGER TYPES........................................................................................................24

EXTENDED REAL TYPES...............................................................................................................25

PROGRAMMING EXERCISE .................................................................... ......................................26THE FOR LOOP.................................................................................................................................27

A COMPOUND PASCAL STATEMENT.........................................................................................28

THE IF STATEMENT ....................................................... ........................................................... .....28

THE IF-THEN-ELSE BLOCK...........................................................................................................29

LOOPS AND IFS TOGETHER ............................................................................. ............................30

FINALLY, A MEANINGFUL PROGRAM.......................................................................................31THE REPEAT UNTIL LOOP ....................................................... .....................................................32

THE WHILE LOOP ................................................... ............................................................ ............33

THE CASE STATEMENT.................................................................................................................34PROGRAMMING EXERCISES........................................................................................................36



CHAPTER 5 : PROCEDURES AND FUNCTIONS ........................................................ .................37

A PASCAL PROGRAM OUTLINE ............................................................... ...................................37

THE FIRST PROCEDURES..............................................................................................................37

DEFINITIONS GO IN THE DEFINITION PART ...................................................................... ......38

HOW TO DEFINE & CALL A PROCEDURE..................................................................................39

THE UNBROKEN RULE OF PASCAL............................................................................................39

MORE PROCEDURE CALLS...........................................................................................................39

FORMAL AND ACTUAL PARAMETERS......................................................................................40

CALL BY VALUE .................................................... ............................................................ .............41

CALL BY REFERENCE....................................................................................................................41

SOME NEW TERMINOLOGY ...................................................... ...................................................41

"CALL BY REFERENCE" OR "CALL BY VALUE"?.....................................................................42

A MULTIPLY DEFINED VARIABLE ...................................................... .......................................43

PROCEDURES CALLING OTHER PROCEDURES .......................................................... .............44

NOW LET'S LOOK AT A FUNCTION ............................................................................ ................45

NOW FOR THE MYSTERY OF RECURSION................................................................................45

ABOUT RECURSIVE PROCEDURES.............................................................................................46

THE FORWARD REFERENCE ....................................................................... .................................46

THE PROCEDURE TYPE .............................................................. ...................................................48PROGRAMMING EXERCISES........................................................................................................50

CHAPTER 6 : ARRAYS, TYPES, CONSTANTS, AND LABELS..................................................51

ARRAYS .............................................................. ........................................................... ...................51

USING THE ARRAY.........................................................................................................................52

DOUBLY INDEXED ARRAYS............................................................ ............................................52

ARRAYS ARE FLEXIBLE................................................................................................................54THE TYPE DEFINITION ............................................................ ......................................................54

PASCAL CHECKS TYPES VERY CAREFULLY ............................................................ ...............54

IS THE CONCEPT OF "TYPES" IMPORTANT?.............................................................................55

THE CONSTANT DECLARATION .......................................................... .......................................55

THE TURBO PASCAL TYPED CONSTANT.................................................................... ..............56

THE LABEL DECLARATION..........................................................................................................56THE PACKED ARRAY.....................................................................................................................57

ONE MORE TURBO PASCAL EXTENSION..................................................................................58


CHAPTER 7 : STRINGS & STRING PROCEDURES ....................................................................59

PASCAL STRINGS............................................................................................................................59

A STRING IS AN ARRAY OF CHAR..............................................................................................59THE TURBO PASCAL STRING TYPE............................................................................................60

STRINGS HAVE VARIABLE LENGTHS........................................................................................60

WHAT'S IN A STRING TYPE VARIABLE? .......................................................... .........................60

PROGRAMMING EXERCISE .................................................................... ......................................61

CHAPTER 8 : SCALARS, SUBRANGES, AND SETS.....................................................................62

PASCAL SCALARS ........................................................... .......................................................... .....62

A BIG SCALAR VARIABLE LOOP.................................................................................................63

LET'S LOOK AT SOME SUBRANGES ........................................................................... ................63

SOME STATEMENTS WITH ERRORS IN THEM. ................................................................. .......64

THREE VERY USEFUL FUNCTIONS ............................................................... .............................65

SETS...................................................................................................................................................65SEARCHING WITH SETS................................................................................................................67


CHAPTER 9 : RECORDS ...................................................... ........................................................ .....69

A VERY SIMPLE RECORD ........................................................ .....................................................69

A SUPER RECORD..................................................... ............................................................ ..........70

HOW TO MANIPULATE ALL OF THAT DATA............................................................................71WHAT IS THE WITH STATEMENT? .............................................. ...............................................72

HOW FAR DOWN CAN YOU NEST THE WITH STATEMENT? ................................................72



SUPER-ASSIGNMENT STATEMENTS ............................................................................. .............72

WHAT GOOD IS ALL OF THIS ....................................................... ................................................72A VARIANT RECORD......................................................................................................................73

WHAT IS A TAG-FIELD?.................................................................................................................74

USING THE VARIANT RECORD....................................................................................................74

NOW TO SEE WHAT WE HAVE IN THE RECORDS .................................................. .................75


CHAPTER 10 : STANDARD INPUT/OUTPUT................................................................................76

WE'VE USED THIS ALREADY ............................................................ ...........................................76

MANY OUTPUT STATEMENTS.....................................................................................................77

NOW FOR SOME INPUT FROM THE KEYBOARD .....................................................................77

TIME TO CRASH THE COMPUTER...............................................................................................79

READING REAL NUMBERS...........................................................................................................79

READING CHARACTER DATA ......................................................... ............................................79BULLET PROOF PROGRAMMING........................................................... .....................................80

HOW DO I PRINT SOMETHING ON THE PRINTER? ................................................................ ..80


CHAPTER 11 : FILE INPUT/OUTPUT ............................................................................................83FILES HANDLE SERIAL DATA......................................................................................................83

A SHORT HISTORY LESSON ................................................................... ......................................83BACK TO THE PRESENT TIME ...................................................... ...............................................83

READING AND DISPLAYING A FILE...........................................................................................83

WHAT ARE THE "EOF" AND "EOLN" FUNCTIONS?..................................................................84

A PROGRAM TO READ ANY FILE................................................................................................85

HOW TO COPY A FILE (SORT OF)................................................................................................86

USING A COMPILER DIRECTIVE .............................................................. ...................................87

WE DO OUR OWN FILE CHECKING.............................................................................................87

HOW TO READ INTEGER DATA FROM A FILE ......................................................... ................87

READ AND READLN ARE SLIGHTLY DIFFERENT ...................................................................89

NOW TO READ SOME REAL VARIABLES FROM A FILE.........................................................89

NOW FOR BINARY INPUT AND OUTPUT...................................................................................90WHY USE A BINARY FILE.............................................................................................................92

READING A BINARY FILE ......................................................... ....................................................92

FILE POINTERS, GET, AND PUT STATEMENTS .......................................................... ..............93


CHAPTER 12 : POINTERS AND DYNAMIC ALLOCATION......................................................94

THIS IS ADVANCED MATERIAL .............................................................. ....................................94

WHAT ARE POINTERS, AND WHAT GOOD ARE THEY? .........................................................94

HOW DO WE USE THE POINTERS? ........................................................... ...................................95

A FEW MORE POINTERS................................................................................................................95

THIS IS FOR TURBO PASCAL ONLY............................................................................................95

OUR FIRST LOOK AT DYNAMIC ALLOCATION .................................................................... ...96

WHAT IS THE HEAP?...................................................... ........................................................... .....97

WHAT IS DYNAMIC ALLOCATION?............................................................................................97

GETTING RID OF DYNAMICALLY ALLOCATED DATA..........................................................98

DYNAMICALLY STORING RECORDS ............................................................ .............................98

WE JUST BROKE THE GREAT RULE OF PASCAL................................................ ...................100

THIS IS A TRICK, BE CAREFUL ........................................................ ..........................................100

WHAT GOOD IS THIS ANYWAY?...............................................................................................101

WHAT IS A LINKED LIST?..................................................................... ......................................101

STILL NO VARIABLES?................................................................................................................103

WHAT IS "NIL" AND WHAT IS IT USED FOR?...........................................................................103

DEFINING THE SECOND RECORD.............................................................................................103

TEN MORE RECORDS...................................................................................................................103

FINALLY, A COMPLETE LINKED LIST ............................................................. ........................104

HOW DO WE GET TO THE DATA NOW?...................................................................................104

PROGRAMMING EXERCISE ............................................................. ...........................................104



CHAPTER 13 : UNITS IN TURBO PASCAL ....................................................... ..........................105

PART OF A PROGRAM..................................................................................................................105

THE INTERFACE PART.................................................................................................................106

THE IMPLEMENTATION PART...................................................................................................107

A LOCAL PROCEDURE.................................................................................................................107

WHAT IS THE BODY USED FOR?................................................................... ............................107

SELECTIVE NAMING OF FUNCTIONS AND PROCEDURES ..................................................107

ANOTHER UNIT.............................................................................................................................108

HOW DO WE USE OUR DEFINED UNITS?.................................................................................109

ONE MORE EXAMPLE OF UNIT USE.........................................................................................109

MULTIPLE USES OF AN IDENTIFIER ................................................................ ........................110

WHY USE UNITS?..........................................................................................................................111

THIS IS INFORMATION HIDING ............................................................... ..................................111

PROGRAMMING EXERCISE ............................................................. ...........................................111

CHAPTER 14 : ENCAPSULATION & INHERITANCE ..............................................................112

OUR FIRST ENCAPSULATION .................................................... ................................................112

WHAT IS A METHOD? ............................................................. .....................................................113

THE METHOD IMPLEMENTATION ....................................................... .....................................114AN INSTANCE OF AN OBJECT....................................................................................................114

NEW TERMINOLOGY ..................................................... ............................................................ ..114

WHAT DID WE ACCOMPLISH?...................................................................................................115

DATA & CODE PROTECTION......................................................................................................115

MORE ENCAPSULATION........................................................... ..................................................115

THE PRIVATE TYPE......................................................................................................................117

A FEW RULES ARE NEEDED.......................................................................................................118WHAT IS A CONSTRUCTOR? ........................................................ ..............................................118

WHAT IS A DESTRUCTOR? .............................................................. ...........................................119

OUR FIRST INHERITANCE ............................................................. .............................................119

HOW DO WE USE THE OBJECTS? .............................................................. ................................121

WHY USE INHERITANCE?...........................................................................................................122

AN OBJECT IN A UNIT .................................................... .......................................................... ...122ANOTHER OBJECT IN A UNIT ........................................................ ............................................123

USING THE OBJECTS DEFINED IN UNITS................................................................................125

AN ARRAY AND A POINTER ................................................ ......................................................126

WHAT IS MULTIPLE INHERITANCE?........................................................................................127

WHAT SHOULD YOU DO NOW?.................................................................................................127

PROGRAMMING EXERCISES......................................................................................................127

CHAPTER 15 : VIRTUAL METHODS ................................................. ..........................................128

WITHOUT A VIRTUAL METHOD................................................................................................128

NOW TO MAKE IT A VIRTUAL METHOD.................................................................................130

ASSIGNING DESCENDANTS TO ANCESTORS? .......................................................................132

WHY USE A CONSTRUCTOR?.....................................................................................................132

VIRTUALS AND POINTERS ................................................................ .........................................133AN ANCESTOR OBJECT ............................................................. ..................................................135

SOME DESCENDENT OBJECTS ............................................................ ......................................136

A COMPLETE EMPLOYEE PROGRAM.......................................................................................137

PROGRAMMING EXERCISES......................................................................................................138

AMORTIZATION TABLE GENERATOR................................................ .....................................139TOP DOWN PROGRAMMING ............................................................ ..........................................139

MOST IMPORTANT - YOUR OWN PROGRAMS................................................................................141



INTRODUCTION

IF YOU KNOW NOTHING ABOUT PASCAL

Assuming you know nothing at all about Pascal, and in fact, that you may know nothing aboutprogramming in general, we will begin to study Pascal. If you are already somewhat familiar with

programming and especially Pascal, you will probably want to skip very quickly through the first few

chapters. You should at least skim these chapters, and you should read the remainder of this

introduction.

A few comments are in order to get us started in the right direction. The sample programs included on

the disks are designed to teach you the basics of Pascal and they do not include any clever or tricky

code. Nearly all of the programs are really quite dumb as far as being useful programs, but all will

teach one or more principles of Pascal. I have seen one tutorial that included a 12 page program as the

first example. In fact there were only 2 example programs in the entire tutorial, and it was impossible to

glean the essentials of programming from that system. For this reason, I will completely bypass any

long programs until the very end of this tutorial. In order to illustrate fundamental concepts used in

Pascal programming, all programs will be very short and concise until we reach the last chapter.

LARGER PASCAL PROGRAMS

Chapter 16 has some rather large programs to illustrate to you how to write a large program. It would

be a disservice to you to show you all of the constructs of Pascal and not show you how to put them

together in a meaningful way to build a large program. After completing all of the fundamentals of

Pascal, it will then be very easy for you to use the tools learned to build as large a program as you

desire or require for your next programming project.

Another problem I have noticed in example programs is the use of one word for all definitions. For

example, a sort program is stored in a file called SORT, the program is named Sort, and various partsof the program are referred to as Sort1, Sort2, etc. This can be confusing since you have no idea if the

program name must be the same as the filename, or if any of the other names were chosen to be the

same because of some obscure rule not clearly documented. For this reason, the example programs use

completely arbitrary names whenever the choice of a name adds nothing to the readability or clarity of

a program. As an illustration of this, the first program is named Puppy_Dog. This adds nothing to the

understanding of the program but does illustrate that the program name means nothing to the Pascal

compiler concerning what the program does.

Due to the fundamental design of the Pascal language, certain words are "reserved" and can only be

used for their defined purposes. These are listed in your TURBO Pascal reference manual. All of the

sample programs in this tutorial are written with the reserved words in all lower-case letters, and the

user variables in lower case with the first letter capitalised since this is becoming the accepted industry

standard. Don't worry about what reserved words are yet, they will be completely defined later.

In this tutorial, all reserved words, type names, variable names, and procedure and function names will

be listed in boldface type within the text as an aid to the student. Because it would add little and could

possible be confusing, the simple predefined types will not be listed in boldface type.

WHAT IS A COMPILER?

There are two methods used to run any computer program that is written in a readable form of English.

The first method is to use an interpreter. An interpreter is a program that looks at each line of the

"English" program, decides what the "English" on that line means, and does what it says to do. If one

of the lines is executed repeatedly, it must be scanned and analysed each time, greatly slowing downthe solution of the problem at hand. A compiler, on the other hand, is a program that looks at each

statement one time and converts it into a code that the computer understands directly. When the



compiled program is actually run, the computer does not have to figure out what each statement means,

it is already in a form that the computer can run directly, resulting in a much faster execution of theprogram.

This tutorial is written especially for Borland International's TURBO Pascal compilers version 5.0

through 6.0. These are very high quality compilers that can do nearly anything you will ask them to do

since they are so flexible. The original intent of this tutorial was to write it in such a way that it wouldbe completely generic and usable with any good Pascal compiler. The programmers at Borlandincluded a great many non-standard aids for the Pascal language and resulted in a very good product

that has dominated the market for microcomputers. To completely omit all of the extensions would do

those of you with the Borland compiler a real disservice, and to include the extensions would not allow

other compilers to be used effectively with this tutorial.

The decision was made to use the Borland extensions and make the tutorial very difficult to use withother compilers. If you have a need to use Pascal with some other compiler, TURBO Pascal is so

inexpensive that it would be a wise decision to purchase a copy solely for the purpose of learning the

Pascal programming language, then moving to a larger compiler on a minicomputer or a mainframe

using the accumulated knowledge to very quickly learn the extensions provided by that particular

compiler. At any rate, this tutorial will not teach you everything you will ever need to know about

Pascal. It will, however, teach you the fundamentals and the advanced features of Pascal, but of evenmore importance is the definition of Pascal terminology needed to progress on your own into more

advanced topics of Pascal and programming in general. You will find that experience will be your best

teacher.

WHICH VERSION OF TURBO PASCAL?

Some of the example programs will not work with some of the earlier versions of TURBO Pascal. This

is primarily due to fact that object oriented programming capabilities were added to version 5.5, and

improved on in version 6.0. Most of the example programs will work with any version however. It

should be pointed out that each successive version of TURBO Pascal has been an improvement over

the previous version since additional capabilities have been added, and each new one compiles a littlefaster and results in smaller but faster executable code than the previous version. Any of the versions of TURBO Pascal can be used to learn to program in Pascal, so whichever version you have on hand will

be adequate. Later, when you become more versed in programming techniques, you may wish to

upgrade to the absolute latest version.

EARLY VERSIONS OF TURBO PASCAL

Most of the files will compile properly with TURBO Pascal versions 2.0 through 4.0. No warning will

be given about which files will not compile with these versions since they have been superseded for so

long. If you are still using one of the earlier versions, it would you to purchase a newer version because

of the flexibility.

WHAT ABOUT TURBO PASCAL VERSION 5.5 & 6.0?

Chapters 14 and 15 of this tutorial are written especially for TURBO Pascal version 5.5 and 6.0 to

discuss the use of object oriented programming and how to use the Borland extensions. Since the topic

of object oriented programming is a very large and diverse field of study and only a limited space is

available to discuss it in this tutorial, these chapters will give you only a brief overview of what it is

and how to use it. You will find 13 complete example programs to get you started in this new and very

meaningful endeavour and this introduction should whet your appetite to continue your study in more

depth.

If you are using an early version of TURBO Pascal without the object oriented extensions, it would payyou to upgrade to learn how to use this new programming method. Object oriented programming has

the potential to greatly improve the quality of your code and to reduce the debugging time required.



PREPARATION FOR USE OF THIS TUTORIAL

Copy the example files into your TURBO Pascal working directory and you are ready to begin,

provided of course that you have already learned how to start the TURBO system and how to edit a

Pascal file. Be sure you make a backup copy of the Pascal source disk so you cannot accidentally lose

all information on the distribution disk. TURBO Pascal version 5.x (5.0 or 5.5) users should read

chapters 1 and 2 of the User's Guide, while version 6.0 users should read chapter 1 and quickly browse

through chapters 7 and 8 of the User's Guide. You should be familiar with the use of the editor supplied

with TURBO Pascal before beginning.

If you are not using TURBO Pascal, you will still be able to compile and execute many of these Pascal

files, since most of the examples use standard Pascal syntax. There will be some statements used which

are unique to TURBO Pascal and will not work with your compiler. This will be especially true when

you come to the chapter on standard input and output since this is where most compilers differ.

Unfortunately, this is one of the most important aspects of any programming language, since it is

required to get data into and out of the computer to do anything useful. You will also find that chapter

13, covering the topic of units, is unique to TURBO Pascal and will not work with any Pascal

compilers other than TURBO Pascal.

WHAT ABOUT THE PROGRAMMING EXERCISES?

It is highly suggested that you do the programming exercises after you complete the study for each

chapter. They are carefully selected to test your understanding of the material covered in that chapter. If

you do not write, enter, debug, and run these programs, you will only be proficient at reading Pascal. If

you do the exercises completely, you will have a good start at being a Pascal program writer.

It should also be mentioned that this tutorial will not teach you everything you will ever need to know

about Pascal. You will continue to learn new techniques as long as you continue to write programs.

Experience is the best teacher here just as it is in any endeavour. This tutorial will teach you enoughabout Pascal that you should feel very comfortable as you search through the reference manual for

some topic. You will also be able to read and understand any Pascal program you find in textbooks or

magazines. Although the primary goal of this tutorial is to teach you the syntax and use of Pascal, the

most important by-product is the knowledge of Pascal terminology you will gain. This terminology will

enable you to learn even more about Pascal and programming in general.

THE ANSWERS DIRECTORY

There is a directory on the distribution disk named ANSWERS which contains an answer to each of the

programming exercises given at the end of the chapters. You should attempt to do original work on

each of the exercises before referring to these answers, in order to gain your own programming

experience. These answers are given for your information in case you are completely stuck on how tosolve a particular problem. These answers are not meant to be the only answer, since there are many

ways to program anything, but they are meant to illustrate one way to solve the suggested programming

problem.

The answers are all in executable files named in the format CHnn_m.PAS where nn is the chapter

number, and m is the exercise number. If there is more than one answer required, an A, B, or C is

included following the exercise number.

A SPECIAL NOTE FOR THE SHAREWARE VERSION

It is impossible to include the graphics diagrams in chapter 12 in a pure ASCII text. They are therefore

omitted from this version of the tutorial. If you need these diagrams, they can be purchased directly

from Coronado Enterprises along with your registration. See the READ.ME file in the TEXT directory

for more information.



Chapter 1 : WHAT IS A COMPUTER PROGRAM?

THIS CHAPTER IS FOR NEW PROGRAMMERS

If you are a complete novice to computers you will find the information in this chapter useful. If however, you have had some experience with programming, you can completely ignore this chapter. It

will deal with a few fundamentals of computers in general and will introduce nothing that is specific to

Pascal.

WHAT IS A COMPUTER PROGRAM?

A computer is nothing but a very dumb machine that has the ability to perform mathematical operations

very rapidly and very accurately, but it can do nothing without the aid of a program written by a human

being. Moreover, if the human being writes a program that turns good data into garbage, the computer

will very obediently, and very rapidly, turn the good data into garbage. It is possible to write acomputer program with one small error in it that will do that very thing, and in some cases appear to be

generating good data. It is up to the human programmer to design a program to achieve the desired

results.

A computer program is simply a "recipe" which the computer will use on the input data to derive the

desired output data. It is similar to the recipe for baking a cake. The input data is comparable to the

ingredients, including the heat supplied by the oven. The program is comparable to the recipe

instructions to mix, stir, wait, heat, cool, and all other possible operations on the ingredients. Theoutput of the computer program can be compared to the final cake sitting on the counter ready to be cut

and served. A computer program is therefore composed of two parts, the data upon which the program

operates, and the program that operates on the data. The data and program are inseparable as implied

by the last sentence.

WHAT ARE CONSTANTS?

Nearly any computer program requires some numbers that never change throughout the program. They

can be defined once and used as often as needed during the operation of the program. To return to the

recipe analogy, once you have defined how big a tablespoon is, you can use the same tablespoon

without regard to what you are measuring with it. When writing a computer program, you can define

the value of PI = 3.141592, and continue to use it wherever it makes sense knowing that it is available,

and correct.

WHAT ARE VARIABLES?

In addition to constants, nearly every computer program uses some numbers that change in value

throughout the program. They can be defined as variables, then changed to any values that make sense

to the proper operation of the program. An example would be the number of eggs in the above recipe.

If a single layer of cake required 2 eggs, then a triple layer cake would require 6 eggs. The number of

eggs would therefore be a variable.

HOW DO WE DEFINE CONSTANTS OR VARIABLES?

All constants and variables have a name and a value. In the last example, the name of the variable was

"eggs", and the value was either 2 or 6 depending on when we looked at the stored data. In a computerprogram the constants and variables are given names in much the same manner, after which they can

store any value within the defined range. Any computer programming language has a means by which



constants or variables can be first named, then assigned a value. The means for doing this in Pascal will

be given throughout the remainder of this tutorial.

WHAT IS SO GOOD ABOUT PASCAL?

Some computer languages allow the programmer to define constants and variables in a very haphazard

manner and then combine data in an even more haphazard manner. For example, if you added the

number of eggs, in the above recipe, to the number of cups of flour, you would arrive at a valid

mathematical addition, but a totally meaningless number. Some programming languages would allow

you to do just such an addition and obediently print out the meaningless answer. Since Pascal requires

you to set up your constants and variables in a very precise manner, the possibility of such a

meaningless answer is minimised. A well written Pascal program has many cross checks to minimise

the possibility of a completely scrambled and meaningless output.

Notice however, in the last statement, that a "well written" Pascal program was under discussion. It is

still up to the programmer to define the data structure in such a way that the program can help prevent

garbage generation. In the end, the program will be no better than the analysis that went into the

program design.

If you are a novice programmer, do not be intimidated by any of the above statements. Pascal is a welldesigned, useful tool that has been used successfully by many computer novices and professionals.

With these few warnings, you are ready to begin.



Chapter 2 : GETTING STARTED IN PASCAL

YOUR FIRST PASCAL PROGRAM

Lets get right into a program that really does nothing, but is an example of the most trivial Pascalprogram. Load Turbo Pascal, then load TRIVIAL.PAS into the integrated environment as a work file.

This assumes you have been successful in learning how to use the TURBO Pascal system.

You should now have the most trivial Pascal program possible on your display, and we can take a look

at each part to define what it does.

The first line is required in the standard Pascal definition and is the program name which can be any

name you like, as long as it follows the rules for an identifier given in the next paragraph. It can have

no blanks, otherwise it would be considered as two words and it would confuse the compiler. The first

word program is the first of the reserved words mentioned earlier and it is the indicator to the Pascal

compiler that this is the name of the program. Notice that the line ends with a semicolon. Pascal uses

the semicolon as a statement separator and although all statements do not actually end in a semicolon,

most do, and the proper use of the semicolon will clear up later in your mind.

TURBO Pascal does not require the program statement, but to remain compatible with standard Pascal,

it will simply ignore the entire statement. It is recommended that you include a program name both to

aid your thinking in standard Pascal, and to add a little more indication of the purpose of each program.

program Puppy_Dog;

begin

end.

{ Result of execution

(There is no output from this program )

}

WHAT IS AN IDENTIFIER?

All identifiers, including the program name, procedure and function names, type definitions, and

constant and variable names, will start with an alphabetical character and be composed of any

combination of alphabetic and numeric characters with no embedded blanks. Upper or lower case

alphabetic characters are not significant and may be mixed at will. (If you find this definition confusing

at this point, don't worry about it, it will be clear later but it must be defined early). The standarddefinition of Pascal requires that any implementation (i.e. any compiler written by some company)

must use at least 8 characters of the identifier as significant and may ignore the remaining characters if

more than 8 are used. Most implementations use far more than 8. All versions of TURBO Pascal use 63

characters in an identifier as being significant.

Standard Pascal does not allow the use of underlines in an identifier but most implementations of

Pascal allow its use after the first character. All versions of TURBO Pascal compilers permit the use of the underline in an identifier, so it will be freely used throughout this tutorial. The underline is used in

the program name Puppy_Dog which should be on your display at this time.

Returning to the example program, line 2 is a blank line which is ignored by all Pascal compilers. More

will be said about the blank line at the end of this chapter.



NOW FOR THE PROGRAM

Lines 3 and 4 comprise the actual Pascal program, which in this case does absolutely nothing. This is

an illustration of the minimum Pascal program. The two words begin and end are the next two reserved

words we will consider. Any logical grouping of Pascal code can be isolated by bracketing it with the

two reserved words begin and end. You will use this construct repeatedly as you write Pascal code so it

is well to learn it thoroughly. Code to be executed by conditional jumps will be bracketed by begin and

end, as will code within a loop, and code contained within a subroutine (although they are called

procedures in Pascal), and in many other ways. In the present program, the begin and end are used to

bracket the main program and every Pascal program will have the main program bracketed in this

manner. Because there is nothing to do in this program, there are no statements between the begin and

end reserved words.

Finally, although it could be very easily overlooked, there is one more very important part of the

program, the period following the reserved word end. The period is the signal to the compiler that it has

reached the end of the executable statements and is therefore finished compiling. Every Pascal program

will have one, and only one period in it and that one period will be at the end of the program. I must

qualify that statement in this regard, a period can be used in comments, and in text to be output. In fact

there are some data formats that require using a period as part of their structure. Think of a Pascal

program as one long sentence with one period at the end. Ignore lines 9 through 13 for a few minutesand we will describe them fully later.

That should pretty well describe our first program. Now it is time for you to compile and run it. Hit

<alt> r to compile and run the program, then <alt> F5 to view the result of execution. Since this

program doesn't do anything, it is not very interesting, so let's look at one that does something.

A PROGRAM THAT DOES SOMETHING

Load the Pascal program WRITESM.PAS and view it on your monitor. The filename is sort of cryptic

for "Write Some" and it will display a little output on the monitor. The program name is Kitty_Cat

which says nothing about the program itself but can be any identifier we choose. We still have thebegin and end to define the main program area followed by the period. However, now we have two

additional statements between the begin and end. Writeln is a special word and it is probably not

surprising that it means to write a line of data somewhere. Without a modifier, which will be fully

explained in due time, it will write to the default device which, in the case of our IBM compatible, is

the video display. The data within the parentheses is the data to be output to the display and although

there are many kinds of data we may wish to display, we will restrict ourselves to the simplest for the

time being. Any information between apostrophes will simply be output as text information.

program Kitty_Cat;

begin

Writeln('This program');

Writeln('actually does something.');

end.


This program

actually does something.

}

The special word Writeln is not a reserved word but is defined by the system to do a very special job

for you, namely to output a line of data to the monitor. It is, in fact, a procedure supplied for you by thewriters of TURBO Pascal as a programming aid for you. You can, if you so desire, use this name for

some other purpose in your program, but doing so will not allow you to use the standard output



procedure. It will then be up to you to somehow get your data out of the program. Note carefully that

some words are reserved and cannot be redefined and used for some other purpose, and some arespecial since they can be redefined. You will probably not want to redefine any of the special words for

a long time. Until you gain considerable programming experience, simply use them as tools.

Notice the semicolon at the end of line 4. This is the statement separator referred to earlier and tells the

Pascal compiler that this line is complete as it stands, nothing more is coming that could be consideredpart of this statement. The next statement, in line 5, is another statement that will be executedsequentially following the statement in line 4. This program will output the two lines of text and stop.

Now it is time to go try it. Compile and run the program in the same manner as you did for the first

example program. You should see the two lines of text output to the video display every time you run

this program. When you grow bored of running WRITESM.PAS let's go on to another example.

ANOTHER PROGRAM WITH MORE OUTPUT

Examine the example program named WRITEMR.PAS. This new program has three lines of output but

the first two are different because another special word is introduced to us, namely Write. Write is a

procedure which causes the text to be output in exactly the same manner as Writeln, but Write does

not cause a carriage return to be output. Writeln causes its output to take place then returns the

"carriage" to the first character of the next line. The end result is that all three of the lines of text will be

output on the same line of the monitor when the program is run. Notice that there is a blank at the end

of each of the first two lines so that the formatting will look nice. Compile and execute the new

program.

Now might be a good time for you to return to editing WRITEMR.PAS and add a few more outputcommands to see if they do what you think they should do. When you tire of that, we will go on to the

next file and learn about comments within a Pascal program.

program Guppy_The_Fish;

begin

Write('This will ');

Write('all be ');

Writeln('on one line.');

end.


This will all be on one line.

}

ADDING COMMENTS IN THE PROGRAM

The file named PASCOMS.PAS is similar to the others except that comments have been added to

illustrate their use. Pascal defines comments as anything between (* and *) or anything between { and

}. Originally only the wiggly brackets were defined, but since many keyboards didn't have them

available, the parenthesis star combination was defined as an extension and is universal by now, so you

can use either. Most of the comments are self explanatory except for the one within the code. Since

comments can go from line to line, lines 11 and 12 that would normally print "send money", are not

Pascal code but are commented out. Try compiling and running this program, then edit the comments

out so that "send money" is printed also.



program Lots_Of_Comments;

begin { This is the start of the main program }

(* This is a comment that is ignored by the Pascal compiler *)

{ This is also ignored }

Writeln('I am in Pascal school, Dad'); (* Comment *)

Writeln('All students are always broke'); {Comment}(*

Writeln('Send money');

Writeln('Send money');

*)

Writeln('I am really getting hungry');

end. (* This is the end of the main program *)


I am in Pascal school, Dad

All students are always broke

I am really getting hungry

}

A fine point should be mentioned here. Even though some compilers allow comments to start with (*

and end with }, or to start with { and end with *), it is very poor programming practice and should be

discouraged. The ANSI Pascal standard allows such usage but TURBO Pascal does not allow thisfunny use of comment delimiters.

TURBO Pascal does not allow you to nest comments using the same delimiters but it does allow you to

nest one type within the other. This could be used as a debugging aid. If you generally use the (* and *)

for comments, you could use the { and } in TURBO Pascal to comment out an entire section of code

during debugging even if it had a few comments in it. This is a trick you should remember when youreach the point of writing programs of significant size.

When you have successfully modified and run the program with comments, we will go on to explain

good formatting practice and how Pascal actually searches through your source file (Pascal program)

for its executable statements.

It should be mentioned that the program named PASCOMS.PAS does not indicate good commenting

style. The program is meant to illustrate where and how comments can be used and looks very choppy

and unorganised. Further examples will illustrate good use of comments to you as you progress through

this tutorial.

THE RESULT OF EXECUTION SECTION

You should now be able to discern the purpose for lines 20 through 26 of this program. Each of the

example programs in this tutorial lists the result of execution in a similar comments section at the end

of the program. This makes it possible to study this tutorial anywhere once you print out the example

programs as described in the READ.ME file on the distribution disk. With this text, and a hard copy of

the example programs containing the result of execution, you do not need access to a computer tostudy. Of course you would need access to a computer to write, compile, and execute the programming

exercises, which you are heartily encouraged to do.

GOOD FORMATTING PRACTICE

Examine GOODFORM.PAS to see an example of good formatting style. It is important to note that

Pascal doesn't give a hoot where you put carriage returns or how many blanks you put in when a blank



is called for as a delimiter. Pascal only uses the combination of reserved words and end-of- statement

semicolons to determine the logical structure of the program. Since we have really only covered twoexecutable statements, I have used them to build a nice looking program that can be easily understood

at a glance. Compile and run this program to see that it really does what you think it should do.

program Good_Programming_Style;

begin

Write('Programming style ');

Write ('is a matter of ');

Writeln ('personal choice');

Write('Each person ');

Write ('can choose ');

Writeln ('his own style');

Write('He can be ');

Write ('very clear, or ');

Writeln ('extremely messy');

end.


Programming style is a matter of personal choice

Each person can choose his own style

He can be very clear, or extremely messy

}

VERY POOR FORMATTING PRACTICE

Examine UGLYFORM.PAS now to see an example of terrible formatting style. It is not really apparent

at a glance but the program you are looking at is exactly the same program as the last one. Pascal

doesn't care which one you ask it to run because to Pascal, they are identical. To you they are

considerably different, and the second one would be a mess to try to modify or maintain sometime in

the future.

program Ugly_Programming_Style;begin Write('Programming style ')

;Write ('is a matter of ');

Writeln('personal choice');Write('Each person ');

Write('can choose ');Writeln

('his own style');Write('He can be ');Write

('very clear, or ');

Writeln('extremely messy');end.


Programming style is a matter of personal choice

Each person can choose his own style

He can be very clear, or extremely messy

}

UGLYFORM.PAS should be a good indication to you that Pascal doesn't care about programming

style or form. Pascal only cares about the structure, including reserved words and delimiters such as

blanks and semicolons. Carriage returns are completely ignored as are extra blanks. You can put extrablanks nearly anywhere except within reserved words or variable names. You should pay some



attention to programming style but don't get too worried about it yet. It would be good for you to

simply use the style illustrated throughout this tutorial until you gain experience with Pascal. As timegoes by you will develop a style of statement indentation, adding blank lines for clarity, and a method

of adding clear comments to Pascal source code. Programs are available to read your source code, and

put it in a "pretty" format, but that is not important now.

Not only is the form of the program important, the names used for variables can be very helpful orhindering as we will see in the next chapter. Feel free to move things around and modify the format of any of the programs we have covered so far and when you are ready, we will start on variables in the

next chapter. Be sure you compile and execute UGLYFORM.PAS.

PROGRAMMING EXERCISES

1. Write a program that displays your name on the video monitor.

2. Modify your program to display your name and address on one line, then modify it by changing

the Write's to Writeln's so that the name and address are on different lines.



Chapter 3 : SIMPLE DATA TYPES

WHAT IS A DATA TYPE?

A type in Pascal, and in several other popular programming languages, defines a variable in such a waythat it defines a range of values which the variable is capable of storing, and it also defines a set of

operations that are permissible to be performed on variables of that type. TURBO Pascal has eight

basic data types which are predefined and can be used anywhere in a program provided you use them

properly. This chapter is devoted to illustrating the use of these eight data types by defining the

allowable range of values that can be assigned to them, and by illustrating the operations that can be

done to variables of these types. The eight types and a very brief description follows;

integer Whole numbers from -32768 to 32767byte The integers from 0 to 255real Floating point numbers from 1E-38 to 1E+38boolean Can only have the value TRUE or FALSEchar Any character in the ASCII character set

shortint The integers from -128 to 127word The integers from 0 to 65535longint The integers from -2147483648 to 2147483647

Please note that four of these types of data (char, shortint, word, and longint) are not a part of

the standard Pascal definition but are included as extensions to the TURBO Pascal compiler.

In addition to the above data types TURBO Pascal version 5.0 and later have the following data types

available;

single Real type with 7 significant digitsdouble Real type with 15 significant digits

extended Real type with 19 significant digitscomp The integers from about -10E18 to 10E18

TURBO Pascal version 5.0 and newer have these four types available which use the 80X87 math

coprocessor. Because TURBO Pascal has a software emulator for the floating point operations, an

80X87 math coprocessor is not absolutely required to use these new types with these versions. Of

course, your resulting program will run much faster if you have the coprocessor available for use by the

program.

A complete definition of the available types for each compiler can be found in the TURBO Pascal

reference manual. It would be good to read these pages now for a good definition prior to learning how

to define and use them in a program. Note that all of these will be used in example programs in this

chapter.

OUR FIRST VARIABLES

The integers are by far the easiest to understand so we will start with a simple program that uses some

integers in a very simple way. Load INTVAR.PAS into your TURBO system and let's take a look at it.

Immediately following the program statement is another reserved word, var. This reserved word is used

to define a variable before it can be used anywhere in the program. There is an unbroken rule of Pascal

that states "Nothing can be used until it is defined." The compiler will complain by indicating a

compilation error if you try to use a variable without properly defining it. It seems a bit bothersome to

have to define every variable prior to its use, but this rule will catch many spelling errors of variablesbefore they cause trouble. Some other languages will simply define a new variable with the new name

and go merrily on its way producing some well formatted garbage for you.



Notice that there is only one use of the reserved word var, but it is used to define three different

variables, Count, X, and Y. Once the word var is recognised, the compiler will continue to recognise

variable definitions line after line until it finds another reserved word. It would be permissible to put a

var on the second line also but it is not necessary. It would also be permissible to put all three

variables on one line but your particular programming style will dictate where you put the three

variables. Following the colon on each line is the word integer which is a standard identifier, and istherefore different from a reserved word. A standard identifier is predefined like a reserved word, butyou can redefine it, thereby losing its original purpose and meaning. For now and for a long time, don't

do that.

(* Chapter 3 - Program 1 *)

program Integer_Variable_Demo;

var Count : integer;

X,Y : integer;

begin

X := 12;

Y := 13;

Count := X + Y;

Writeln('The value of X is',X:4);Writeln('The value of Y is',Y:5);

Writeln('And Count is now ',Count:6);

end.


The value of X is 12

The value of Y is 13

And Count is now 25

}

OUR FIRST ARITHMETIC

Now that we have three variables defined as integer type variables, we are free to use them in aprogram in any way we desire as long as we use them properly. If we tried to assign a real value to X,

the compiler will generate an error, and prevent a garbage output. Observe the start of the main body of

the program. There are three statements assigning values to X, Y, and Count. A fine point of

mathematics would state that Count is only equal to the value of X+Y until one of them is modified,

therefore the equal sign used in so many other languages is not used here. The sign := is used, and can

be read as "is replaced by the value of," when reading a listing. Another quicker way is to use the word"gets". Thus X := X + 1 would be read, "X gets the value of X plus 1". We will see later that the simple

equal sign is reserved for another use in Pascal.

The first three statements give X the value of 12, Y the value of 13, and Count the value of 12 + 13 or

25. If we have a requirement to get those values out of the computer, we need another extension to the

Writeln statement. The first part of the data within the parentheses should be very familiar to you

now, but the second part is new.

Multiple outputs can be handled within one Writeln if the fields are separated by a comma. To

output a variable, simply write the variable's name in the output field. The number following the

variable in each case is the number of output columns to be used by the output data. This number is

optional and can be omitted allowing the system to use as many columns as it needs. For purposes of

illustration, they have all been assigned different numbers of columns. At this point, you can compile

and run INTVAR.PAS and examine its output.

To illustrate the various ways to output data, load INTVAR2.PAS and observe that even though the

output is identical, it is output in a completely different manner. The Writeln statements are broken



into pieces and the individual pieces are output with Write and Writeln statements. Observe

especially that a Writeln all by itself simply moves the cursor to the beginning of a new line on the

video monitor.

Compile and run this program and observe its output after you are certain that the two programs are

actually identical.


program More_Integer_Demos;

var X,Y : integer;

Count : integer;

begin

X := 12;

Y := 13;

Count := X + Y;

Write('The value of X is');

Writeln(X:4);

Write('The value of Y is');

Writeln(Y:5);

Write('And Count is now ');

Write(Count:6);Writeln;

end.


The value of X is 12

The value of Y is 13

And Count is now 25

}

NOW LET'S USE LOTS OF VARIABLES

Load ALLVAR.PAS to observe a short program using five of the basic data types. The variables are

simply assigned values and the values are printed. A complete and detailed description of the options

available in the Write statement is given in the TURBO Pascal reference manual. Check the index to

find this information for the version you are using. It would be to your advantage to read this section at

this time since very little explanation will be given about Write statements from this point on. We will

discuss the method by which we can write to disk files or other output devices in a later chapter of this

tutorial.

Back to the basic types. Pascal does lots of cross checking for obvious errors. It is illegal to assign the

value of any variable with a value that is of the wrong type or outside the allowable range of thatvariable. There are routines to convert from one system to another when that is necessary. Suppose, for

example, that you wished to use the value of an integer in a calculation of real numbers. That is

possible by first converting the integer into a real number of the same value and using the new real type

variable in the desired calculations. The new real type variable must of course be defined in a var

statement as a real type variable before it can be used. Details of how to do several conversions of this

kind will be given in the example program named CONVERT.PAS later in this chapter.

Since we have some variables defined, it would be nice to use the properties of computers for which

they are famous, namely some arithmetic. Two programs are available for your observation to illustrate

the various kinds of math available, REALMATH.PAS using real variables, and INTMATH.PAS using

integer variables. You can edit, compile, and run these on your own with no comment from me except

the comments embedded into the source files. You should output some of the results using the method

of outputting illustrated in line 24 of the previous example program. Read the definition of how to dothis in your TURBO Pascal User's Guide.




program All_Simple_Variable_Types;

var A,B : integer;

C,D : byte;

Dog_Tail : real;

Puppy : boolean;

Animal_Cookies : char;

begin

A := 4;

B := 5;

C := 212;

D := C + 3;

Dog_Tail := 345.12456;

Puppy := B > A; (* since B is greater than A, Puppy

will be assigned the value TRUE *)

Animal_Cookies := 'R'; (* this is a single character *)

Writeln('The integers are',A:5,B:5);

Writeln('The bytes are', C:5,D:5); (* notice that the spaces

prior to the C will be

ignored on output *)

Writeln('The real value is',Dog_Tail:12:2,Dog_Tail:12:4);

Writeln;Writeln('The boolean value is ',Puppy,Puppy:13);

Writeln('The char variable is an ',Animal_Cookies);

end.


The integers are 4 5

The bytes are 212 215

The real value is 345.12 345.1246

The boolean value is TRUE TRUE

The char variable is an R

}


program Real_Math_Demo;

var A,B,C,D : real;

begin

A := 3.234; {simply assigning a value}

B := A + 1.101; {adding a constant}

C := A + B; {summing two variables}

D := 4.234*A*B; {multiplication}

D := 2.3/B; {division}

D := A - B; {subtraction}

D := (A + B)/(12.56-B); {example of a multiple expression}

{It will be left up to you to print out some of the above values}

end.


(There is no output from this program )

}

The example program named INTMATH.PAS illustrates some of the math capabilities of Pascal whenusing integer class variables. A byte type variable is used just like an integer variable but with a much



smaller allowable range. Only one byte of computer memory is used for each variable defined as a byte

type variable, but 2 are used for each integer type variable.


program Integer_Math_Demo;

var A,B,C,D : integer;

E : real;

begin

A := 9; (* Simple assignment *)

B := A + 4; (* simple addition *)

C := A + B; (* simple addition *)

D := 4*A*B; (* multiplication *)

E := A/B; (* integer division with the result

expressed as a real number *)

D := B div A; (* integer division with the result

expressed as a truncated integer

number *)

D := B mod A; (* d is the remainder of the division,

in this case d = 4 *)

D := (A + B) div (B + 7); (* composite math statement *)

(* It will be up to you to print out some of these values *)

end.


(There is no output from this program)

}

BOOLEAN VARIABLES

Let's take a look at a boolean variable, which is only allowed to take on two different values, TRUE or

FALSE. This variable is used for loop controls, end of file indicators or any other TRUE or FALSE

conditions in the program. Variables can be compared to determine a boolean value. A complete list of

the relational operators available with Pascal is given in the following list.

= equal to<> not equal to

> greater than

< less than

>= greater than or equal to

<= less than or equal to

These operators can be used to compare any of the simple types of data including integer, char, byte,

and real type variables or constants, and they can be used to compare boolean variables. An illustration

is the best way to learn about the boolean variable so load BOOLMATH.PAS and observe it.

In BOOLMATH.PAS we define a few boolean variables and two integer type variables for use in theprogram and begin by assigning values to the two integer variables. The expression Junk = Who in line

14 is actually a boolean operation that is not true since the value of Junk is not equal to the value of

Who, The result is therefore FALSE and that value is assigned to the boolean variable A. The boolean

variable B is assigned the value of TRUE because the boolean expression Junk = (Who - 1) is true. The

boolean variables C and D are likewise assigned some values in a manner that should not need any

comment. After assigning a value to the variable with the big name, the values are all printed out. Notethat if either A or B is TRUE, the result is TRUE in line 18.




program Illustrate_What_Boolean_Math_Looks_Like;

(* notice the program name, it can be up to 63 characters long.

Variables can also be very long as we will see below *)

var A,B,C,D : boolean;

A_Very_Big_Boolean_Name_Can_Be_Used : boolean;

Junk,Who : integer;

begin

Junk := 4;

Who := 5;

A := Junk = Who; {since Junk is not equal to Who, A is false}

B := Junk = (Who - 1); {This is true}

C := Junk < Who; {This is true}

D := Junk > 10; {This is false}

A_Very_Big_Boolean_Name_Can_Be_Used := A or B; {Since B is true,

the result is true}

Writeln('result A is ',A);

Writeln('result B is ',B);

Writeln('result C is ',C);

Writeln('result D is ',D:12); {This answer will be right justified

in a 12 character field}

Writeln('result A_Very_Big_Boolean_Name_Can_Be_Used is ',A_Very_Big_Boolean_Name_Can_Be_Used);

(* Following are a few boolean expressions. *)

A := B and C and D;

A := B and C and not D;

A := B or C or D;

A := (B and C) or not (C and D);

A := (Junk = Who - 1) or (Junk = Who);

end.


result A is FALSE

result B is TRUE

result C is TRUE

result D is FALSE

result A_Very_Big_Boolean_Name_Can_Be_Used is TRUE

}

WHERE DO WE USE THE BOOLEAN VARIABLES?

We will find many uses for the boolean type variable when we study the loops and conditional

statements soon, but until then we can only learn what they are. Often, in a conditional statement, you

will want to do something if both of two things are true, in which case you will use the reserved wordand with two boolean expressions. If both are true, the result will be true. Line 29 is an example of this.

If the boolean variables B, C, and D, are all true, then the result will be true and A will be assigned the

value of TRUE. If any one of them is false, the result will be false and A will be assigned the value of

FALSE.

In Line 31, where the or operator is illustrated, if any of the three boolean variables is true, the result

will be true, and if all three are false, the result will be false. Another boolean operator is the not which

is illustrated in line 30. Examine line 33 which says the result is true only if the variable Junk is one

less than Who, or if Junk is equal to Who. This should indicate the level of flexibility available with a

boolean variable.

Compile and run this program, then add some additional printout to see if the boolean variables change

in the manner you think they should in the last few statements.



SHORT CIRCUIT OR COMPLETE EVALUATION?

Suppose you have several boolean expressions "and"ed together, and when evaluation starts, the first

expression results in a FALSE. Since the first expression is FALSE, it is impossible for the following

expressions to ever allow the final result to be TRUE because the first FALSE will force the answer to

be FALSE. It seems like a waste of execution time to continue evaluating terms if the final result is

already known, but that is exactly what standard Pascal will do because of the language definition. This

is known as complete evaluation of a boolean expression. If the system is smart enough to realise that

the final result is known, it could stop evaluation as soon as the final result is known. This is known as

short circuit evaluation of a boolean expression, and could also be applied if a term of an "or"ed

boolean expression resulted in a TRUE, since the result would always be TRUE.

TURBO Pascal versions 5.0 and later, allows you to choose between complete evaluation or short

circuit evaluation. The default for these compilers is the short circuit form but it can be changed

through the Options menu when you are using the integrated environment, or through use of a compilerdirective.

LET'S LOOK AT THE CHAR TYPE VARIABLE

A char type variable is a very useful variable, but usually not when used alone. It is very powerful

when used in an array or some other user defined data structure which is beyond the scope of thischapter. A very simple program, CHARDEMO.PAS is included to give you an idea of how a char type

variable can be used. Study, then compile and run CHARDEMO.PAS for a very brief idea of what the

char type variable is used for.


program Char_Demonstration;

var Letter : char;

Number : char;

Dogfood : char;

begin

Letter := 'P';

Number := 'A';

Dogfood := 'S';

Write(Letter,Number,Dogfood);

Letter := Number; (* This is now the 'A' *)

Number := 'L';

Dogfood := 'C';

Write(Dogfood,Letter,Number);

Writeln;

end.


PASCAL

}

Examine the sample program CONVERT.PAS for several examples of converting data from one

simple variable to another. The comments make the program self explanatory.




program Convert_From_Type_To_Type;

var Index,Count : integer;

Error_Ind : integer;

Size,Cost : real;

Letter : char;

Name,Amount : string[12];

begin

Index := 65;

Count := 66;

Cost := 124.678;

Amount := '12.4612';

Letter := Chr(Index); (* convert integer to char *)

Size := Count; (* convert integer to real *)

Index := Round(Cost); (* real to integer, rounded *)

Count := Trunc(Cost); (* real to integer, truncated *)

Index := Ord(Letter); (* convert char to integer *)

Str(Count,Name); (* integer to string of char *)

Val(Amount,Size,Error_Ind); (* string to real note that

"Error_Ind" is used forreturning an error code *)

Writeln('Name is ',Name,' and Size is ',Size:10:4);

end.


Name is 124 and Size is 12.4612

}

EXTENDED INTEGER TYPES

Display the program EXTINT.PAS for an example of using the extended integer types available with

the Pascal compiler. Four variables are defined and values assigned to each, then the results are

displayed. When you compile and run the program, you will see that the variable Big_int can indeed

handle a rather large number.


program Extended_Integer_Types;

var Index : integer;

Big_int : longint;

Small_int : shortint;

Pos_int : word;

begin

Index := MaxInt;

Small_int := 127;

Pos_int := Index + 256 * Small_int;

Big_int := 1000 * MaxInt + 1234;

Writeln('Index = ',Index:12);

Writeln('Small_int = ',Small_int:12);

Writeln('Pos_int = ',Pos_int:12);

Writeln('Big_int = ',Big_int:12);

Writeln;

Big_int := 1000 * Index + 1234;

Writeln('Big_int = ',Big_int:12);

end.




Index = 32767

Small_Int = 127

Pos_Int = 65279

Big_Int = 32768234

Big_Int = 234

}

It must be pointed out that the calculation in lines 13 and 21 result in a different answer even though

they appear to be calculating the same thing. An explanation is in order. The quantity named MaxInt

used in lines 10 and 13 is a constant built into the system that represents the largest value that an

integer type variable can store. On the first page of this chapter we defined that as 32767 and when

running the program you will find that Index displays that value as it should. The constant MaxInt has

a type that is of a universal_integer type as do all of the numeric constants in line 13. The result

then is calculated to the number of significant digits dictated by the left hand side of the assignmentstatement which is of type longint resulting in a very large number.

When we get to line 21, however, the variable Index is of type integer so the calculations are done as

though the constants were of type integer also which causes some of the more significant digits to be

truncated. The truncated result is converted to type longint and assigned to the variable Big_int

and the truncated value is displayed by line 22.

After that discussion it should be apparent to you that it is important what types you use for your

variables. It must be emphasised that it would not be wise to use all large type variables because they

use more storage space and slow down calculations. Experience will dictate the proper data types to use

for each application.

EXTENDED REAL TYPES

Display the program EXTREAL.PAS for an example using the new "real" types available with the

newer versions of TURBO Pascal.

If you are using a version of TURBO Pascal which is 5.0 or newer, you can use the 80X87 mathcoprocessor. If you do not have a math coprocessor, TURBO Pascal version 5.0 and newer has an

emulator mode which can be used as instructed in the User's Guide. Keep in mind that, even though the

emulator will allow you to use these newer data types, the resulting program will execute much slower

due to the extra calculations required.

This program should be self explanatory so nothing will be said except that when you run it, you can

observe the relative accuracy of each of the variable types. Once again, you should keep in mind that

use of the larger "real" types costs you extra storage space and reduced run-time speed, but gives you

more accuracy.




program Extended_Real_Types;

(* Note: If you are using TURBO Pascal Version 5.0 or newer *)

(* and you do not have a Math Co_Processor, you can *)

(* still compile and run this program by using the *)

(* compiler directive as explained in the User's Guide. *)

var Number : real;Small_Number : single;

Big_Number : double;

Huge_Number : extended;

Whole_Number : comp;

begin

Number := 100000000000000000000000000.0;

Small_Number := 100000000000000000000000000.0;

Big_Number := 100000000000000000000000000.0;

Huge_Number := 100000000000000000000000000.0;

Whole_Number := 1000000000000000000.0;

Writeln('Number = ',Number :40:3);

Writeln('Small_Number = ',Small_Number:40:3);

Writeln('Big_Number = ',Big_Number :40:3);

Writeln('Huge_Number = ',Huge_Number :40:3);Writeln('Whole_Number = ',Whole_Number:40:3);

end.


Number = 99999999999985900100000000.000

Small_Number = 100000002537764290000000000.000

Big_Number = 100000000000000005000000000.000

Huge_Number = 100000000000000000000000000.000

Whole_Number = 1000000000000000000.000

}

PROGRAMMING EXERCISE

1. Write a program containing several variable definitions and do some math on them, printing out

the results.



Chapter 4 : LOOPS & CONTROL STRUCTURES

Every program we have examined to this point has been a simple one pass through with no statements

being repeated. As in all other languages, Pascal has extensive capabilities to do looping andconditional branching. We will look at these now.

THE FOR LOOP

We will start with what may be the easiest structure to understand, the for loop. This is used to repeat a

single Pascal statement any number of times we desire. Load LOOPDEMO.PAS and we will discuss

the loops presented there.

The example illustrated in lines 13 and 14, is the simplest loop and does nothing more than execute a

Writeln 7 times. We have three new reserved words, for, to, and do which are used as shown. Any

simple variable of type integer, byte, or char can be used for the loop index but due to the requirement

that everything must be defined prior to use in Pascal, the loop index must be defined in a var

statement. Following the reserved word do is any single Pascal statement that will be repeated the

specified number of times. Note that the loop is an incrementing loop but substitution of downto forto will make it a decrementing loop as is illustrated in the last example in this program. It should be

pointed out that the loop control variable can only be incremented or decremented by 1 each time

through the loop in Pascal.


program Demonstrate_Loops;


Start : integer;

Ending : integer;

Total : integer;

Alphabet : char;

begin

Start := 1;Ending := 7;

for Count := Start to Ending do (* Example 1 *)

Writeln('This is a count loop and we are in pass',Count:4);

Writeln;

Total := 0;

for Count := 1 to 10 do begin (* Example 2 *)

Total := Total + 12;

Write('Count =',Count:3,' Total =',Total:5);

Writeln;

end;

Writeln;

Write('The alphabet is ');

for Alphabet := 'A' to 'Z' do (* Example 3 *)

Write(Alphabet);Writeln;

Writeln;

for Count := 7 downto 2 do (* Example 4 *)

Writeln('Decrementing loop ',Count:3);

end.


This is a count loop and we are in pass 1









Count = 1 Total = 12






Count = 7 Total = 84Count = 8 Total = 96



The alphabet is ABCDEFGHIJKLMNOPQRSTUVWXYZ

Decrementing loop 7

Decrementing loop 6

Decrementing loop 5

Decrementing loop 4

Decrementing loop 3

Decrementing loop 2

}

A COMPOUND PASCAL STATEMENT

The example given in lines 18 through 22 contains our first compound Pascal statement. It was

mentioned in Chapter 1 that the begin end pair of reserved words could be used to mark the limits of a

compound statement. In this case, the single compound statement starting with the begin at the end of

line 18 and extending through and including the end statement in line 22 is the single Pascal statement

that will be executed 10 times. A second variable Total has been introduced to simply add another

operation to the loop. Any valid Pascal operation can be performed within the begin end pair, including

another for loop, resulting in nested loops to whatever depth you desire.

The third example shows how the char type variable could be used in a for loop. Pascal requires thatthe loop index, the starting point, and the ending point all be of the same type or it will generate an

error message during compilation. In addition, they must be variables of type integer, byte, or char. The

starting point and ending point can be constants or expressions of arbitrary complexity.

The fourth example is a decrementing loop as mentioned earlier. It uses the reserved word downto,

and should be self explanatory.

THE IF STATEMENT

Pascal has two conditional branching capabilities, the if and the case statements. We will look at the

simplest of the two now, the if statement. Load IFDEMO.PAS for an onscreen look at the if then pair

of reserved words first illustrated in lines 11 and 12. Any condition that can be reduced to a booleananswer is put between the if then pair of words. If the resulting expression resolves to TRUE, then the

single Pascal statement following the reserved word then is executed, and if it resolves to FALSE, then

the single statement is skipped over. Of course, you can probably guess that the single statement can be

replaced with a compound statement bracketed with a begin end pair and you are correct. Study

example 1 and you will see that the line will always be printed in this particular fragment because

Three is equal to One + Two. It is very difficult to come up with a good example without combiningsome of the other control structures but we will do so in the next example program.

The second example in lines 14 through 19, is similar to the first but has the single statement replaced

with a compound statement and should be simple for you to understand.




program Demonstrate_Conditional_Branching;

var One,Two,Three : integer;

begin (* main program *)

One := 1; (* these are to have some numbers *)

Two := 2; (* to use for illustrations *)

Three := 3;

if Three = (One + Two) then (* Example 1 *)

Writeln('three is equal to one plus two');

if Three = 3 then begin (* Example 2 *)

Write('three is ');

Write('equal to ');

Write('one plus two');

Writeln;

end;

if Two = 2 then (* Example 3 *)

Writeln('two is equal to 2 as expected')

else

Writeln('two is not equal to 2... rather strange');

if Two = 2 then (* Example 4 *)

if One = 1 then

Writeln('one is equal to one')

else

Writeln('one is not equal to one')

else

if Three = 3 then

Writeln('three is equal to three')

else

Writeln('three is not equal to three');

end. (* main program *)


three is equal to one plus two

three is equal to one plus two

two is equal to 2 as expected

one is equal to one

}

The third example in lines 21 through 24, contains a new reserved word, else. When the if condition is

FALSE, the single statement following the word then is skipped and if a semicolon is encountered, the

if clause is totally complete. If instead of a semicolon, the reserved word else is encountered, then the

single Pascal statement following else is executed. One and only one of the two statements will beexecuted every time this if statement is encountered in the program. Examination of the third example

should clear this up in your mind.

Notice that the Pascal compiler is looking for either a semicolon to end the if, or the reserved word else

to continue the logic. It is therefore not legal to use a semicolon immediately preceding the reserved

word else. You will get a compiler error if you include the semicolon. This is a common error, but easy

to fix, so you will get used to writing it correctly.

THE IF-THEN-ELSE BLOCK

Put on your thinking cap because the next principle is difficult to grasp at first but will suddenly clearup and be one of the most useful facts of Pascal programming. Since the entire if then else block of

code is itself a single Pascal statement by definition, it can be used anywhere that an executable



statement is legal without begin end separators. This is shown in the fourth example of the

IFDEMO.PAS Pascal example program. Lines 27 through 30 comprise a single Pascal statement, andlines 32 through 35 comprise another. The if statement begun in line 26 therefore has a single statement

in each of its branches, and it is a single Pascal statement itself beginning in line 26 and ending in line

35. Reread this paragraph until you understand it completely, because it is a very important concept.

The if then else construct is one of the most used, most useful, and therefore most important aspects of Pascal. For this reason you should become very familiar with it.

Try changing some of the conditions in the example program to see if you can get it to print when you

expect it to for your own practice. When you are ready, we will go on to a program with loops and

conditional statements combined and working together.

LOOPS AND IFS TOGETHER

Load LOOPIF.PAS and study it for a few minutes. It contains most of what you have studied so far and

should be understandable to you at this point. It contains a loop (lines 7 through 17) with two if

statements within it (lines 8 & 9 and lines 10 through 16), and another loop (lines 11 through 15) within

one of the if statements.


program Loops_And_Ifs;

var Count,Index : integer;

begin (* Main program *)

for Count := 1 to 10 do begin (* Main loop *)

if Count < 6 then

Writeln('The loop counter is up to ',Count:4);

if Count = 8 then begin

for Index := 8 to 12 do begin (* Internal loop *)

Write('The internal loop index is ',Index:4);

Write(' and the main count is ',Count:4);

Writeln;end; (* Internal loop *)

end; (* if Count = 8 condition *)

end; (* Main loop *)

end. (* Main program *)


The loop counter is up to 1





The internal loop index is 8 and the main count is 8





}

You should make careful note of the formatting used here. The begin is at the end of the line which

starts the control and the end is lined up under the control word such that it is very clear which controlword it is associated with. All statements within each control structure are indented three spaces which

greatly adds to the readability. You will develop your own clear method of formatting your code in

time but until then it is suggested that you follow this example which is written in a manner which is

acceptable within the general Pascal programming community.



An easily made error should be pointed out at this time. If an extraneous semicolon were put at the end

of the if statement in line 8, the code following the statement would always be executed because thenull statement (the nothing statement between the then and the semicolon) would be the conditional

statement. The compiler would not generate an error and you would get no warning.

Add a semicolon at the end of line 8 to see the error. Of course, you will need to compile and execute

the program to see line 9 print for all 10 values of Count.

FINALLY, A MEANINGFUL PROGRAM

Load TEMPCONV.PAS and study its structure. Notice the header block that defines the program and

gives a very brief explanation of what the program does. This program should pose no problem to you

in understanding what it does since it is so clearly documented. If you study the style of indenting used

here, you will learn to appreciate the clarity afforded by the indentation. Compile and run this program

and you will have a list of Centigrade to Fahrenheit temperature conversions with a few added notes.


(**************************************************************)

(* Centigrade to Farenheight temperature conversion *)(* *)

(* This program generates a list of temperature conversions *)

(* with a note at the freezing point of water, and another *)

(* note at the boiling point of water. *)

(**************************************************************)

program Temperature_Conversion;

var Count,Centigrade,Farenheight : integer;

begin

Writeln('Centigrade to farenheight temperature table');

Writeln;

for Count := -2 to 12 do begin

Centigrade := 10*Count;

Farenheight := 32 + Centigrade*9 div 5;Write(' C =',Centigrade:5);

Write(' F =',Farenheight:5);

if Centigrade = 0 then

Write(' Freezing point of water');

if Centigrade = 100 then

Write(' Boiling point of water');

Writeln;

end;

end.


Centigrade to farenheight temperature table

C = -20 F = -4

C = -10 F = 14

C = 0 F = 32 Freezing point of water

C = 10 F = 50

C = 20 F = 68

C = 30 F = 86

C = 40 F = 104

C = 50 F = 122

C = 60 F = 140

C = 70 F = 158

C = 80 F = 176

C = 90 F = 194

C = 100 F = 212 Boiling point of water

C = 110 F = 230

C = 120 F = 248

}



Load, examine, and run DUMBCONV.PAS for a good example of poor variable naming. The structure

of the program is identical to the last program and when you run it, you will see that it is identical in

output, but compared to the last program, it is difficult to understand what it does by studying the

listing. We studied UGLYFORM.PAS in chapter 2 of this tutorial and it illustrated really stupid

formatting that nobody would ever use. The poor choice of variable names and lack of comments in thepresent program is nearly as unreadable, but many programmers are content to write code similar tothis example. You should be conscious of good formatting style and naming conventions from the start

and your programs will be clear, easy to understand, and will run efficiently. This program, like the last

should be easily understood by you, so we will go on to our next Pascal control structure.


program Temperature_Conversion;

var A1,A2,A3 : integer;

begin

Writeln('Centigrade to farenheight temperature table');

Writeln;

for A1 := -2 to 12 do begin

A2 := 10*A1;A3 := 32 + A2*9 div 5;

Write(' C =',A2:5);

Write(' F =',A3:5);

if A2 = 0 then

Write(' Freezing point of water');

if A2 = 100 then

Write(' Boiling point of water');

Writeln;

end;

end.


Centigrade to farenheight temperature table

C = -20 F = -4

C = -10 F = 14

C = 0 F = 32 Freezing point of water

C = 10 F = 50

C = 20 F = 68

C = 30 F = 86

C = 40 F = 104

C = 50 F = 122

C = 60 F = 140

C = 70 F = 158

C = 80 F = 176

C = 90 F = 194

C = 100 F = 212 Boiling point of water

C = 110 F = 230

C = 120 F = 248

}

THE REPEAT UNTIL LOOP

The next two Pascal constructs are very similar because they are both indefinite loops (indefinitebecause they are not executed a fixed number of times). One of the loops is evaluated at the top and the

other at the bottom. It will probably be easier to start with the repeat until construct which is the loop

that is evaluated at the bottom.

Examine the file named REPEATLP.PAS to see an example of a repeat loop. Two more reservedwords are defined here, namely repeat and until. This rather simple construct simply repeats all



statements between the two reserved words until the boolean expression following the until is found to

be TRUE. This is the only expression in Pascal that operates on a range of statements rather than asingle statement and begin end delimiters are not required.

A word of caution is in order here. Since the loop is executed until some condition becomes TRUE, it

is possible that the condition will never be TRUE and the loop will never terminate. It is up to you, the

programmer, to insure that the loop will eventually terminate. Compile and execute REPEATLP.PASto observe the output.


program Repeat_Loop_Example;


begin

Count := 4;

repeat

Write('This is in the ');

Write('repeat loop, and ');

Write('the index is',Count:4);

Writeln;

Count := Count + 2;

until Count = 20;Writeln(' We have completed the loop ');

end.


This is in the repeat loop, and the index is 4







This is in the repeat loop, and the index is 18We have completed the loop

}

THE WHILE LOOP

The file WHILELP.PAS contains an example of another new construct, the while loop. This uses the

while do reserved words and will execute one Pascal statement (or one compound statement bounded

with begin and end) continuously until the boolean expression between the two words becomes

FALSE. This loop is also indeterminate and could, like the repeat until loop, never terminate. You

should therefore exercise care in using it.

There are two basic differences in the last two loops. The repeat until loop is evaluated at the bottom of

the loop and must therefore always go through the loop at least one time. The while loop is evaluated at

the top and may not go through even once. This gives you flexibility when choosing the loop to do the

job at hand. Compile, run, and examine the output from the example program WHILELP.PAS.




program While_Loop_Example;

var Counter : integer;

begin

Counter := 4;

while Counter < 20 do begin

Write('In the while loop, waiting ');Write('for the counter to reach 20. It is',Counter:4);

Writeln;

Counter := Counter + 2;

end;

end.


In the while loop, waiting for the counter to reach 20. It is 4




In the while loop, waiting for the counter to reach 20. It is 12In the while loop, waiting for the counter to reach 20. It is 14



}

THE CASE STATEMENT

The final control structure introduces one more reserved word, case. The case construct actually

should be included with the if statement since it is a conditional execution statement, but we saved it

for last because it is rather unusual and will probably be used less than the others we have discussed in

this chapter.

The case statement is used to select one of many possible simple Pascal statements to execute based

on the value of a simple variable. Load the file CASEDEMO.PAS and observe the program for an

example of a case statement. The variable between the case and of reserved words in line 9 is the

variable used to make the selection and is called the case selector. Following that, the various selectionsare listed as a possible value or range, followed by a colon, a single Pascal statement, and a semicolon

for each selector. Following the list of selections, an else can be added to cover the possibility that none

of the selections were executed. Finally, an end statement is used to terminate the case construct. Note

that this is one of the few places in Pascal that an end is used without a corresponding begin.

The example file uses Count for the case selector, prints the numbers one through five in text form, and

declares that numbers outside this range are not in the allowable list. The program should be self

explanatory beyond that point. Be sure to compile and run this example program.

Load and display the sample program BIGCASE.PAS for another example of a case statement with a

few more added features. This program uses the identical structure as the previous program but in line

11 a range is used as the selector so that if the value of Count is 7, 8, or 9 this selection will be made. In

line 12, three different listed values will cause selection of this part of the code. Of greater importance

are the compound statements used in some of the selections. If the variable Count has the value of 2, 4,

or 6, a compound statement will be executed and if the value is 3, a for loop is executed. If the value is1, an if statement is executed which will cause a compound statement to be executed. In this case the if

statement will always be executed because TRUE will always be true, but any Boolean expression

could be used in the expression. Be sure to compile and run this program, then study the output untilyou understand the result thoroughly.




program Demonstrate_Case;



for Count := 0 to 8 do begin

Write(Count:5);

case Count of1 : Write(' One'); (* Note that these do not have *)

2 : Write(' Two'); (* to be in consecutive order *)

3 : Write(' Three');

4 : Write(' Four');

5 : Write(' Five');

else Write(' This number is not in the allowable list');

end; (* of case structure *)

Writeln;

end; (* of Count loop *)

end. (* of main program *)


0 This number is not in the allowable list

1 One

2 Two

3 Three

4 Four

5 Five




}




program A_Bigger_Case_Example;


Index : byte;



Write(Count:5);

case Count of

7..9 : Write(' Big Number');

2,4,6 : begin Write(' Small');

Write(' even');

Write(' number.');

end;

3 : for Index := 1 to 3 do Write(' Boo');

1 : if TRUE then begin Write(' TRUE is True,');

Write(' and this is dumb.');

end;

else Write(' This number is not in the allowable list');

end; (* of case structure *)

Writeln;

end; (* of Count loop *)



1 TRUE is true, and this is dumb.

2 Small even number.

3 Boo Boo Boo




7 Big number

8 Big number

9 Big number


}

This brings us to the end of chapter 4 and you now have enough information to write essentially any

program desired in Pascal. You would find however, that you would have a few difficulties if you

attempted to try to write a very big program without the topics coming up in the next few chapters. The

additional topics will greatly add to the flexibility of Pascal and will greatly ease programming with it.


1. Write a program that lists the numbers from 1 to 12 and writes a special message beside the

number representing your month of birth.

2. Write a program that lists all of the numbers from 1 to 12 except for the numbers 2 and 9.



Chapter 5 : PROCEDURES AND FUNCTIONS

A PASCAL PROGRAM OUTLINE

In order to properly define procedures and functions we need to lay some groundwork in the form of afew definitions. These are important concepts, so pay close attention. Every Pascal program is

composed of three fundamental parts which can be defined as follows.

Program Heading

This is the easiest part since it is only one line, at least it has been in all of our programs up to this

point. It is simply the program line, and it never needs to be any more involved or complicated than ithas been up to this point in TURBO Pascal. You may remember that we said it is not even necessary in

TURBO Pascal.

Declaration Part

This is the part of the Pascal source code in which all constants, variables, and user defined auxiliary

operations are defined. Some of the programs we have examined have had one or more vardeclarations. These are the only components of the declaration part we have used to this point. There

are actually five components in the declaration part, and procedures and functions which are the topics

of this chapter, are the fifth part. We will cover the other components in the next chapter.

Statement Part

This is the last part of any Pascal program, and it is what we have been calling the main program. It is

one compound statement bracketed with the reserved words begin and end. Note that neither of these

two words are optional since both are required to have a Pascal program.

It is very important that you grasp the above definitions because we will be referring to them constantly

during this chapter and throughout the remainder of this tutorial. With that introduction, let's go on to

our first Pascal program with a procedure in it, in fact it will have three procedures.

THE FIRST PROCEDURES

Examine PROCED1.PAS as your first example program with a procedure and display it on your

monitor. You will notice that it doesn't look like anything you have seen up to this point because it has

procedures in it. Let's go back to our definitions from above. The first line is the Program Heading

which should pose no difficulty. The Declaration Part begins with the var statement in line 4 and

continues down through and including all three procedures ending in line 19. Lines 21 through 26

constitute the Statement Part. It may seem strange that what appears to be executable Pascal statements,

and indeed they are executable statements, are contained in the Declaration Part rather than the

Statement Part. This is because of the Pascal definition and it will make sense when we have completed

our study of procedures and functions.

Continuing to examine PROCED1.PAS, we will make note of the program itself, which is the

Statement Part. The program, due to the nature of Pascal and the carefully chosen procedure names,

clearly tells us what it will do. It will write a header, eight messages, and an ending. The only problem

we are faced with is, how will it write these messages? This is where the Declaration Part is called

upon to define these operations in detail. The Declaration Part contains the three procedures which will

completely define what is to be done by the procedure calls in the main program.




program First_Procedure_Call;


procedure Write_A_Header;begin

Writeln('This is the header');

end;

procedure Write_A_Message;

begin

Writeln('This is the message and the count is',Count:4);

end;

procedure Write_An_Ending;

begin

Writeln('This is the ending message');

end;


Write_A_Header;

for Count := 1 to 8 do

Write_A_Message;

Write_An_Ending;



This is the header

This is the message and the count is 1





This is the message and the count is 6This is the message and the count is 7


This is the ending message

}

DEFINITIONS GO IN THE DEFINITION PART

It should be clear to you that the definitions of the procedures should be in the Definition Part of the

program because that is exactly what they do. In the case of a var, a variable is defined for later use by

the main program, and in the case of a procedure, the procedure itself is defined for later use by the

main program.

Let's arbitrarily pick one of the procedures, the first, and examine it in detail. The first executable

statement we come to in the main program is line 22 which says simply, Write_A_Header,

followed by the usual statement separator, the semicolon. This is a simple procedure call. When the

compiler finds this statement, it goes looking for a predefined procedure of that name which it can

execute. If it finds one in the Declaration Part of the program, it will execute that procedure. If it

doesn't find a user defined procedure, it will search the Pascal library for a system defined procedure

and execute it. The Write and Writeln statements are system procedures, and you have already

been using them quite a bit, so procedures are not completely new to you. If the compiler doesn't find

the procedure defined in either place, it will generate an error message. Depending on which version of TURBO Pascal you are using, the system may search several libraries to find the procedures called by

the program. Much more will be said about this later in this tutorial.



HOW TO DEFINE & CALL A PROCEDURE

To call a procedure, we simply need to state its name. To define a simple procedure, we use the

reserved word procedure followed by its calling name, with a semicolon as a terminator. Following the

Procedure Heading, there is the Declaration Part of the procedure followed by a body which is nothing

more than a compound statement bracketed by the reserved words begin and end. This is identical to

the Statement Part of the main program except that the procedure ends with a semicolon instead of a

period. Any valid Pascal statements can be put between the begin and end, and in fact, there is no

difference in what can be put in a procedure and what can be put in the main program.

The program we are examining would be no different if we would eliminate the first procedure

completely and move the Writeln contained in it down to the Statement Part in place of

Write_A_Header . If that statement is not clear, go back and reread the last two paragraphs until it

is.

Lines 23 and 24 will cause the procedure named Write_A_Message to be called 8 times, each time

writing a line of output to the monitor. Suffice it to say at this time, that the value of the variable Count,

as defined here, is available globally, meaning anywhere in the entire Pascal program. We will define

the scope of variables shortly. Finally, the last procedure call is made, causing the ending message to bedisplayed, and the program execution is complete.

THE UNBROKEN RULE OF PASCAL

Having examined your first Pascal procedures, there is a fine point that is obvious but could be easily

overlooked. We mentioned the unbroken rule of Pascal in an earlier chapter and it must be followed

here too. "Nothing can be used in Pascal until it has been defined". The procedures must all be defined

ahead of any calls to them, once again emphasising the fact that they are part of the Declaration Part of

the program, not the Statement Part.

Compile and run PROCED1.PAS to verify that it does what you expect it to do.

MORE PROCEDURE CALLS

Assuming you have run PROCED1.PAS successfully and understand its output, let's go on toPROCED2.PAS and examine it. In this program we will see how to call a procedure and take along

some data for use within the procedure. To begin with, notice that there are three procedure calls in the

Statement Part of the program and each has an additional term not contained in the calls in the last

program, namely the variable name Index within brackets. This is Pascal's way of taking along a

variable parameter to the procedure.

You will notice that the variable Index is defined as an integer type variable in the very top of the

Declaration Part. Since we are taking an integer type variable along when we visit the procedurePrint_Data_Out , it had better be expecting an integer variable as input or we will have a type

mismatch. In fact, observing the procedure heading itself in line 7, indicates that it is indeed expecting

an integer variable but it prefers to call the variable Puppy inside of the procedure. Calling it

something different poses no problem as long as the main program doesn't try to call its variable

Puppy, and the procedure doesn't try to use the name Index. Both are actually referring to the same

piece of data but they simply wish to refer to it by different names.




program Another_Procedure_Example;


Index : integer;

procedure Print_Data_Out(Puppy : integer);

begin

Writeln('This is the print routine',Puppy:5);

Puppy := 12;

end;

procedure Print_And_Modify(var Cat : integer);

begin

Writeln('This is the print and modify routine',Cat:5);

Cat := 35;

end;



Index := Count;

Print_Data_Out(Index);

Writeln('Back from the print routine, Index =',Index:5);

Print_And_Modify(Index);Writeln('Back from the modify routine, Index =',Index:5);

Print_Data_Out(Index);

Writeln('Back from print again and the Index =',Index:5);

Writeln; (* This is just for formatting *)

end;



This is the print routine 1

Back from the print routine, Index = 1

This is the print and modify routine 1

Back from the modify routine, Index = 35This is the print routine 35

Back from print again and the Index = 35




Back from the modify routine, Index = 35






Back from the modify routine, Index = 35



}

FORMAL AND ACTUAL PARAMETERS

The parameters listed within the parentheses of the procedure header are called the formal parameters,

whereas the parameters listed within the parentheses of the procedure call in the main program are

referred to as the actual parameters. Observe that the next procedure is called with Index as the actual

parameter and the procedure prefers to use the name Cat as the formal parameter name. In both cases,

the procedures simply print out the parameter passed to it, and each then try to modify the value passedto it before passing it back. We will see that one will be successful and the other will not.



We are in a loop in which Count is incremented from 1 to 3 and Pascal does not allow us to modify

the loop variable so we make a copy of the value in line 21 and call it Index. We can then modify

Index in the main program if we desire.

CALL BY VALUE

In line 7, the procedure heading does not contain the reserved word var in front of the passed

parameter and therefore the parameter passing is only one way because of the way Pascal is defined.

Without the reserved word var in front of the variable Puppy, the system makes a copy of Index,

and passes the copy to the procedure which can do anything with it, using its new name, Puppy, but

when control returns to the main program, the original value of Index is still there. The copy of Index

named Puppy is modified in the procedure, but the original variable named Index remains

unchanged. You can think of the passed parameter without the var as one way parameter passing. This

is a "call by value" because only the value of the actual variable is passed to the procedure.

CALL BY REFERENCE

In line 13, the second procedure has the reserved word var in front of its desired name for the variable,

namely Cat, so it can not only receive the variable, it can modify it, and return the modified value to

the main program. A copy is not made, but the original variable named Index is actually passed to

this procedure and the procedure can modify it, therefore communicating with the main program. The

formal parameter name, Cat in the procedure, is actually another name for the actual variable named

Index in the main program. A passed parameter with a var in front of it is therefore a two way

situation. This is a "call by reference" since a reference to the original variable is passed to the

procedure.

SOME NEW TERMINOLOGY

To restate some of the new terminology in the last few paragraphs, the parameter name in the calling

program is referred to as the actual parameter, and the parameter name in the procedure is referred to as

the formal parameter. In the last example then, the actual parameter is named Index and the formal

parameter in the procedure is named Cat. It should be pointed out that it is called a formal parameter

whether it is a "call by reference" or a "call by value". This terminology is used in many other

programming languages, not only in Pascal.

When you run this program, you will find that the first procedure is unable to return the value of 12

back to the main program, but the second procedure does in fact succeed in returning its value of 35 to

the main program. Spend as much time as you like studying this program until you fully understand it.

It should be noted that as many parameters as desired can be passed to and from a procedure by simply

making a list separated by commas in the calls, and separated by semicolons in the procedure. This willbe illustrated in the next example program.

Compile and run PROCED2.PAS and study the output. You should be able to comprehend all of the

output. If it is not clear, reread the last few paragraphs.

To add to your knowledge of Pascal, examine the program PROCED3.PAS for an example of a

procedure call with more than one variable in the call. Normally, you would group the three input

values together to make the program more readable, but for purposes of illustration, they are separated.

Observe that the variable Fruit is a two way variable because it is the 3rd variable in the actual

parameter list and corresponds to the 3rd formal parameter in the procedure header. Since the third

variable is a "call by reference", it has the ability to return the sum of the other 3 variables to the mainprogram. This is the reason it was defined this way.

Compile and run PROCED3.PAS to see that it does what you expect it to do based on the above

explanation.




program Make_A_Fruit_Salad;

var Apple,Orange,Pear,Fruit : integer;

procedure Add_The_Fruit(Value1,Value2 : integer; (* one-way *)

var Total : integer; (* two-way *)

Value3 : integer); (* one-way *)

begin

Total := Value1 + Value2 + Value3;

end;


Apple := 4;

Orange := 5;

Pear := 7;

Add_The_Fruit(Apple,Orange,Fruit,Pear);

Writeln('The fruit basket contains ',Fruit:3,' fruits');



The fruit basket contains 16 fruits

}

"CALL BY REFERENCE" OR "CALL BY VALUE"?

It may seem to you that it would be a good idea to simply put the word var in front of every formal

parameter in every procedure header to gain maximum flexibility, but using all "call by references"

could actually limit your flexibility. There are two reasons to use "call by value" variables when you

can. The first is simply to shield some data from being corrupted by the procedure. This is becoming avery important topic in Software Engineering known as "information hiding" and is the primary basis

behind Object Oriented Programming which will be discussed in chapters 14 and 15 of this tutorial.

Secondly is the ability to use a constant in the procedure call. Modify line 17 of PROCED3.PAS as

follows;

Add_The_Fruit(12,Orange,Fruit,Pear);

and compile and run the program. Since Value1 is a "call by value", the constant 12 can be used and

the program will compile and run. However, if you change line 17 to;

Add_The_Fruit(Apple,Orange,32,Pear);

you will find that it will not compile because Total is a "call by reference" and the system must be able

to return a value for the formal parameter Total. It cannot do this because 32 is a constant, not a

variable. A call by reference must have a variable for the actual parameter. As a programming exercise,make Value1 a call by reference by adding the word var in front of it in line 6, and you will find that

the compiler will not allow you to replace the variable Apple with the constant 12 as was suggested

earlier in this section.

The prior discussion should indicate to you that both "call by value" and "call by reference" have a

useful place in Pascal programming and it is up to you to decide which you should use. When you aresatisfied with the present illustration and you have compiled and executed PROCED3.PAS several

times to study the results of the suggested changes, we will go on to study the scope of variables using

PROCED4.PAS.



A MULTIPLY DEFINED VARIABLE

If you will examine PROCED4.PAS, you will notice that the variable Count is defined twice, once in

the main program var block and once in the var block contained within the procedure named

Print_Some_Data. This is perfectly legal and is within the Pascal definition.


program Scope_Of_Variables;


Index : integer;

procedure Print_Some_Data;

var Count, More_Stuff : integer;

begin

Count := 7;

Writeln('In Print_Some_Data Count =',Count:5,

' Index =',Index:5);

end; (* of Print_Some_Data procedure *)


for Index := 1 to 3 do begin

Count := Index;Writeln('In main program Count =',Count:5,


Print_Some_Data;

Writeln('In main program Count =',Count:5,


Writeln;

end; (* Count loop *)



In main program Count = 1 Index = 1

In Print_Some_Data Count = 7 Index = 1In main program Count = 1 Index = 1


In Print_Some_Data Count = 7 Index = 2



In Print_Some_Data Count = 7 Index = 3


}

The variable Index is defined only in the main program var block and is valid anywhere within the

entire Pascal program, including the procedures and is said to be a global variable. The variable Count

is also defined in the main program var block and is valid anywhere within the entire Pascal program,

except within the procedure where another variable is defined with the same name Count. The two

variables with the same name are in fact, two completely different variables, one being available onlyoutside of the procedure and the other being available only within the procedure. The variable

More_Stuff is defined within the procedure, so it is invisible to the main program, since it is defined

at a lower level than that of the main program. It is only available for use within the procedure in which

it is defined.

Any variable is available at any point in the program following its definition but only at the level of

definition or below. This means that any procedure in the Declaration Part of a program can use any

variable defined in the Declaration Part of the program provided that the definition occurs prior to the

procedure. Any variable defined within a procedure cannot be used by the main program since thedefinition is at a lower level than the main program.



Be sure to compile and run PROCED4.PAS before continuing on to the next example program.

PROCEDURES CALLING OTHER PROCEDURES

Load and examine PROCED5.PAS to see an example of procedures that call other procedures. Keep in

mind that, "Nothing can be used in Pascal until it has been previously defined", and the order of

procedures will be clear in this example. Note that procedure Three calls procedure Two which in turn

calls procedure One.

Compile and run PROCED5.PAS and study the output until you understand why it outputs each line in

the order that it does.


program Procedure_Calling_A_Procedure;

procedure One;

begin

Writeln('This is procedure one');

end;

procedure Two;

begin

One;

Writeln('This is procedure two');

end;

procedure Three;

begin

Two;

Writeln('This is procedure three');

end;


One;

Writeln;

Two;Writeln;

Three;



This is procedure one


This is procedure two


This is procedure two

This is procedure three

}

Now that you have a good working knowledge of procedures, we need to make another important

point. Remember that any Pascal program is made up of three parts, the Program Heading, the

Declaration Part, and the Statement Part. The Declaration Part is composed of five unique components,

four of which we will discuss in detail in the next chapter, and the last component, which is composed

of some number of procedures and functions. We will cover functions in the next example, so for now

simply accept the fact that it is like a procedure. A procedure is also composed of three parts, a

Procedure Heading, a Declaration Part, and a Statement Part. A procedure, by definition, is therefore

nothing more or less than another complete Pascal program embedded within the main program, andany number of procedures can be located in the Declaration Part of the main program. These

procedures are all in a line, one right after another.



Since a procedure is defined like the main program, it would seem to be possible to embed anotherprocedure within the Declaration Part of any procedure. This is perfectly valid and is often done, but

remember that the embedded procedure can only be called by the procedure in which it is embedded,

not by the main program. This is a form of information hiding which is becoming popular in modern

software engineering.

The previous paragraph is probably a bit difficult to grasp. Don't worry about it too much now, as youbecome proficient as a Pascal programmer, you will very clearly see how embedded procedures are

used.

NOW LET'S LOOK AT A FUNCTION

Now to keep a promise, let's examine the program named FUNCTION.PAS to see what a function is

and how to use it. In this very simple program, we have a function that simply multiplies the sum of

two variables by 4 and returns the result. The major difference between a function and a procedure is

that the function returns a single value and is called from within a mathematical expression, a

Writelncommand, or anywhere that it is valid to use a variable, since it is really a variable itself.Observing the Function Heading of the function, in line 6, you will notice that a function begins with

the reserved word function. Further observation reveals the two input variables inside the parenthesis

pair being defined as integer variables, and following the parenthesis is a colon and another integer.

The last integer is used to define the type of the variable being returned to the main program.


program Example_Function;

var Dogs,Cats,Feet : integer;

function Quad_Of_Sum(Number1,Number2 : integer) : integer;

begin

Quad_Of_Sum := 4*(Number1 + Number2);

end;


Dogs := 4;

Cats := 3;

Feet := Quad_Of_Sum(Dogs,Cats);

Writeln(' There are a total of',Feet:3,' paws.');



There are a total of 28 paws.

}

Any call to this function is actually replaced by an integer value upon completion of the call. Therefore

in line 14, the function is evaluated and the value returned is used in place of the function call. The

value is returned by assigning the return value to the name of the function as illustrated in line 8. The

result of the function is therefore assigned to the variable named Feet.

Note that a function always returns a value and it may return additional values if some of its formalparameters are defined as "call by reference". Be sure to compile and run this program.

NOW FOR THE MYSTERY OF RECURSION



One of the great mysteries of Pascal and several other popular programming languages, is the recursion

of procedure calls. Simply defined, recursion is the ability of a procedure to call itself. Examine thePascal example file RECURSON.PAS for an example of recursion. The main program is very simple,

it sets the variable named Count to the value 7 and calls the procedure Print_And_Decrement.

The procedure prefers to refer to the variable by the name Index but that poses no problem for us

because we understand that the name of the formal parameter can be any legal identifier. The procedure

writes a line to the video display with the value of Index written within the line, and decrements thevariable.


program Try_Recursion;


procedure Print_And_Decrement(Index : integer);

begin

Writeln('The value of the index is ',Index:3);

Index := Index - 1;

if Index > 0 then

Print_And_Decrement(Index);

end;


Count := 7;

Print_And_Decrement(Count);



The value of the index is 7







}

The if statement introduces the interesting part of this program. If the variable is greater than zero, and

it is now 6, then the procedure Print_And_Decrement is called once again. This might seem to

create a problem except for the fact that this is perfectly legal in Pascal. Upon entering the procedure

the second time, the value of Index is printed as 6, and it is once again decremented. Since it is now 5,

the same procedure will be called again, and it will continue until the value of Index is reduced to zero

when each procedure call will be completed one at a time and control will return to the main program.

ABOUT RECURSIVE PROCEDURES

This is really a stupid way to implement this particular program, but it is the simplest recursive

program that can be written and therefore the easiest to understand. You will have occasional use for

recursive procedures, so don't be afraid to try them. Remember that the recursive procedure must have

some variable converging to something, or you will have an infinite loop. Compile and run this

program and observe the value decrementing as the recursion takes place.

THE FORWARD REFERENCE

Occasionally you will have a need to refer to a procedure before you can define it. In that case you willneed a forward reference. The example program named FORWARD.PAS has an example of a forwardreference in it. In this program, each one of the procedures calls the other, a form of recursion. This



program, like the last, is a very stupid way to count from 7 to 0, but it is the simplest program possible

with the forward reference.


program Forward_Reference_Example;

var Number_Of_Times : integer;

procedure Write_A_Line(var Count : integer); forward;

procedure Decrement(var Index : integer);

begin

Index := Index - 1;

if Index > 0 then

Write_A_Line(Index);

end;

procedure Write_A_Line;

begin

Writeln('The value of the count is now ',Count:4);

Decrement(Count);

end;

begin (* main program *)Number_Of_Times := 7;

Decrement(Number_Of_Times);

Writeln;

Number_Of_Times := 7;

Write_A_Line(Number_Of_Times);



The value of the count is now 6




The value of the count is now 2The value of the count is now 1








}

The first procedure, Write_A_Line, has its header defined in exactly the same manner as any otherprocedure but instead of the normal procedure body, only the reserved word forward is given in line 6.

This tells the compiler that the procedure will be defined later. The next procedure is defined as usual,

then the body of Write_A_Line is given with only the reserved word procedure and the procedure

name. The variable reference has been defined earlier. In this way, each of the procedure names aredefined before they are called.

It would be possible, by using the forward reference in great numbers, to move the main program ahead

of all procedure definitions and have the program structured like some other languages. This style of

programming would be perfectly legal as far as the compiler is concerned, but the resulting program

would be very non-standard and confusing. You would do well to stick with conventional Pascalformatting techniques and use the forward reference sparingly. Be sure you compile and run this

program.



THE PROCEDURE TYPE

Examine the program named PROCTYPE.PAS. This is a new extension to the Pascal language by

Borland beginning with version 5.0. In this program, the procedure Do_Math is defined as a procedure

type in line 12, and three regular procedures are defined each of which have the same structure of

formal parameters. In the program, since Do_Math is a procedure type that is compatible with each of

the defined procedures, it can be assigned one of the other procedures, and a call to Do_Math is

identical to a call to that procedure to which it is currently assigned. The program should be self

explanatory with those few comments so you will be left to study the details on your own. It is

important that you recognise that the procedure type variable is used to call several different

procedures.


program Procedure_Type_Example;

{$F+} (* This forces far calls and is required by TURBO *)

(* Pascal to use a procedure type. *)

type Procedure_Type = procedure(In1, In2, In3 : integer;

var Result : integer);

var Number1, Number2, Number3 : integer;

Final_Result : integer;

Do_Math : Procedure_Type;

procedure Add(In1, In2, In3 : integer;


begin

Result := In1 + In2 + In3;

Writeln('The sum of the numbers is ',Result:6);

end;

procedure Mult(In1, In2, In3 : integer;


begin

Result := In1 * In2 * In3;

Writeln('The product of the numbers is',Result:6);

end;

procedure Average(In1, In2, In3 : integer;


begin

Result := (In1 * In2 * In3) div 3;

Writeln('The Average of the numbers is',Result:6);

end;

begin

Number1 := 10;

Number2 := 15;

Number3 := 20;

Do_Math := Add;

Do_Math(Number1, Number2, Number3, Final_Result);

Do_Math := Mult;


Do_Math := Average;


end.


The sum of the numbers is 45

The product of the numbers is 3000

The average of the numbers is 1000

}



Note the comments in lines 4 and 5 of the program. When using a procedure type or a function type,

which is the topic of the next example program, TURBO Pascal requires that you use the compiler

directive F+, which forces the system to use far calls for all procedure calls. Study the documentation

for your version of TURBO Pascal to obtain more information on compiler directives.

Examine the program named FUNCTYPE.PAS for an example of a program using some of the sametechniques as the last program but instead uses a function type for the subprogram variable. This

program should be simple for you to study on your own concerning the details of operation.


program Function_Type_Example;

{$F+} (* This forces far calls and is required by TURBO *)

(* Pascal to use a function type. *)

type Function_Type = function(In1, In2, In3 : integer) : integer;

var Number1, Number2, Number3 : integer;

Final_Result : integer;

Do_Math : Function_Type;

function Add(In1, In2, In3 : integer) : integer;

var Temp : integer;

begin

Temp := In1 + In2 + In3;

Writeln('The sum of the numbers is ',Temp:6);

Add := Temp;

end;

function Mult(In1, In2, In3 : integer) : integer;

var Temp : integer;

begin

Temp := In1 * In2 * In3;

Writeln('The product of the numbers is',Temp:6);

Mult := Temp;

end;

function Average(In1, In2, In3 : integer) : integer;

var Temp : integer;

begin

Temp := (In1 * In2 * In3) div 3;

Writeln('The Average of the numbers is',Temp:6);

Average := Temp;

end;

begin

Number1 := 10;

Number2 := 15;

Number3 := 20;

Do_Math := Add;

Final_Result := Do_Math(Number1, Number2, Number3);

Do_Math := Mult;


Do_Math := Average;


end.


The sum of the numbers is 45

The product of the numbers is 3000

The average of the numbers is 1000

}



The only rule concerning the procedure and function types which must be stated, is that a subprogram

type variable can only be assigned subprogram names if the list of actual parameters are identical for

the type and the subprogram. This includes the type of the return value for a function. Since this is a

new extension to the TURBO Pascal languages, it has not been used much, so don't worry about it too

much. You should know that it can be done, because someday you will find a piece of Pascal code withthis construct used. Of course, you will someday find a good use for it yourself.


1. Write a program to write your name, address, and phone number with each Writeln in a

different procedure.

2. Add a statement to the procedure in RECURSON.PAS to display the value of Index after the

call to itself so you can see the value increasing as the recurring calls are returned to the next

higher level.

3. Rewrite TEMPCONV.PAS putting the centigrade to Fahrenheit formulas in a function call.



Chapter 6 : ARRAYS, TYPES, CONSTANTS, AND LABELS

ARRAYS

At the beginning of this tutorial we said that a computer program is composed of data and executablestatements to do something with that data. Having covered nearly all of the programming statements,

we must now go back and fill in some gaps in our data definition and look at the array in particular.

One of the most useful Pascal data structures is the array, which is, in the simplest terms, a group of 2

or more identical terms, all having the same type. Let's go directly to an example to see what an array

looks like. Display the Pascal program ARRAYS.PAS and notice line 5 starting with the word

Automobiles. The variable Automobiles is defined as an integer variable but in addition, it is

defined to have twelve different integer variables, namely Automobile[1], Automobile[2],

Automobile[3], .. Automobile[12].

The square braces are used in Pascal to denote a subscript for an array variable. The array definition

given in line 5 is the standard definition for an array, namely a variable name, followed by a colon and

the reserved word array, with the range of the array given in square brackets followed by another

reserved word of and finally the type of variable for each element of the array.


program Simple_Arrays;

var Count,Index : integer;

Automobiles : array[1..12] of integer;

begin

for Index := 1 to 12 do

Automobiles[Index] := Index + 10;

Writeln('This is the first program with an array');

Writeln;

for Index := 1 to 12 doWriteln('automobile number',Index:3,' has the value',

Automobiles[Index]:4);

Writeln;

Writeln('End of program');

end.


This is the first program with an array

automobile number 1 has the value 11


automobile number 3 has the value 13automobile number 4 has the value 14









End of program

}



USING THE ARRAY

In using the elements of the array in a program, each of the elements of the array are required to be

used in exactly the same manner as any simple variable having the same type. Each time one of the

variables is used, it must have the subscript since the subscript is now part of the variable name. The

subscript moreover, must be of the type used in the definition and it must be within the range defined or

it will be construed as an error.

Now consider the program itself. As Index is varied from 1 to 12, the range of the subscripts of the

variable Automobile, the 12 variables are set to the series of values 11 to 22. Any integer values could

be used, this was only a convenient way to set the values to some well defined numbers. With the

values stored, a header is now printed and the list of values contained in the array is printed. Note

carefully that, although the subscripts are limited to 1 through 12, the values stored in each of the 12

variables are limited only by the range of integers, namely –32768 to 32767. Review this material and

this program as long as needed to fully understand it, as it is very important.

Keep in mind that the array is actually composed of 12 different integer type variables that can be used

in any way that it is legal to use any other integer type variable. Compile and run this program.

DOUBLY INDEXED ARRAYS

After understanding the above example program, load the program ARRAYS2.PAS to see the next

level of complexity of arrays. You will see that Checkerboard is defined as an array from 1 to 8, but

instead of it being a simple data type, it is itself another array from 1 to 8 of type integer. The variable

Checkerboard is actually composed of 8 elements, each of which is 8 elements, leading to a total of 64

elements, each of which is a simple integer variable. This is called a doubly subscripted array and it can

be envisioned in exactly the same manner as a real checker board, an 8 by 8 matrix. Another way to

achieve the same end is to define the double array as in the next line of the program where Value is

defined as a total of 64 elements.

To use either of the two variables in a program, we must add two subscripts to the variable name to tellthe program which element of the 64 we desire to use. Examining the program will reveal two loops,

one nested within the other, and both ranging in value from 1 to 8. The two loop indices can therefore

be used as subscripts of the defined array variables. The variable Checkerboard is subscripted by

both of the loop indices and each of the 64 variables is assigned a value as a function of the indices.

The assigned value has no real meaning other than to illustrate to you how it is done. Since the value of

Checkerboard is now available, it is used to define some values to be used for the variable Value in line

12 of the program.

After defining all of those variables, and you should understand that we have defined a total of 128

variables in the double loop, 64 of Checkerboard and 64 of Value, they can be printed out. The

next section of the program does just that, by using another doubly nested loop, with a Write

statement in the centre. Each time we go through the centre of the loop we tell it to print out one of the

64 variables in the Checkerboard matrix with the indices Index and Count defining which of the

variables to write each time. Careful study of the loop should reveal its exact operation.

After printing out the matrix defined by the variable Checkerboardwe still have the matrix defined

by the variable Value intact (In fact, we still have all of Checkerboard available because we

haven't changed any of it). Before printing out the matrix defined by Value, let's change a few of the

elements just to see how it is done. The code in lines 24 to 26 simply change three of the variables to

illustrate that you can operate on all of the matrix in loops, or on any part of the matrix in simple

assignment statements. Notice especially line 26, in which Value[3,6] (which was just set to the value

of 3), is used as a subscript. This is perfectly legal since it is defined as a simple integer variable and is

within the range of 1 to 8, which is the requirement for a subscript of the variable Value. The last part

of the program simply prints out the 64 values of the variable Value in the same manner as above.

Notice that when you run the program, the three values are in fact changed as expected.




program Multiple_Arrays;

var Index,Count : integer;

Checkerboard : array[1..8] of array[1..8] of integer;

Value : array[1..8,1..8] of integer;


for Index := 1 to 8 do begin (* index loop *)for Count := 1 to 8 do begin

Checkerboard[Index,Count] := Index + 3*Count;

Value[Index,Count] := Index + 2*Checkerboard[Index,Count];

end;

end; (* of index loop *)

Writeln(' Output of checkerboard');

Writeln;



Write(Checkerboard[Index,Count]:7);

Writeln;

end;

Value[3,5] := -1; (* change some of the value matrix *)

Value[3,6] := 3;Value[Value[3,6],7] := 2; (* This is the same as writing

Value[3,7] := 2; *)


Writeln; (* Three blank lines *)

Writeln('Output of value');

Writeln;



Write(Value[Count,Index]:7);

Writeln;

end;



Output of checkerboard

4 7 10 13 16 19 22 25

5 8 11 14 17 20 23 26

6 9 12 15 18 21 24 27

7 10 13 16 19 22 25 28

8 11 14 17 20 23 26 29

9 12 15 18 21 24 27 30

10 13 16 19 22 25 28 31

11 14 17 20 23 26 29 32

Output of value

9 15 21 27 33 39 45 51

12 18 24 30 36 42 48 54

15 21 27 33 -1 3 2 57

18 24 30 36 42 48 54 60

21 27 33 39 45 51 57 63

24 30 36 42 48 54 60 66

27 33 39 45 51 57 63 69

30 36 42 48 54 60 66 72

}



ARRAYS ARE FLEXIBLE

A few more words about arrays before we go on. The arrays in the last program were both defined to

be square, namely 8 by 8, but that choice was purely arbitrary. The subscripts were chosen to go from 1

to 8 but they could have been chosen to go from 101 to 108 or any other range needed to clearly define

the problem at hand. And, as you may have guessed, you are not limited to a doubly subscripted matrix

but you can define a variable with as many subscripts as you need to achieve your desired end. There is

a practical limit to the number of subscripts because you can very quickly use up all of your available

memory with one large subscripted variable.

THE TYPE DEFINITION

Now that you understand arrays, let's look at a more convenient way to define them by examining the

Pascal file TYPES.PAS. You will notice a new section at the beginning of the listing which begins with

the word type. The word type is another reserved word which is used at the beginning of a section to

define "user-defined types". Beginning with the simple predefined types we studied earlier, we can

build up as many new types as we need and they can be as complex as we desire. The six names (fromArray_Def to Boat) in the type section are not variables, but are defined to be types and can be used

in the same manner as we use integer, byte, real, etc.


program Example_Of_Types;

type Array_Def = array[12..25] of integer;

Char_Def = array[0..27] of char;

Real_Array = array[-17..42] of real;

Dog_Food = array[1..6] of boolean;

Airplane = array[1..12] of Dog_Food;

Boat = array[1..12,1..6] of boolean;

var Index,Counter : integer;

Stuff : Array_Def;Stuff2 : Array_Def;

Puppies : Airplane;

Kitties : Boat;



for Counter := 1 to 6 do begin

Puppies[Index,Counter] := TRUE;

Kitties[Index,Counter] := Puppies[Index,Counter];

end;

Writeln(Puppies[2,3]:7,Kitties[12,5]:7,Puppies[1,1]:7);



TRUE TRUE TRUE

}

PASCAL CHECKS TYPES VERY CAREFULLY

This is a very difficult concept, but a very important one. The Pascal compiler is very picky about the

types you use for variables in the program, doing lots of checking to insure that you don't use the wrong

type anywhere in the program. Because it is picky, you could do very little without the ability to define

new types when needed, and that is the reason Pascal gives you the ability to define new types to solvea particular problem.



Some of these types are used in the var declaration part of the program. Notice that since Airplane is

an array of Dog_Food and Dog_Food is in turn an array of boolean, then Airplane defines a

doubly subscripted array, each element being a boolean variable. This does not define any variables,

only a user defined type, which can be used in a var to define a matrix of boolean variables. This is

in fact done in the definition of Puppies, which is an array composed of 72 (6 times 12) boolean

variables. In the same manner, Stuff is composed of an array of 14 variables, each being an integer

variable. The elements of the array are, Stuff[12], Stuff[13], .. Stuff[25]. Notice also that

Stuff2 is also defined in exactly the same manner and is also composed of 14 variables.

Careful inspection will reveal that Kitties is a variable which has the same definition as Puppies.

It would probably be poor programming practice to define them in different manners unless they were

in fact totally disassociated. In this example program, it serves to illustrate some of the ways user-

defined types can be defined. Be sure to compile and run this program.

IS THE CONCEPT OF "TYPES" IMPORTANT?

If you spend the time to carefully select the types for the variables used in the program, the Pascal

compiler will do some debugging for you since it is picky about the use of variables with differenttypes. Any aid you can use to help find and remove errors from your program is useful and you should

learn to take advantage of type checking. The type checking in Pascal is relatively weak compared to

some other languages such as Modula-2 or Ada, but still very useful.

In a tiny program like this example, the value of the type declaration part cannot be appreciated, but in

a large program with many variables, the type declaration can be used to great advantage. This will be

illustrated later.

THE CONSTANT DECLARATION

Examining the Pascal example program CONSTANT.PAS will give us an example of a constantdefinition. The reserved word const is the beginning of the section that is used to define constants

that can be used anyplace in the program as long as they are consistent with the required data typing

limitations. In this example, Max_Size is defined as a constant with the value of 12. This is not a

variable and cannot be changed in the program, but is still a very valuable number.

For the moment ignore the next two constant definitions. As we inspect the type declarations, we see

two user-defined types, both of which are arrays of size 1 to 12 since Max_Size is defined as 12.

Then when we get to the var declaration part, we find five different variables, all defined as arrays

from 1 to 12 (some are type integer and some are type char). When we come to the program we find

that it is one big loop which we go through 12 times because the loop is executed Max_Size times.

In the above definition, there seems to be no advantage to using the constant, and there is none, until

you find that for some reason you wish to increase the range of all arrays from 12 to 18. In order to do

so, you only need to redefine the value of the constant, recompile, and the whole job is done. Without

the constant definition, you would have had to change all type declarations and the upper limit of the

loop in the program. Of course that would not be too bad in the small example program, but could be a

real mess in a 2000 line program, especially if you missed changing one of the 12's to an 18. That

would be a good example of data in and garbage out. This program should give you a good idea of

what the constant can be used for, and as you develop good programming techniques, you will use the

constant declaration to your advantage.




program Example_Of_Constants;

const Max_Size = 12; (* Pascal assumes this is a byte type, but it

can be used as an integer also *)

Index_Start : integer = 49; (* This is a typed constant *)

Check_It_Out : boolean = TRUE; (* Another typed constant *)

type Bigarray = array[1..Max_Size] of integer;

Chararray = array[1..Max_Size] of char;

var Airplane : Bigarray;

Seaplane : Bigarray;

Helicopter : Bigarray;

Cows : Chararray;

Horses : Chararray;

Index : integer;


for Index := 1 to Max_Size do begin

Airplane[Index] := Index*2;

Seaplane[Index] := Index*3 + 7;

Helicopter[Max_Size - Index + 1] := Index + Airplane[Index];

Horses[Index] := 'X';Cows[Index] := 'R';

end;



(There is no output from this program)

}

THE TURBO PASCAL TYPED CONSTANT

We skipped over the second and third constant declarations for a very good reason. They are not

constant declarations. TURBO Pascal has defined, as an extension, the "typed constant". Using the

syntax shown, Index_Start is defined as an integer type variable and is initialised to the value of

49. This is a true variable and can be used as such in the program. The same effect can be achieved by

simply defining Index_Start as an integer type variable in the var declaration part and setting it to

the value of 49 in the program itself. Since it does not really fit the definition of a constant, it's use is

discouraged until you gain experience as a Pascal programmer. Until then it will probably only be

confusing to you. In like manner, Check_It_Out is a boolean type variable initialised to the value

TRUE. It is not a constant.

The typed constants defined in the last paragraph have one additional characteristic, they are initialisedonly once, when the program is loaded. Even when used in a procedure or function, they are only

initialised when the program is loaded, not upon each call to the procedure or function. Don't worry too

much about this at this point, when you gain experience with Pascal, you will be able to use thisinformation very effectively.

THE LABEL DECLARATION

Finally, the example program LABELS.PAS will illustrate the use of labels. In the Pascal definition, a

label is a number from 0 to 9999 that is used to define a point in the program to which you wish to

jump. All labels must be defined in the label definition part of the program before they can be used.

Then a new reserved word goto is used to jump to that point in the program. The best way to see howthe goto is used with labels is to examine the program before you.



TURBO Pascal has an extension for labels. Any valid identifier, such as used for variables, can be used

as a label in addition to the values from 0 to 9999. These are illustrated in the example program.

When you compile and run this program, the output will look a little better than the program does.


program Label_Illustration;

label 274,Repeat_Loop,Help,Dog;

var Counter : byte; (* This limits us to a maximum of 255 *)

begin

Writeln('Start here and go to "help"');

goto Help;

Dog:

Writeln('Now go and end this silly program');

goto 274;

Repeat_Loop:

for Counter := 1 to 4 do Writeln('In the repeat loop');

goto Dog;

Help:

Writeln('This is the help section that does nothing');

goto Repeat_Loop;

274:

Writeln('This is the end of this spaghetti code');

end.


Start here and go to "help"

This is the help section that does nothing

In the repeat loopIn the repeat loop

In the repeat loop

In the repeat loop

Now go and end this silly program

This is the end of this spaghetti code

}

THE PACKED ARRAY

When Pascal was first defined in 1971, many of the computers in use at that time used very large

words, 60 bits being a typical word size. Memory was very expensive, so large memories were not toocommon. A Pascal program that used arrays was inefficient because only one variable was stored in

each word. Most of the bits in each word were totally wasted, so the packed array was defined in which

several variables were stored in each word. This saved storage space but took extra time to unpack each

word to use the data. The programmer was given a choice of using a fast scheme that wasted memory,

the array, or a slower scheme that used memory more efficiently, the packed array.

The modern microcomputer has the best of both schemes, a short word, usually 16 bits, and a large

memory. The packed array is therefore not even implemented in many compilers and will be ignored

during compilation. The packed array is specifically ignored by all versions of TURBO Pascal.



ONE MORE TURBO PASCAL EXTENSION

Standard Pascal, as defined by Nicklaus Wirth, requires that the various fields in the definition part of

the program come in a specific order and each must appear only once. The specific order is, label,

const, type, var, and finally the procedures and functions. Of course, if any are not needed, they

are simply omitted. This is a rather rigid requirement but it was required by the pure Pascal definition

probably to teach good programming techniques to beginning students.

All versions of TURBO Pascal are not nearly as rigid as the standard Pascal requirement. You arepermitted to use the fields in any order and as often as you wish provided that you define everything

before you use it, which is the unbroken rule of Pascal. It sometimes makes sense to define a few

variables immediately after their types are defined to keep them near their type definitions, then define

a few more types with the variables that are associated with them also. TURBO Pascal gives you this

extra flexibility that can be used to your advantage.


1. Write a program to store the integers 201 to 212 in an array then display them on the monitor.

2. Write a program to store a 10 by 10 array containing the products of the indices, therefore a

multiplication table. Display the matrix on the video monitor.

3. Modify the program in 2 above to include a constant so that by simply changing the constant, the

size of the matrix and the range of the table will be changed.



Chapter 7 : STRINGS & STRING PROCEDURES

PASCAL STRINGS

According to the Pascal definition, a string is simply an array of 2 of more elements of type char, and is

contained in an array defined in a var declaration as a fixed length. Examine the example program

named STRARRAY.PAS. Notice that the strings are defined in the type declaration even though they

could have been defined in the var part of the declaration. This is to begin getting you used to seeing

the type declaration. The strings defined here are nothing more than arrays with char type variables.


program Pure_Pascal_Strings;

type Long_String = array[1..25] of char;

String10 = array[1..10] of char;

String12 = array[1..12] of char;

var First_Name : String10;Initial : char;

Last_Name : String12;

Full_Name : Long_String;

Index : integer;


First_Name := 'John ';

Initial := 'Q';

Last_Name := 'Doe ';

Writeln(First_Name,Initial,Last_Name);


Full_Name[Index] := First_Name[Index];

Full_Name[11] := Initial;


Full_Name[Index + 11] := Last_Name[Index];

for Index := 24 to 25 do Full_Name[Index] := ' ';

Writeln(Full_Name);



John QDoe

John QDoe

}

A STRING IS AN ARRAY OF CHAR

The interesting part of this file is the executable program. Notice that when the variable First_Name

is assigned a value, the value assigned to it must contain exactly 10 characters or the compiler will

generate an error. If you edit out a blank, you will get an invalid type error. Pascal is neat in allowing

you to write out the values in the string array without specifically writing each character in a loop as

can be seen in the Writeln statement. To combine the data, called concatenation, requires the use of

the rather extensive looping and subscripting seen in the last part of the program. It would be even

messier if we were to consider variable length fields which is nearly always the case in a real program.

Two things should be observed in this program. First, notice the fact that the string operations are truly

array operations and will follow all of the characteristics discussed in the last chapter. Secondly, it is

very obvious that Pascal is rather weak when it comes to its handling of text type data. Pascal willhandle text data, even though it may be difficult to do so using the standard description of Pascal as



illustrated in this program. We will see next that TURBO Pascal really shines when it is desired to

manipulate text. Compile and run STRARRAY.PAS and observe the output.

THE TURBO PASCAL STRING TYPE

Examine the example program STRINGS.PAS. You = will see a much more concise program that

STRINGS.PAS actually does more. TURBO Pascal has, as an = extension to standard Pascal, the string

type of variable. It is used as shown, and the number in the square brackets in the var declaration is

the maximum length of the string. In actual use in the program, the variable can be used as any length

from zero characters up to the maximum given in the declaration. The variable First_Name, for

example, actually has 11 locations of storage for its data. The current length is stored in

First_Name[0] and the data is stored in First_Name[1] through First_Name[10]. All data are

stored as byte variables, including the size, so the length is limited to a maximum of 255 characters.

STRINGS HAVE VARIABLE LENGTHS

Now look at the program itself. Even though the variable First_Name is defined as 10 characters

long, it is perfectly legal to assign it a 4 character constant, with First_Name[0] automatically set to

4 by the system and the last six characters undefined and unneeded. When the program is run, the three

variables are printed out all squeezed together indicating that the variables are indeed shorter than their

full size as defined in the var declaration.

Using the string type is even easier when you desire to combine several fields into one as can be seen in

the assignment to Full_Name. The concatenation operator is the plus sign and is used to combine

strings and individual characters as indicated in line 14. Notice that there are even two blanks, in the

form of constant fields, inserted between the component parts of the full name. When it is written out,

the full name is formatted neatly and is easy to read. Compile and run STRINGS.PAS and observe theoutput.

WHAT'S IN A STRING TYPE VARIABLE?

The next example program named WHATSTRG.PAS, is intended to show you exactly what is in a

string variable. This program is identical to the last program except for some added statements at the

end. Notice the assignment to Total. The function Length is available in TURBO Pascal to return

the current length of any string type variable. It returns a byte type variable with the value contained in

the [0] position of the variable. We print out the number of characters in the string at this point, and

then print out each character on a line by itself to illustrate that the TURBO Pascal string type variable

is simply an array variable.

The TURBO Pascal reference manual has a full description of several more procedures and functions

for use with strings which are available in TURBO Pascal only. Refer to your TURBO Pascal reference

manual for complete details. The use of these should be clear after you grasp the material covered here.




program What_Is_In_A_String;

var First_Name : string[10];

Initial : char;

Last_Name : string[12];

Full_Name : string[25];

Index,Total : integer;


First_Name := 'John';

Initial := 'Q';

Last_Name := 'Doe';

Writeln(First_Name,Initial,Last_Name);

Full_Name := First_Name + ' ' + Initial + ' ' + Last_Name;

Writeln(Full_Name);

Total := Length(Full_Name);

Writeln('The string contains ',Total:4,' characters');

for Index := 1 to Length(Full_Name) do

Writeln(Full_Name[Index]);

Writeln('End of program');



JohnQDoe

John Q Doe

The string contains 10 characters

J

o

h

n

Q

Do

e

End of program

}


1. Write a program in which you store your first, middle, and last names as variables, then displaythem one to a line. Concatenate the names with blanks between them and display your full name as

a single variable.



Chapter 8 : SCALARS, SUBRANGES, AND SETS

PASCAL SCALARS

A scalar, also called an enumerated type, is a list of values which a variable of that type may assume.Examine the Pascal program ENTYPES.PAS for an example of some scalars. The first type declaration

defines Days as being a type which can take on any one of seven values. Since, within the var

declaration, Day is assigned the type of Days, Day is then a variable which can assume any one of

seven different values. Moreover Day can be assigned the value Mon, or Tue, etc., which is

considerably clearer than using 0 to represent Monday, 1 for Tuesday, etc. This makes the program

easier to follow and understand.


program Enumerated_Types;

type Days = (Mon,Tue,Wed,Thu,Fri,Sat,Sun);

Time_Of_Day = (Morning,Afternoon,Evening,Night);

var Day : Days;

Time : Time_Of_Day;

Regular_Rate : real;

Evening_Premium : real;

Night_Premium : real;

Weekend_Premium : real;

Total_Pay : real;


Writeln('Pay rate table':33);

Writeln;

Write(' DAY Morning Afternoon');

Writeln(' Evening Night');

Writeln;

Regular_Rate := 12.00; (* This is the normal pay rate *)Evening_Premium := 1.10; (* 10 percent extra for working late *)

Night_Premium := 1.33; (* 33 percent extra for graveyard *)

Weekend_Premium := 1.25; (* 25 percent extra for weekends *)

for Day := Mon to Sun do begin

case Day of

Mon : Write('Monday ');

Tue : Write('Tuesday ');

Wed : Write('Wednesday');

Thu : Write('Thursday ');

Fri : Write('Friday ');

Sat : Write('Saturday ');

Sun : Write('Sunday ');

end; (* of case statement *)

for Time := Morning to Night do begincase Time of

Morning : Total_Pay := Regular_Rate;

Afternoon : Total_Pay := Regular_Rate;

Evening : Total_Pay := Regular_Rate * Evening_Premium;

Night : Total_Pay := Regular_Rate * Night_Premium;


case Day of

Sat : Total_Pay := Total_Pay * Weekend_Premium;

Sun : Total_Pay := Total_Pay * Weekend_Premium;


Write(Total_Pay:10:2);

end; (* of "for Time" loop *)

Writeln;

end; (* of "for Day" loop *)





Pay rate table

DAY Morning Afternoon Evening Night

Monday 12.00 12.00 13.20 15.96Tuesday 12.00 12.00 13.20 15.96

Wednesday 12.00 12.00 13.20 15.96

Thursday 12.00 12.00 13.20 15.96

Friday 12.00 12.00 13.20 15.96

Saturday 15.00 15.00 16.50 19.95

Sunday 15.00 15.00 16.50 19.95

}

Internally, Pascal does not actually assign the value Mon to the variable Day, but it uses an integer

representation for each of the names. This is important to understand because you need to realise that

you cannot print out Mon, Tue, etc., but can only use them for indexing control statements.

The second line of the type definition defines Time_Of_Day as another scalar which can have any of

four different values, namely those listed. The variable Time can only be assigned one of four values

since it is defined as the type Time_Of_Day. It should be clear that even though it can be assigned

Morning, it cannot be assigned Morning_time or any other variant spelling of Morning, since it is

simply another identifier which must have an exact spelling to be understood by the compiler. Several

real variables are defined to allow us to demonstrate the use of the scalar variables. After writing a

header in lines 16 through 20, the real variables are initialised to some values that are probably not real

life values, but will serve to illustrate the scalar variable.

A BIG SCALAR VARIABLE LOOP

The remainder of the program is one large loop being controlled by the variable Day as it goes through

all of its values, one at a time. Note that the loop could have gone from Tue to Sat or whatever portionof the range desired. It does not have to go through all of the values of Day. Using Day as the case

variable of a case statement, the name of one of the days of the week is written out each time we go

through the loop. Another loop controlled by Time is executed four times, once for each value of Time.

The two case statements within the inner loop are used to calculate the total pay rate for each time

period and each day.

The data is formatted carefully to make a nice looking table of pay rates as a function of Time and Day.

Take careful notice of the fact that the scalar variables never entered into the calculations, and they

were not printed out. They were only used to control the flow of logic. It was much neater than trying

to remember that Mon is represented by a 0, Tue is represented by a 1, etc. In fact, those numbers are

used for the internal representation of the scalars but we can relax and let Pascal worry about the

internal representation of our scalars. Compile and run this program and observe the output.

LET'S LOOK AT SOME SUBRANGES

Examine the program SUBRANGE.PAS for an example of sub-ranges and some additional instruction

on scalar variables. It may be expedient to define some variables that only cover a part of the full range

as defined in a scalar type. Notice that Days is declared a scalar type as in the last program, and Work

is declared a type with an even more restricted range. In the var declaration, Day is once again

defined as the days of the week and can be assigned any of the days by the program. The variable

Workday, however, is assigned the type Work, and can only be assigned the days Mon through Fri.If an attempt is made to assign Workday the value Sat, a run-time error will be generated. A



carefully written program will never attempt that, and it would be an indication that something is

wrong with either the program or the data. This is one of the advantages of Pascal over older languagesand is a reason for the relatively strong type checking built into the language.

Further examination will reveal that Index is assigned the range of integers from 1 through 12. During

execution of the program, if an attempt is made to assign Index any value outside of that range, a run

time error will be generated. Suppose the variable Index was intended to refer to your employees, andyou have only 12. If an attempt was made to refer to employee number 27, or employee number -8,there is clearly an error somewhere in the data and you would want to stop running the payroll to fix

the problem. Pascal would have saved you a lot of grief.


program Scaler_Operations;

type Days = (Mon,Tue,Wed,Thu,Fri,Sat,Sun);

Work = Mon..Fri;

Rest = Sat..Sun;

var Day : Days; (* This is any day of the week *)

Workday : Work; (* These are the the working days *)

Weekend : Rest; (* The two weekend days only *)

Index : 1..12;Alphabet : 'a'..'z';

Start : 'a'..'e';


(* The following statements are commented out because they contain

various errors that will halt compilation.

Workday := Sat; Sat is not part of Workday's subrange.

Rest := Fri; Fri is not part of Weekend's subrange.

Index := 13; Index is only allowed to go up to 12,

Index := -1; and down to 1.

Alphabet := 'A' Alphabet, as defined, includes only the

lower case alphabet.

Start := 'h' h is not in the first five letters.

End of commented out section. *)

Workday := Tue;

Weekend := Sat;

Day := Workday;

Day := Weekend;

Index := 3+2*2;

Start := 'd';

Alphabet := Start;

(* since Alphabet is "d" *)

Start := Succ(Alphabet); (* Start will be 'e' *)

Start := Pred(Alphabet); (* Start will be 'c' *)

Day := Wed;

Day := Succ(Day); (* Day will now be 'Thu' *)

Day := Succ(Day); (* Day will now be 'Fri' *)

Index := Ord(Day); (* Index will be 4 (Fri = 4) *)



(There is no output from this porgram.)

}

SOME STATEMENTS WITH ERRORS IN THEM.

In order to have a program that would compile without errors, and yet show some errors, the section of the program in lines 16 through 27 is not really a part of the program since it is within a comment area.

This is a trick to remember when you are debugging a program, a troublesome part can be commented



out until you are ready to include it with the rest of the code. The errors are self explanatory and it

would pay for you to spend enough time to understand each of the errors.

There are seven assignment statements as examples of sub-range variable use in lines 29 through 35.

Notice that the variable Day can always be assigned the value of either Workday or Weekend, but the

reverse is not true because Day can assume values that would be illegal to assign to the others.

THREE VERY USEFUL FUNCTIONS

Lines 37 through 42 of the example program demonstrate the use of three very important functions

when using scalars. The first is the Succ function that returns the value of the successor to the scalar

used as an argument, the next value. If the argument is the last value, a run time error is generated. The

next function is the Pred function that returns the predecessor to the argument of the function. Finally

the Ord function which returns the ordinal value of the scalar.

All scalars have an internal representation starting at 0 and increasing by one until the end is reached.

In our example program, Ord(Day) is 5 if Day has been assigned Sat, but Ord(Weekend) is 0 if

Weekend has been assigned Sat. As you gain experience in programming with scalars and sub-ranges,you will realise the value of these three new functions.

A few more thoughts about sub-ranges are in order before we go on to another topic. A sub-range is

always defined by two predefined constants, and is always defined in an ascending order. A variable

defined as a sub-range type is actually a variable defined with a restricted range. Good programming

practice would dictate that subranges should be used as often as possible in order to prevent garbage

data. There are actually very few variables ever used that cannot be restricted by some amount. The

limits may give a hint at what the program is doing and can help in understanding the program

operation. Subrange types can only be constructed using the simple types, integer, char, byte, or scalar.

Compile and run this program even though it has no output. Add some output statements to see what

values some of the variables assume.

SETS

Now for a new topic, sets. Examining the example Pascal program SETS.PAS will reveal some sets. A

scalar variable is defined first, in this case the scalar type named Goodies. A set is then defined with

the reserved words set of followed by a predefined scalar type. Several variables are defined as sets of

Treat, after which they can individually be assigned portions of the entire set.

Consider the variable Ice_Cream_Cone which has been defined as a set of type Treat. This

variable is composed of as many elements of Goodies as we care to assign to it. In the program, wedefine it as being composed of Ice_Cream, and Cone. The set Ice_Cream_Cone is therefore

composed of two elements, and it has no numerical or alphabetic value as most other variables have.

In lines 21 through 26, you will see four more delicious deserts defined as sets of their components.

Notice that the banana split is first defined as a range of terms, then another term is added to the group

illustrating how you can add to a set. All five are combined in the set named Mixed, then Mixed is

subtracted from the entire set of values to form the set of ingredients that are not used in any of the

deserts. Each ingredient is then checked to see if it is in the set of unused ingredients, and printed out if

it is. Note that in is another reserved word in Pascal. Running the program will reveal a list of unused

elements.

In this example, better programming practice would have dictated defining a new variable, possibly

called Remaining for the ingredients unused in line 32. It was desirable to illustrate that Mixed



could be assigned a value based on subtracting itself from the entire set, so the poor variable name was

used.

When you compile and run this program you will see that this example results in some nonsense results

but hopefully it led your thinking toward the fact that sets can be used for inventory control, possibly a

parts allocation scheme, or some other useful system.


program Define_Some_Sets;

type Goodies = (Ice_Cream,Whipped_Cream,Banana,Nuts,Cherry,

Choc_Syrup,Strawberries,Caramel,Soda_Water,

Salt,Pepper,Cone,Straw,Spoon,Stick);

Treat = set of Goodies;

var Sundae : Treat;

Banana_Split : Treat;

Soda : Treat;

Ice_Cream_Cone : Treat;

Nutty_Buddy : Treat;

Mixed : Treat;

Index : byte;

begin

(* define all ingredients used in each treat *)

Ice_Cream_Cone := [Ice_Cream,Cone];

Soda := [Straw,Soda_Water,Ice_Cream,Cherry];

Banana_Split := [Ice_Cream..Caramel];

Banana_Split := Banana_Split + [Spoon];

Nutty_Buddy := [Cone,Ice_Cream,Choc_Syrup,Nuts];

Sundae := [Ice_Cream,Whipped_Cream,Nuts,Cherry,Choc_Syrup,

Spoon];

(* combine for a list of all ingredients used *)

Mixed := Ice_Cream_Cone + Soda + Banana_Split + Nutty_Buddy +

Sundae;

Mixed := [Ice_Cream..Stick] - Mixed; (* all ingredients not used *)

if Ice_Cream in Mixed then Writeln('Ice cream not used');

if Whipped_Cream in Mixed then Writeln('Whipped cream not used');

if Banana in Mixed then Writeln('Bananas not used');

if Nuts in Mixed then Writeln('Nuts are not used');

if Cherry in Mixed then Writeln('Cherrys not used');

if Choc_Syrup in Mixed then Writeln('Chocolate syrup not used');

if Strawberries in Mixed then Writeln('Strawberries not used');

if Caramel in Mixed then Writeln('Caramel is not used');

if Soda_Water in Mixed then Writeln('Soda water is not used');

if Salt in Mixed then Writeln('Salt not used');

if Pepper in Mixed then Writeln('Pepper not used');

if Cone in Mixed then Writeln('Cone not used');

if Straw in Mixed then Writeln('Straw not used');

if Spoon in Mixed then Writeln('Spoon not used');

if Stick in Mixed then Writeln('Stick not used');

end.


Salt not used

Pepper not used

Stick not used

}



SEARCHING WITH SETS

The Pascal program FINDCHRS.PAS is more useful than the last one. In it we start with a short

sentence and search it for all lower case alphabetic letters and write a list of those used. Since we are

using a portion of the complete range of char, we do not need to define a scalar before defining the set,

we can define the set using the range 'a'..'z'. The set Data_Set is assigned the value of no elements in

the first statement of the program, and the print string, named Print_Group, is set to blank in the

next. The variable Storage is assigned the sentence to search, and the search loop is begun. Each

time through the loop, one of the characters is checked. It is either declared as a non-lower-case

character, as a repeat of one already found, or as a new character to be added to the list.

You are left to decipher the details of the program, which should be no problem since there is nothing

new here. Run the program and observe how the list grows with new letters as the sentence is scanned.


program Find_All_Lower_Case_Characters;

const String_Size = 30;

type Low_Set = set of 'a'..'z';

var Data_Set : Low_Set;

Storage : string[String_Size];

Index : 1..String_Size;

Print_Group : string[26];


Data_Set := [];

Print_Group := '';

Storage := 'This is a set test.';

for Index := 1 to Length(Storage) do begin

if Storage[Index] in ['a'..'z'] then begin

if Storage[Index] in Data_Set then

Writeln(Index:4,' ',Storage[Index],

' is already in the set')

else begin

Data_Set := Data_Set + [Storage[Index]];

Print_Group := Print_Group + Storage[Index];


' added to group, complete group = ',

Print_Group);

end;

end

else


' is not a lower case letter');

end;



1 T is not a lower case letter

2 h added to group, complete group = h

3 i added to group, complete group = hi

4 s added to group, complete group = his

5 is not a lower case letter

6 i is already in the set

7 s is already in the set


9 a added to group, complete group = hisa



12 e added to group, complete group = hisae

13 t added to group, complete group = hiseat14 is not a lower case letter

15 t is already in the set



16 e is already in the set


18 t is already in the set

19 . is not a lower case letter

}


1. Modify FINDCHRS.PAS to search for upper-case letters.



Chapter 9 : RECORDS

A VERY SIMPLE RECORD

We come to the grandaddy of all data structures in Pascal, the record. A record is composed of anumber of variables any of which can be of any predefined data type, including other records. Rather

than spend time trying to define a record in detail, lets go right to the first example program,

SMALLREC.PAS. This is a program using nonsense data that will illustrate the use of a record.


program A_Small_Record;

type Description = record

Year : integer;

Model : string[20];

Engine : string[8];

end;

var Truck : Description;

Cars : array[1..10] of Description;

Index : integer;


Truck.Year := 1988;

Truck.Model := 'Pickup';

Truck.Engine := 'Diesel';


Cars[Index].Year := 1930 + Index;

Cars[Index].Model := 'Duesenburg';

Cars[Index].Engine := 'V8';

end;

Cars[2].Model := 'Stanley Steamer';Cars[2].Engine := 'Coal';

Cars[7].Engine := 'V12';

Cars[9].Model := 'Ford';

Cars[9].Engine := 'rusted';

Write('My ',Truck.Year:4,' ');

Write(Truck.Model,' has a ');

Writeln(Truck.Engine,' engine.');


Write('My ',Cars[Index].Year:4,' ');

Write(Cars[Index].Model,' has a ');

Writeln(Cars[Index].Engine,' engine.');

end;



My 1988 Pickup has a Diesel engine.

My 1931 Duesenburg has a V8 engine.

My 1932 Stanley Steamer has a Coal engine.







My 1939 Ford has a rusted engine.


}



There is only one entry in the type declaration part of the program, the record identified by the name

Description. The record is composed of three fields, the Year, Model, and Engine variables. Notice that

the three fields are each of a different type, indicating that the record can be of mixed types. You have a

complete example of the way a record is defined before you. It is composed of the identifier

Description, the = sign, the reserved word record, the list of elements, and followed by the reservedword end. This is one of the places in Pascal where an end is used without a corresponding begin.

Notice that this only defines a type, it does not define any variables. That is done in the var

declaration where the variable Truck is defined as a record of type Description and Cars is defined to

have 10 complete records of the type Description. The variable Truck has three components, Year,

Model, and Engine, and any or all of these components can be used to store data pertaining to Truck.

When assigning data to the variable Truck, for example, there are actually three parts to the variable, so

we use three assignment statements, one for each of the sub-fields. In order to assign values to the

various sub-fields, the variable name is followed by the sub-field name with a separating period. The

"var.sub_field" combination is a variable name.

Keep in mind that Truck is a complete record containing three variables, and to assign or use one of the

variables, you must designate which sub-field you are interested in. Examine lines 16 through 18 of theprogram where the three fields are assigned meaningless data for illustration. The Year field is assigned

an integer number, the Model field is assigned the name Pickup, and the Engine variable is assigned the

value Diesel.

A loop is then used to assign data to all 10 records of Cars. In order to further illustrate that there are

actually 30 variables in use here, a few are changed at random in lines 26 through 30, being very

careful to maintain the required types as defined in the type declaration part of the program. Finally, all

ten composite variables, consisting of 30 actual variables in a logical grouping are printed out using the

same "var.sub-field" notation described above.

If the preceding description of a record is not clear in your mind, review it very carefully. It's a veryimportant concept in Pascal, and you won't have a hope of a chance of understanding the next example

until this one is clear. Be sure to compile and run SMALLREC.PAS so you can study the output.

A SUPER RECORD

Examine the Pascal example file BIGREC.PAS for a very interesting record. First we have a constant

defined. Ignore it for the moment, we will come back to it later. Within the type declaration we havethree records defined, and upon close examination, you will notice that the first two records are

included as part of the definition of the third record. The record identified as Person, actually

contains 9 variable definitions, three within the Full_Name record, three of its own, and three within

the Date record. Once again, this is a type declaration and does not actually define any variables, that is

done in the var part of the program.

The var part of the program defines some variables beginning with the array of Friend containing

50 (because of the constant definition in the const part) records of the user defined type, Person.

Since the type Person defines 9 fields, we have now defined 9 times 50 = 450 separate and distinct

variables, each with its own defined type. Remember that Pascal is picky about assigning data by the

correct type. Each of the 450 separate variables has its own type associated with it, and the compiler

will generate an error if you try to assign any of those variables the wrong type of data. Since Person

is a type definition, it can be used to define more than one variable, and in fact it is used again to define

three more records, Self, Mother, and Father. These three records are each composed of 9

variables, so we have 27 more variables which we can manipulate within the program. Finally we have

the variable Index defined as a simple byte type variable.




program A_Larger_Record;

const Number_Of_Friends = 50;

type Full_Name = record

First_Name : string[12];

Initial : char;

Last_Name : string[15];end;

Date = record

Day : byte;

Month : byte;

Year : integer;

end;

Person = record

Name : Full_Name;

City : string[15];

State : string[2];

Zipcode : string[5];

Birthday : Date;

end;

var Friend : array[1..Number_Of_Friends] of Person;

Self,Mother,Father : Person;

Index : byte;


Self.Name.First_Name := 'Charley';

Self.Name.Initial := 'Z';

Self.Name.Last_Name := 'Brown';

with Self do begin

City := 'Anywhere';

State := 'CA';

Zipcode := '97342';

Birthday.Day := 17;

with Birthday do begin

Month := 7;

Year := 1938;

end;

end; (* all data for self now defined *)

Mother := Self;

Father := Mother;

for Index := 1 to Number_Of_Friends do

Friend[Index] := Mother;

Write(Friend[27].Name.First_Name,' ');

Write(Friend[33].Name.Initial,' ');

Write(Father.Name.Last_Name);

Writeln;



Charley Z Brown

}

HOW TO MANIPULATE ALL OF THAT DATA

In the program we begin by assigning data to all of the fields of Self in lines 31 through 43.

Examining the first three statements of the main program, we see the construction we learned in the last

example program being used, namely the period between descriptor fields. The main record is named

Self, and we are interested in the first part of it, specifically the Name part of the Person record. Sincethe Name part of the Person record is itself composed of three parts, we must designate which

component of it we are interested in. Self.Name.First_Name is the complete description of the



first name of Self and is used in the assignment statement in line 31 where it is assigned the name of

"Charley". The next two fields are handled in the same way and are self explanatory.

WHAT IS THE WITH STATEMENT?

Continuing on to the fourth field, the City, there are only two levels required because City is not

another record definition. The fourth field is therefore completely defined by Self.City. Notice the

with Self do statement. This is a shorthand notation used with record definitions to simplify coding.

From the begin in line 34 to the matching end in line 43, any variables within the Self record are used

as though they had a Self. in front of them. It greatly simplifies coding to be able to omit the leading

identifier within the with section of code. You will see that City, State, and Zipcode are easily

assigned values without further reference to the Self variable. When we get to the Day part of the

birthday, we are back to three levels and the complete definition is Self.Birthday.Day but once

again, the Self. part is taken care of automatically because we are still within the with Self do area.

To illustrate the with statement further, another is introduced in line 39, with Birthday do, and

an area is defined by the begin end pair which extends from line 39 through line 42. Within this area

both leading identifiers are handled automatically to simplify coding, and Month is equivalent to

writing Self.Birthday.Month if both with statements were removed.

HOW FAR DOWN CAN YOU NEST THE WITH STATEMENT?

You may be wondering how many levels of nesting are allowed in record definitions. There doesn't

appear to be a limit according to the Pascal definition, but we do get a hint at how far it is possible to

go. In TURBO Pascal, you are allowed to have with statements nested to nine levels, and it would be

worthless to nest with statements deeper than the level of records. Any program requiring more levels

than nine is probably far beyond the scope of your programming ability, and mine, for a long time.

After assigning a value to Year, the entire record of Self is defined, all nine variables. It should bepointed out that even though Self is composed of nine separate variables, it is proper to call Self a

variable itself because it is a record variable.

SUPER-ASSIGNMENT STATEMENTS

The statement in line 45, "Mother := Self;" is very interesting. Since both of these are records,

both are the same type of record, and both therefore contain 9 variables, Pascal is smart enough to

recognise that, and assign all nine values contained in Self to the corresponding variables of

Mother. So after one statement, the record variable Mother is completely defined. The statement in

line 46 assigns the same values to the nine respective variables of Father, and the next two lines

assign all 50 Friend variables the same data. By this point in the program, we have thereforegenerated 450 + 27 = 477 separate pieces of data so far in this program. We could print it all out, but

since it is nonsense data, it would only waste time and paper. Lines 49 through 52 write out three

sample pieces of the data for your inspection.

WHAT GOOD IS ALL OF THIS

It should be obvious to you that what this program does, even though the data is nonsense, appears to

be the beginning of a database management system, which indeed it is. Instead of assigning nonsense

data, a list could be read in and stored for manipulation. It is a crude beginning, and has a long way to

go to be useful, but you should see a seed for a useful program.

Now to go back to the const in line 4 as promised. The number of friends was defined as 50 and used

for the size of the array and in the assignment loop in line 47. You can now edit this number and see



how big this database can become on your computer. If you are using TURBO Pascal, you will be

limited to slightly more than 1000 because of the 64K limitation of an executable program, and the factthat all of this data is stored within that 64K boundary. It should be noted that TURBO Pascal allows a

program larger than 64K but still places a limitation of 64K on each compilation unit. See how big you

can make the number of friends before you get the memory overflow message. Keep the number in

mind because when we get to the chapter on Pointers and Dynamic Allocation, you will see a marked

increase in allowable size, especially if you have a large amount of RAM installed in your computer.

A VARIANT RECORD

If any part of this chapter is still unclear, it would be good for you to go back and review it at this time.

The next example will really tax your mind to completely understand it, and this will be true especially

if the prior material is not clear.

Examine the Pascal program VARREC.PAS for an example of a program with a variant record

definition. In this example, we first define a scalar type, namely Kind_Of_Vehicle for use within

the record. Then we have a record defining Vehicle, intended to define several different vehicles,

each with different kinds of data. It would be possible to define all variables for all types of vehicles,but it would be a waste of storage space to define the number of tires for a boat, or the number of

propeller blades used on a car or truck. The variant record lets us define the data precisely for eachvehicle without wasting data storage space.


program Variant_Record_Example;

type Kind_Of_Vehicle = (Car,Truck,Bicycle,Boat);

Vehicle = record

Owner_Name : string[25];

Gross_Weight : integer;

Value : real;

case What_Kind : Kind_Of_Vehicle of

Car : (Wheels : integer;

Engine : string[8]);

Truck : (Motor : string[8];

Tires : integer;

Payload : integer);

Bicycle : (Tyres : integer);

Boat : (Prop_Blades : byte;

Sail : boolean;

Power : string[8]);

end; (* of record *)

var Sunfish,Ford,Schwinn,Mac : Vehicle;


Ford.Owner_Name := 'Walter'; (* fields defined in order *)

Ford.Gross_Weight := 5750;

Ford.Value := 2595.00;

Ford.What_Kind := Truck;Ford.Motor := 'V8';

Ford.Tires := 18;

Ford.Payload := 12000;

with Sunfish do begin

What_Kind := Boat; (* fields defined in random order *)

Sail := TRUE;

Prop_Blades := 3;

Power := 'wind';

Gross_Weight := 375;

Value := 1300.00;

Owner_Name := 'Herman and George';

end;

Ford.Engine := 'flathead'; (* tag-field not defined yet but it *)

Ford.What_Kind := Car; (* must be before it can be used *)Ford.Wheels := 4;

(* notice that the non variant part is not redefined here *)



Mac := Sunfish; (* entire record copied, including the tag-field *)

if Ford.What_Kind = Car then (* this should print *)

Writeln(Ford.Owner_Name,' owns the car with a ',Ford.Engine,

' engine.');

if Sunfish.What_Kind = Bicycle then (* this should not print *)

Writeln('The sunfish is a bicycle which it shouldn''t be');

if Mac.What_Kind = Boat then (* this should print *)

Writeln('The mac is now a boat with',Mac.Prop_Blades:2,

' propeller blades.');



Walter owns the car with the flathead engine.

The mac is now a boat with three propeller blades.

}

WHAT IS A TAG-FIELD?

In the record definition we have the usual record header followed by three variables defined in the

same manner as the records in the last two example programs. Then we come to the case statement.

Following this statement, the record is different for each of the four types defined in the associated

scalar definition. The variable What_Kind is called the tag-field and must be defined as a scalar type

prior to the record definition. The tag-field is used to select the variant, when the program uses one of

the variables of this record type. The tag-field is followed by a colon and its type definition, then the

reserved word of. A list of the variants is then given, with each of the variants having the variables forits particular case defined. The list of variables for one variant is called the field list.

A few rules are in order at this point. The variants do not have to have the same number of variables in

each field list, and in fact, one or more of the variants may have no variables at all in its variant part. If

a variant has no variables, it must still be defined with a pair of empty parentheses followed by a semi-

colon. All variables in the entire variant part must have unique names. The three variables, Wheels,

Tires, and Tyres, all mean the same thing to the user, but they must be different for the compiler. You

may use the same identifiers again in other records and for simple variables anywhere else in the

program. The Pascal compiler can tell which variable you mean by its context. Using the same variable

name should be discouraged as bad programming practice because it may confuse you or another

person trying to understand your program at a later date.

The final rule is that the variant part of the record must be the last part of it, and in fact, the last part of any or all variants can itself have a variant part to it. That is getting pretty advanced for our level of use

of Pascal at this time however.

USING THE VARIANT RECORD

We properly define four variables with the record type Vehicle in line 22 and go on to examine the

program itself.

We begin by defining one of our variables of type Vehicle, namely the variable named Ford. The

seven lines assigning values to Ford are similar to the prior examples with the exception of line 28. In

that line the tag-field which selects the particular variant used is set equal to the value Truck, which is

a scalar definition, not a variable. This means that the variables named Motor, Tires, and Payload



are available for use with the record Ford, but the variables named Wheels, Engine, Tyres, etc.

are not available in the record named Ford.

Next, we will define the record Sunfish as a Boat, and define all of its variables in lines 33 through

41. All of Sunfish's variables are defined but in a rather random order to illustrate that they need not

be defined in a particular order. You should remember the with statement from the last example

program.

To go even further in randomly assigning the variables to a record, we redefine Ford as having an

Engine which it can only have if it is a car. This is one of the fine points of the Pascal record. If you

assign any of the variant variables, the record is changed to that variant, but it is the programmers

responsibility to assign the correct tag-field to the record, not Pascal's. Good programming practice

would be to assign the tag-field before assigning any of the variant variables. The remainder of the

Ford variables are assigned to complete that record, the non-variant part remaining from the lastassignment.

The variable Mac is now set equal to the variable Sunfish in line 48. All variables within the record

are copied to Mac including the tag-field, making Mac a Boat.

NOW TO SEE WHAT WE HAVE IN THE RECORDS

We have assigned Ford to be a car, and two boats exist, namely Sunfish and Mac. Since Schwinn

was never defined, it has no data in it, and is at this point useless. The Ford tag-field has been defined

as a car, so it should be true in the if statement, and the message in line 51 should print. The Sunfish

is not a bicycle, so it will not print. The Mac has been defined as a boat in the single assignment

statement, so it will print a message with an indication that all of the data in the record was transferredto its variables.

Even though we can make assignment statements with records, they cannot be used in any

mathematical operations such as addition, or multiplication. They are simply used for data storage. It is

true however, that the individual elements in a record can be used in any mathematical statements legal

for their respective types.

One other point should be mentioned. The tag-field can be completely eliminated resulting in a "free

union" variant record. This is possible because Pascal, as you may remember from above, will

automatically assign the variant required when you assign data to one of the variables within a variant.

This is the reason that all variables within any of the variants must have unique names. The free union

record should be avoided in your early programming efforts because you cannot test a record to seewhat variant has been assigned to it. It is definitely an advanced technique in Pascal.

Be sure you compile and run VARREC.PAS and study the output until you understand it completely.


1. Write a simple program with a record to store the names of five of your friends and display the

names.



Chapter 10 : STANDARD INPUT/OUTPUT

WE'VE USED THIS ALREADY

During the course of this tutorial we have been using the Write and Writeln procedures to display

data, and it is now time to discuss them fully. Actually there is little to be said about them that has not

already been said, but in order to get all of the data in one place, they will be redefined here.

As mentioned earlier, Write and Writeln are not actually reserved words but are procedure calls.

They are therefore merely identifiers that could be changed, but there should never be a reason to do so.

Let's get on to our first example program WRITELNX.PAS which has lots of output.


program Examples_Of_Write_Statements;

var Index : byte;

Count : real;

What : boolean;Letter : char;

Name : string[10];

begin

Writeln('Integer');

Index := 17;

Writeln(Index,Index);

Writeln(Index:15,Index:15);

Writeln;

Writeln('Real');

Count := 27.5678;

Writeln(Count,Count);

Writeln(Count:15,Count:15);

Writeln(Count:15:2,Count:15:2);

Writeln(Count:15:3,Count:15:3);Writeln(Count:15:4,Count:15:4);

Writeln;

Writeln('Boolean');

What := FALSE;

Writeln(What,What);

Writeln(What:15,What:15);

Writeln('Char');

Letter := 'Z';

Writeln(Letter,Letter);

Writeln(Letter:15,Letter:15);

Writeln('String');

Name := 'John Doe';

Writeln(Name,Name);Writeln(Name:15,Name:15);

Writeln;

Writeln('Text output','Text output');

Writeln('Text output':15,'Text output':15);

end.


Integer

1717

17 17

Real2.7567800000E+01 2.7567800000E+01

2.75678000E+01 2.75678000E+01



27.57 27.57

27.568 27.568

27.5678 27.5678

Boolean

FALSEFALSE

FALSE FALSE

Char

ZZZ Z

String

John DoeJohn Doe

John Doe John Doe

Text OutputText Output

Text Output Text Output

}

MANY OUTPUT STATEMENTS

Pascal has two output statements with only slight differences in the way they work. The Writeln

statement outputs all of the data specified within it, then returns the cursor to the beginning of the next

line. The Write statement outputs all of the data specified within it, then leaves the cursor at the next

character where additional data can be output. The Write statement can therefore be used to output a

line in bits and pieces if desired for programming convenience. The first example program for this

chapter, WRITELNX.PAS, has many output statements for your observation. All outputs are repeated

so you can observe where the present field ends and the next starts.

Observe the two integer output statements in lines 13 and 14. The first simply directs the system to

output Index twice, and it outputs the value with no separating blanks. The second statement says to

output Index twice also, but it instructs the system to put each output in a field 15 characters wide with

the data right justified in the field. This makes the output look much better. This illustrates that youhave complete control over the appearance of your output data.

The real output statements in lines 19 and 20 are similar to the integer except that when the data is put

into a field 15 characters wide, it is still displayed in scientific format. Adding a second field descriptor

as illustrated in lines 21 through 23, tells the system how many digits you want displayed after the

decimal point.

The boolean, char, and string examples should be self explanatory. Notice that when the string is

output, even though the string has been defined as a maximum of 10 characters, it has been assigned a

string of only 8 characters, so only 8 characters are output. Compile and run this program and observe

the results.

The new data types in TURBO Pascal which were described in chapter 3 of this tutorial are output inthe same manner as those illustrated in the program WRITELNX.PAS.

NOW FOR SOME INPUT FROM THE KEYBOARD

The example file READINT.PAS will illustrate reading some integer data from the keyboard. A

message is output by line 8 with an interesting fact that should be pointed out. Anyplace where Pascal

uses a string variable or constant, it uses the apostrophe for a delimiter. Therefore, anyplace where an

apostrophe is used in a string, it will end the string. Two apostrophes in a row will be construed as a

single apostrophe within the string and will not terminate the string. The term 'Read' within the string

will therefore be displayed as shown earlier in this sentence.




program Read_Some_Variables;

var Index : byte;

Number1,Number2,Number3 : integer;


Writeln('This is the ''Read'' portion of the program');

for Index := 1 to 5 do beginWrite('Enter up to three integers ');

Read(Number1,Number2,Number3);

Writeln('Thank you');

Writeln('The numbers entered were ',Number1:6,Number2:6,

Number3:6);

end;

Writeln;

Writeln('This is the ''Readln'' portion of the program');


Write('Enter up to three integers ');

Readln(Number1,Number2,Number3);

Writeln('Thank you');

Writeln('The numbers entered were ',Number1:6,Number2:6,

Number3:6);

end;end. (* of main program *)


(The results are dependent on the data entered at the keyboard)

}

The variable Index is used to loop five times through a sequence of statements with one Read statement

in it. The three integer values are read in and stored in their respective variables with the one statement.If less than three are entered at the keyboard, only as many as are read in will be defined, the rest will

be unchanged. Following completion of the first loop, there is a second loop in lines 19 through 25 that

will be executed 5 times with only one minor change, the Read statement is replaced by the Readln

statement. At this point it would be best run this program trying several variations with input data.

When you run READINT.PAS, it will request three integers. Reply with three small integers of your

choice with as many blank spaces between each as you desire, followed by a carriage return. The

system will echo your three numbers back out, and request three more. Respond with only one number

this time, different from each of the first three, and a carriage return. You will get your new number

followed by your previous second and third number indicating that you did not re-enter the last twointeger variables. Enter three more numbers, this time including a negative number and observe the

echo once again.

Continue entering numbers until the system outputs the message indicating that it will now be using the

Readln for reading data. At this point enter the same numbers that you did in the previous section and

notice the difference, which is only very slight. Each time you hit the enter key to cause the computerto process the data you have just given it, it will echo the carriage return to the display, and the "Thank

you" message will be on a new line. When entering data from the keyboard, the only difference in Read

and Readln is whether or not the carriage return is echoed to the display following the data read

operation.

It should not be a surprise to you that after you enter the data, the data is stored within the program and

can be used anywhere that integer data is legal for use. Thus, you could read in an integer, and use the

integer to control the number of times through a loop, as a case selector, etc.



TIME TO CRASH THE COMPUTER

Crashing the computer will not hurt a thing. Rerun the above program and instead of entering integer

data, enter some real data with decimal points, or even some character data. The computer should

display some kind of message indicating that you have caused an I/O error (Input/Output), and TURBO

Pascal will abort operation (that simply means to stop the program and return control to the operating

system). No harm has been done, simply start it again to enter more numbers or errors.

READING REAL NUMBERS

The example program READREAL.PAS will illustrate how to read real numbers into the computer. It

will read an integer and three real numbers each time through the loop. It is perfectly fine to give the

system a number without a decimal point for a real number. The computer will simply read it as a

decimal number with zeros after the decimal point and consider it as a real number internally. As you

found out in the last example program, however, it is not permissible to include a decimal point in the

data if the computer is looking for an integer variable. Include some character data for a real number

and crash the system in this program too.


program Read_Some_Real_Variables;

var Index : byte;

Number : integer;

Real1,Real2,Real3 : real;

begin

Writeln('Read some Real variables');


Write('Enter an integer and 3 reals ');

Readln(Number,Real1,Real2,Real3);

Writeln(Number:20,Real1:12:3,Real2:12:3,Real3:12:3);

end;

end.


(The results depend on the data entered at the keyboard)

}

READING CHARACTER DATA

The next example program, READCHAR.PAS, will read in one character each time through the loop

and display it for you. Try entering more than one character and you will see that the extra characters

will simply be ignored. It is not possible to crash this program because any character you enter will bevalid.

Finally, READSTRG.PAS will also read up to 10 characters, but since a string is a dynamic length

variable, it will only print out the characters you input each time, up to the maximum of 10 as defined

in the var declaration. It will display trailing blanks if you type them in because blanks are valid

characters.




program Read_Some_Char_Data;

var Letter : char;

Index : byte;

begin


Write('Input a character ');Readln(Letter);

Writeln('The character input was an ',Letter);

end;

end.


( The results depend on the data entered at the keyboard)

}


program Read_A_String;

var Index : byte;

Field : string[10];

begin


Write('Enter up to 10 characters ');

Readln(Field);

Writeln('The data you entered was (',Field,')');

end;

end.


(The output depends on the data entered at the keyboard)

}

BULLET PROOF PROGRAMMING

It can be frustrating to be running a program and have it declare an I/O error and terminate operation

simply because you have entered an incorrect character. The integer and real data inputs defined earlierin this chapter are fine for quick little programs to do specific calculations, but if you are writing a large

applications program it is better to use another technique. Since the character and string inputs cannot

abort operation of the program, it is best to use them to input the variable data and check the data

internally under your own program control. An error message can then be given to the operator and

another opportunity granted to input the correct data. All well written large application programs usethis technique.

HOW DO I PRINT SOMETHING ON THE PRINTER?

With all of the Pascal knowledge you now have, it is the simplest thing in the world to get data to the

printer. The example file PRINTOUT.PAS will show you graphically how to do it. Every Write orWriteln statement is required to have a device identifier prior to the first output field. If there is

none, it is automatically defaulted to the standard output device, the display monitor. The example



program has a few outputs to the monitor in lines 9 and 10 with the device identifier included, namely

Output. This is only done to show you the general form of the Write statements, but if you desire, youcan add the standard device identifier to every monitor output.


program Printout_Example;

uses Printer;

var Index : byte;

begin

Writeln(Output,'Printer program example');

Writeln(Output,'Turn on your printer and install paper in it.');

Writeln(Lst,'This is to demonstrate printing in Pascal');

Writeln(Lst);


Write(Lst,'The index value is ');

Write(Lst,Index:3);

Writeln(Lst,' at this point');

end;

end.


Printer example program

Turn on your printer and install paper in it

(The following is output to the printer)

This is to demonstrate printing in Pascal

The index value is 1 at this point















}

There are many statements in this program with the device identifier Lst, which is the standard name

for the list device or the printer. It should be obvious to you that the first field is the device selector

which is used to direct the output to the desired device.

Compile and run this program with your printer turned on for some printer output. Just to supply you

with a bit more information, every Read and Readln statement is also required to have a device

identifier prior to the first input field. As you may suspect, it is also defaulted to Input if none is

specified, and the standard input device is the keyboard.




3. Write a program containing a loop to read in a character string up to 60 characters long, then print

the string on your printer. When you run the program, you will have the simplest word processingprogram in the world. Be sure to include a test to end the loop, such as when "END" is typed in.



Chapter 11 : FILE INPUT/OUTPUT

FILES HANDLE SERIAL DATA

One of the most common operations when using a computer is to either read from, or write to a file.You are already somewhat experienced in file handling from the last chapter, because in computer

terminology, the keyboard, terminal, and printer are all classified as files. A file is any serial input or

output device that the computer has access to. Since it is serial, only one piece of information is

available to the computer at any instant of time. This is in contrast to an array, for example, in which all

elements of the array are stored internally and are all available at any time.

A SHORT HISTORY LESSON

Several years ago computers were all large cumbersome machines with large peripheral devices such as

magnetic tape drives, punch card readers, paper tape readers or punches, etc. It was a simple task toassign the paper tape reader a symbol and use that symbol whenever it was necessary to read a paper

tape. There was never more than one file on the paper tape being read, so it was simply read

sequentially, and hopefully the data was the desired data. With the advent of floppy disks, and hard

disks, it became practical to put several files of data on one disk, none of which necessarily had

anything to do with any of the other files on that disk. This led to the problem of reading the proper file

from the disk, not just reading the disk.

Pascal was originally released in 1971, before the introduction of the compact floppy disk. The originalrelease of Pascal had no provision for selecting a certain file from among the many included on the

disk. Each compiler writer had to overcome this deficiency and he did so by defining an extension to

the standard Pascal system. Unfortunately, all of the extensions were not the same, and there are now

several ways to accomplish this operation. There are primarily two ways, one using the Assign

statement, and the other using the Open statement. They are similar to each other and they accomplishthe same end result.

BACK TO THE PRESENT TIME

All of the above was described to let you know that we will have a problem in this chapter, namely,

how do we cover all of the possible implementations of Pascal available? The answer is, we can't. Most

of what is covered in this chapter will apply to all compilers, and all that is covered will apply to the

TURBO Pascal compilers. As we have mentioned before, this tutorial is especially written for the

TURBO Pascal compilers, but you should be warned that you will find differences in Pascal

implementations if you have occasion to use a different Pascal compiler someday. You may, for

example, need to use a mini-computer or a mainframe computer someday to complete a programming

assignment. When that happens, you will find that the area of input/output control will probably be the

biggest difference in the implementations of Pascal.

READING AND DISPLAYING A FILE

Examine the file READFILE.PAS for an example of a program that can read a text file from the disk.

In fact, it will read itself from the disk and display itself on the video monitor. The first statement in the

program is the Assign statement. This is TURBO Pascal's way of selecting which file on the disk will

be either read from or written to. In this case we will read from the disk. The first argument in the

Assign statement is the device specifier similar to Lst used in the last chapter for the printer. We

have chosen to use the name Turkey for the device identifier, but could have used any valid identifier.

This identifier must be defined in a var declaration as a text type variable as illustrated in line 4. The



next argument is the filename desired. The filename can be defined as a string constant, as it is here, or

as a string variable.


program Read_A_File;

var Turkey : text;

Big_String : string[80];


Assign(Turkey,'READFILE.PAS');

Reset(Turkey);

while not Eof(Turkey) do begin

Readln(Turkey,Big_String);

Writeln(Big_String);

end; (* of while loop *)

Close(Turkey);

end. (* of program *)


(This file is displayed on the monitor)

}

The text type is a predefined type and is used to define a file identifier. It is predefined as a "file

of char", so it can only be used for a text file. We will see later that there is another type of file, a

binary file.

You will find that the operating system that you are using requires a file name to follow certainconventions when it is named, and the Pascal programming language has a set of rules by which an

identifier can be named. Since these two conventions are not necessarily the same, it is necessary to

give each file an external name which the operating system is happy with, and an internal filenamewhich Pascal is happy with. It now becomes necessary to tie these two names together, and this is the

primary job of the Assign statement.

Now that we have a file identified, it is necessary to prepare it for reading by executing a reset

statement in line 9. The Reset statement positions the read pointer at the beginning of the file, ready to

read the first piece of information in the file. Once we have done that, data is read from the file in the

same manner as it was when reading from the keyboard. In this program, the input is controlled by the

while loop which is executed until we exhaust the data in the file.

WHAT ARE THE "EOF" AND "EOLN" FUNCTIONS?

The Eof function is new and must therefore be defined. When we read data from the file, we move

closer and closer to the end of the file, until finally we reach the end and there is no more data to read.

This is called "end of file" and is abbreviated Eof. Pascal has this function available as a part of the

standard library which returns FALSE until we reach the last line of the file. When there is no moredata to read left in the file, the function Eof returns TRUE. To use the function, we merely give it our

file identifier as an argument. It should be clear to you that we will loop in this program until we read

all of the data available in the input file.

The Eoln function is not used in this program but is a very useful function. If the input pointer is

anywhere in the text file except at the end of a line, the Eoln returns FALSE, but at the end of a line, it

returns a value of TRUE. This function can therefore be used to find the end of a line of text for

variable length text input.



To actually read the data, we use the Readln procedure, giving it our identifier Turkey and the name

of the variable we want the data read into. In this case, we read up to 80 characters into the string and if

more are available, ignore them. You should remember when we did this in the last chapter from the

keyboard input. We are using the same technique here except we are reading from a file this time.

Since we would like to do something with the data, we output the line to the default device, the video

monitor. It should be clear to you by now that the program will read the entire file and display it on the

monitor.

Finally, we Close the file Turkey. It is not really necessary to close the file because the system will

close it for us automatically at program termination, but it is a good habit to get into. It must be

carefully pointed out here, that you did not do anything to the input file, you only read the data and left

it intact. You could Reset it and reread it again in this same program. Compile and run this program

to see if it does what you expect it to do.

A PROGRAM TO READ ANY FILE

Examine the next program READDISP.PAS for an improved file reading program. This is very similar

to the previous program, except that it asks you for the name of the file which you wish to display.After you enter the filename, it enters the name into a 12 character string which is named

Name_Of_File_To_Inputwhich will be the external filename. This is then used in the Assign

statement to select the file to be read, and to connect the external filename to the internal filename,

Chicken. The file is then reset as before. Lines 15 through 18 display a header, and from that point

on, the program is identical to the last one with a few small additions. In order to demonstrate the use

of a function within the Writeln specification, the program calls for the length of the input string in

line 23 and displays it before each line. The lines are counted as they are read and displayed, and the

line count is displayed at the end of the listing. Both of these operations are done only to illustrate toyou how they can be done.


program Read_And_Display;

var Chicken : text;

Name_Of_File_To_Input : string[12];

Line_Count : integer;



Write('Enter input file name ');

Readln(Name_Of_File_To_Input);

Assign(Chicken,Name_Of_File_To_Input);

Reset(Chicken);

Writeln;

Writeln('Program listing with character count per');

Writeln('line and total line count');

Writeln;

Line_Count := 0;while not Eof(Chicken) do begin

Readln(Chicken,Big_String);

Writeln(Length(Big_String):5,' ',Big_String);

Line_Count := Line_Count + 1;

end;

Close(Chicken);

Writeln;

Writeln('The line count is ',Line_Count:3);



(The selected file is displayed on the monitor)

}



You should be able to understand clearly how each of these operations is accomplished. Compile and

run this program, entering any filename we have used so far as the file to be listed on the monitor (be

sure to include the .PAS extension). After a successful run, enter a non-existent filename and see the

I/O error generated by the Pascal runtime system. The next example program will illustrate a more

graceful method of detecting this error.

HOW TO COPY A FILE (SORT OF)

Examine the file READSTOR.PAS for an example of reading from a file and writing to another one. In

this program we request an operator input for the filename to read, after which we Assign the name to

the file and Reset it. When we reset the file however, we go to a bit of extra trouble to assure that the

file actually exists. Note that this is an extension to TURBO Pascal and will probably not work with

other Pascal compilers.

Suppose we input a filename, and the file did not exist because the file was actually missing, or because

we entered the name of the file wrong. Without this extra effort, the TURBO Pascal runtime system

would indicate a run-time error, and terminate the program returning us to the operating system. In

order to make a program easier to use, it would be nice to tell the operator that the file didn't exist andgive him the opportunity to try again with another file name. The method given in lines 16 through 20

of this program will allow you to do just that.


program Read_And_Store_A_File;

var Read_File : text;

Input_File_Name : string[12];

Write_File : text;

Output_File_Name : string[12];

Line_Number : integer;


Read_File_OK : boolean;

begin

Write('Enter input file name ');

Readln(Input_File_Name);

Assign(Read_File,Input_File_Name);

{$I-}

Reset(Read_File);

{$I+}

Read_File_OK := (IOResult = 0);

if Read_File_OK then begin

Write('Enter output file name ');

Readln(Output_File_Name);

Assign(Write_File,Output_File_Name);

Rewrite(Write_File);

Line_Number := 1;

while not Eof(Read_File) do beginReadln(Read_File,Big_String);

Write(Write_File,Line_Number:5,' ');

Writeln(Write_File,Big_String);

Line_Number := Line_Number + 1;

end;

Close(Read_File);

Close(Write_File);

end

else

Writeln('Input file doesn''t exist, execution aborted');



(The selected file is copied to the selected output file)

}



USING A COMPILER DIRECTIVE

First you must disable the built in TURBO Pascal I/O checking by inserting the compiler directive in

line 16. This tells the system to ignore any I/O errors from this point on and if the file doesn't exist, the

system will not abort when you attempt to reset it in line 17. Another compiler directive is given in line

18 to enable I/O checking again for the remainder of the program.

WE DO OUR OWN FILE CHECKING

If the file didn't exist and could not therefore be reset, we have a problem because the program thinks

the file is available for use but it actually isn't. Fortunately, TURBO Pascal has a built in variable,

named IOResult that informs us of the result of each I/O operation. Following any I/O operation, if this

variable contains the value of zero, the I/O operation was correct, and if it contains any other value, the

operation had some sort of error. In our case, we simply compare it to zero to generate a boolean value,

then based on the boolean value we either give an error message and stop, or perform the desired file

operations.

It would be good programming practice to check all file openings in this manner to allow the operatorto recover from a simple oversight or spelling error.

If the file was opened properly, then in line 21 through 24 we request a different filename to write to,

which is assigned to a different identifier. Note that the output file is not checked for a valid opening in

this example as it should be. The statement in line 24 is new to us, the Rewrite statement. This name

apparently comes from the words REset for WRITEing because that is exactly what it does. It clears

the entire file of any prior data and prepares to write into the very beginning of the file. Each time you

write into it, the file grows by the amount of the new data written.

Once the identifier has been defined, and the Rewrite has been executed, writing to the file is identical

to writing to the display with the addition of the identifier being specified prior to the first output field.

With that in mind, you should have no trouble comprehending the operation of the program. This

program is very similar to the last, except that it numbers the lines as the file is copied. After runningthe program, look in your default directory for the new filename which you input when the system

asked for the output filename. Examine that file to see if it is truly a copy of the input file with line

numbers added.

Actually a much better style would result if the logic for each file opening was put into a procedure of

its own. In addition to this being easier to debug and understand, the completed procedures could be

reused in other programs in the future.

One word of caution. If you used an existing filename for the output file, the file was overwritten, and

the original destroyed. In that case, it was good that you followed instructions at the beginning of this

tutorial and made a working copy of the distribution disk. You did do that, didn't you?

Compile and run this program two different ways, once with a valid input filename that should runproperly, and the second time with an input filename that doesn't exist to prove to yourself that the test

actually does work correctly.

HOW TO READ INTEGER DATA FROM A FILE

It is well and good to be able to read text from a file, but now we would like to read other forms of data

from a file. First we will look at an example program to read data from a text file, then later we will see

an example program that reads from a binary file.

Examine the program READINTS.PAS for an example of reading data from a text file. A text file is an

ASCII file that can be read by a text editor, printed, displayed, or in some cases, compiled andexecuted. It is simply a file made up of a long string of char type data, and usually includes linefeeds,

carriage returns, and blanks for neat formatting. Nearly every file on the Tutorial disk you received



with this package is a text file. One notable exception is the file named LIST.EXE, which is an

executable program file.


program Read_Integers_From_File;

var Cupcake : text;

Index : byte;Var1,Var2,Var3,Var4,Var5 : integer;

begin

Assign(Cupcake,'INTDATA.TXT');

Reset(Cupcake);


Read(Cupcake,Var1,Var2,Var3);

Writeln(Var1:6,Var2:6,Var3:6);

end;

Reset(Cupcake);

Writeln;


Readln(Cupcake,Var1,Var2,Var3);

Writeln(Var1:6,Var2:6,Var3:6);end;

Reset(Cupcake);

Writeln;


Read(Cupcake,Var1,Var2,Var3,Var4,Var5);

Writeln(Var1:6,Var2:6,Var3:6,Var4:6,Var5:6);

end;

Reset(Cupcake);

Writeln;


Readln(Cupcake,Var1,Var2,Var3,Var4,Var5);

Writeln(Var1:6,Var2:6,Var3:6,Var4:6,Var5:6);

end;

Close(Cupcake);


{ Result of execution (This data is read from INTDATA.TXT)

101 102 103

104 105 106

107 108 109

110 111 112

113 114 115

101 102 103

105 106 107

109 110 111

113 114 115117 118 119

101 102 103 104 105

106 107 108 109 110

111 112 113 114 115

116 117 118 119 120

121 122 123 124 125

101 102 103 104 105

109 110 111 112 113

117 118 119 120 121

125 126 127 128 129

133 134 135 136 137

}



The example program has nothing new, you have seen everything in it before. We have an assignment,

followed by a reset of our file, followed by four read and write loops. Each of the loops has a subtle

difference to illustrate the Read and Readln statements. Notice that the same file is used for reading

four times with a Reset prior to each, illustrating the non-destructive read mentioned a few paragraphs

ago.

The file we will be using is named INTDATA.TXT and is on your disk. You could display it at thistime using the program READDISP.PAS we covered recently. Notice that it is simply composed of the

integer values from 101 to 148 arranged four to a line with a couple of spaces between each for

separation and a neat appearance. The important thing to remember is that there are four data points per

line.

READ AND READLN ARE SLIGHTLY DIFFERENT

As variables are read in with either procedure, the input file is scanned for the variables using blanks asdelimiters. If there are not enough data points on one line to satisfy the arguments in the input list, the

next line is searched also, and the next, etc. Finally when all of the arguments in the input list are

satisfied, the Read is complete, but the Readln is not. If it is a Read procedure, the input pointer is leftat that point in the file, but if it is a Readln procedure, the input pointer is advanced to the beginning

of the next line. The next paragraph should clear that up for you.

The input data file INTDATA.TXT has four data points per line but the first loop in the program

READINTS.PAS requests only three each time through the loop. The first time through, it reads the

values 101, 102, and 103, and displays those values, leaving the input pointer just prior to the 104,

because it is a Read procedure. The next time through, it reads the value 104, advances to the next line

and reads the values 105, and 106, leaving the pointer just prior to the 107. This continues until the 5passes through the loop are completed.

The loop in lines 19 through 22 contains a Readln procedure and also reads the values 101, 102, and

103, but when the input parameter list is satisfied, it moves the pointer to the beginning of the next line,

leaving it just before the 105. The values are printed out and the next time we come to the Readln, weread the 105, 106, and 107, and the pointer is moved to the beginning of the next line. It would be goodto run the program now to see the difference in output data for the two loops. Remember that the only

difference is that the first loop uses the Read procedure, and the second uses the Readln procedure.

When you come back to the program again, observe the last two loops, which operate much like the

first two except that there are now five requested integer variables, and the input file still only has four

per line. This is no problem. Both input procedures will simply read the first four in the first line,advance to the second line for its required fifth input, and each will do its own operation next. The

Read procedure will leave the input pointer just before the second data point of the second line, and

the Readln will advance the input pointer to the beginning of the third line. Compile and run this

program and observe the four output fields to see an illustration of these principles.

NOW TO READ SOME REAL VARIABLES FROM A FILE

Examine the file named REALDATA.TXT supplied on your Pascal Tutorial disk. You will see 8 lines

of what appears to be scrambled data, but it is good data that Pascal can read. Notice especially line 4

which has some data missing, and line 6 which has some extra data.

Examine the program file READDATA.PAS which will be used to illustrate the method of reading real

type data. Everything should be familiar to you, since there is nothing new here. The Readln

statement is requesting one integer variable, and three real variables, which is what most of the input

file contains. When we come to the fourth line, there are not enough data points available, so the first

two data points of the next line are read to complete the fourth pass through the loop. Since the file

pointer is advanced to the beginning of the next line, we are automatically synchronised with the dataagain. When we come to the sixth line, the last two data points are simply ignored. Run the program to

see if the results are as you would predict.




program Read_Real_Data_From_A_File;

var Hot_Dog : text;

Index : byte;

Line_Number : integer;

Real1,Real2,Real3 : real;


Assign(Hot_Dog,'REALDATA.TXT');

Reset(Hot_Dog);


Readln(Hot_Dog,Line_Number,Real1,Real2,Real3);

Writeln(Line_Number:7,Real1:12:3,Real2:12:3,Real3:12:3);

end;

Close(Hot_Dog);



1 23.600 145.450 234.800

2 -0.400 -0.050 0.345

3 4.000 3456.000 123.000

4 77.000 5.000 -0.003

6 1.000 2.000 3.000

7 12.120 13.110 14.140

8 2.000 3.000 4.000

}

If a Read were substituted for the Readln in line 14 of the program, the file pointer would not be

advanced to the beginning of line 6 after the fourth pass through the loop. The next attempt to read

would result in trying to read the value 0.0006 as an integer, and a run time error would result. Modifythe program, substituting a Read for the Readln in line 14, and see if this is not true. It will be left as

an exercise for the diligent student to add code to detect and act on the error in a manner similar to thatillustrated in READSTOR.PAS earlier in this chapter.

It should be pointed out that TURBO Pascal requires a digit both before and after the decimal point in

all data that is to be read in as real type data or it will be flagged as a run-time error and the program

will be halted. The digits can be zero as they are in several places in the example file but they must be

there.

That is all there is to reading and writing text files. If you learn the necessities, you will not be

stumbling around in the area of input/output which is very intimidating to many people. Remember to

Assign, then Reset before reading, Rewrite before writing, and Close before quitting. It is of the

utmost importance to close a file you have been writing to before quitting to write the last few buffers

to the file, but it is not as important to close read files unless you are using a lot of them, as there is animplementation dependent limit of how many files can be open at once. It is possible to read from a

file, close it, reopen it, and write to it in one program. You can reuse a file as often as you desire in a

program, but you cannot read from and write into a file at the same time.

NOW FOR BINARY INPUT AND OUTPUT

Examine the file BINOUT.PAS for an example of writing data to a file in binary form. First there is a

record defined in the type declaration part composed of three different variable types. In the var part,



Output_File is defined as a "file of Dat_Rec", the record defined earlier. The variable

Dog_Food is then defined as an array of the record, and a simple variable is defined.


program Binary_Output_Example;

type Dat_Rec = record

Count : integer;Size : real;

Name : string[30];

end;

var Output_File : file of Dat_Rec;

Dog_Food : array[1..20] of Dat_Rec;

Index : byte;


Assign(Output_File,'KIBBLES.BIT');

Rewrite(Output_File);


Dog_Food[Index].Count := Index;

Dog_Food[Index].Size := 12345.6789;

Dog_Food[Index].Name := 'Large size Kibbles & Bits';end;

Writeln('Begin outputting data');


Write(Output_File,Dog_Food[Index]);

Close(Output_File);

Writeln('End of output');



Begin outputting data

End of output

(In addition to the above output to the monitor, the file

named KIBBLES.BIT is created and filled with binary data.)

}

Any file assigned a type of text, which is a "file of char", is a text file. A text file can be read

and modified with a text editor, printed out, displayed on the monitor, etc. If a file is defined with any

other definition, it will be a binary file and will be in an internal format as defined by the Pascal

compiler and may not be readable by any compiler other than the one used to write it. Attempting to

display such a file will result in very strange looking gibberish on the monitor.

When we get to the program, the output file is assigned a name in line 15, and a Rewrite is

performed on it to reset the input pointer to the beginning of the file, empty the file, and prepare for

writing data into it. The loop in lines 18 through 22 simply assigns nonsense data to all of the variables

in the 20 records so we have something to work with.

We write a message to the display that we are ready to start outputting data, then we output the data

one record at a time with the standard Write statement. A few cautions are in order here. The output

file can be defined to store any simple variable type, integer, byte, real, or a record, but the

types cannot be mixed. The record itself however, can be any combination of data including other

records if desired, but any file can only have one type of record written to it.

A Writeln statement is illegal when writing to a binary file because a binary file is not line oriented.

A Write statement is limited to one output field per statement. This is not a serious limitation since it isa simple matter to put one Write statement in the program for each variable you wish to write out to

the file. It is important to Close the file when you are finished writing to it.



WHY USE A BINARY FILE

A binary file written by a Pascal program cannot be read by a word processor, a text editor or any other

application program such as a database or spreadsheet, and it may not even be readable by a Pascal

program compiled by a different companies compiler because the actual data structure is

implementation dependent. It can't even be read by a Pascal program using the same compiler unless

the data structure is identical to the one used to write the file. With all these rules, it seems like a silly

way to output data, but there are advantages to using a binary output.

A binary file uses less file space than a corresponding text file because the data is stored in a packed

mode. Since all significant digits of real data are stored, it is more precise unless you are careful to

output all significant data to the corresponding text file. Finally, since the binary data does not require

formatting into ASCII characters, it will be considerably faster than outputting it in text format. When

you run this example program, it will create the file KIBBLES.BIT, and put 20 records in it. Return to

DOS and look for this file and verify its existence. If you try to TYPE it, using the DOS TYPE

command, you will have a real mess on your monitor because it does not contain char type data, but

that might be a good exercise.

READING A BINARY FILE

BININ.PAS is another example program that will read in the file we just created. Notice that the

variables are named differently, but the types are all identical to those used to write the file and they are

in the same order. An additional line is found in the program, the if statement. We must check for the

"end of file" marker to stop reading when we find it or Pascal will list an error and terminate operation.

Three pieces of information are written out to verify that we actually did read the data file in.

Once again, a few rules are in order. A Readln is illegal since there are no lines in a binary file, and

only one variable or record can be read in with each Read statement.


program Binary_Input;

type Input_Record = record

Number : integer;

Amount : real;

Name : string[30];

end;

var Input_File : file of Input_Record;

Bird_Food : array[1..20] of Input_Record;

Index : byte;


Assign(Input_File,'KIBBLES.BIT');

Reset(Input_File);


if not Eof(Input_File) then

Read(Input_File,Bird_Food[Index]);

Close(Input_File);

Writeln(Bird_Food[6].Number:6,Bird_Food[20].Amount:13:5,

' ',Bird_Food[1].Name);



6 12345.67890 Large size Kibbles & Bits

}



FILE POINTERS, GET, AND PUT STATEMENTS

File pointers and the Get and Put procedures are a part of standard Pascal, but since they are redundant

and therefore not needed, they are not a part of TURBO Pascal. The standard Read and Write

procedures are more flexible, more efficient, and easier to use. The use of Get and Put will not be

illustrated or defined here. If you ever have a need for them, they should be covered in detail in your

Pascal reference manual for the particular implementation you are using.

Pointers will be covered in detail in the next chapter of this tutorial.


1. Modify READFILE.PAS so that after reading and displaying the file, the file is reset, then read

and displayed again. This was suggested in the text.

2. Write a program to read the data from any text file, and display it on the monitor with line numbersand the number of characters in each line. Finally display the number of lines found in the file, and

the total number of characters in the entire file. Compare this number with the filesize given by the

DOS command DIR.



Chapter 12 : POINTERS AND DYNAMIC ALLOCATION

THIS IS ADVANCED MATERIAL

For certain types of programs, pointers and dynamic allocation can be a tremendous advantage, butmany programs do not need such a high degree of data structure. For that reason, it would probably be

to your advantage to lightly skim over these topics and come back to them later when you have a

substantial base of Pascal programming experience. It would be good to at least skim over this material

rather than completely neglecting it, so you will have an idea of how pointers and dynamic allocation

work and that they are available for your use when needed.

A complete understanding of this material will require deep concentration as it is complex and not at all

intuitive. Nevertheless, if you pay close attention, you will have a good grasp of pointers and dynamic

allocation in a short time.

WHAT ARE POINTERS, AND WHAT GOOD ARE THEY?

Examine the program named POINT.PAS for your first example of a program using pointers. In the

var declaration you will see two variables named Where and Who that have the symbol ^ in front of

their types. This defines them, not as variables, but as pointers to integer type variables and since they

are pointers, they store the address of data rather than the data itself. Note that three additional pointers

are defined in line 9 which are also pointers defined with a pointer type. The pointer type is defined

earlier in line 4. Either method of definition can be used and gives the same result. Figure 12-1 is a

graphical representation of the data space prior to beginning execution of the program. A box

represents a variable, and a box with a dot in it represents a pointer. In line 12 of the program, the

variable Index is assigned the value of 17 for purposes of illustration. The pointer named Where is

then assigned the address of the variable Index which means that it does not contain the value of 17, it

contains the address of the storage location where the variable Index is stored. In like manner, weassign the address of Index to the pointer named Who. It should be obvious to you that Addr is a

TURBO Pascal function that returns the address of its argument.


program First_Pointer_Example;

type Int_Point = Înteger;

var Index : Integer;

Where : Înteger;

Who : Înteger;

Pt1, Pt2, Pt3 : Int_Point;

begin

Index := 17;Where := Addr(Index);

Who := Addr(Index);

Writeln('The values are ',Index:5,Where^:5,Who^:5);

Where^ := 23;


Pt1 := Addr(Index);

Pt2 := Pt1;

Pt3 := Pt2;

Pt2^ := 15;

Writeln('The Pt values are',Pt1^:5,Pt2^:5,Pt3^:5);

end.




The values are 17 17 17


The Pt values are 15 15 15

}

HOW DO WE USE THE POINTERS?

It should be clear to you that we now have a single variable named Index with two pointers pointing

at it as depicted in figure 12-2. If the pointers are useful, we should be able to do something with them

now, so we simply print out the same variable three different ways in line 15. When we write

"Where^", we are telling the system that we are not interested in the pointer itself, but instead we are

interested in the data to which the pointer points. This is referred to as dereferencing the pointer.

Careful study of the output fields in line 15 will reveal that we first display the value of Index, then

the value to which the pointer Where points, and finally the value to which the pointer Who points.

Since both pointers point to the variable Index, we are essentially displaying the value of Index three

times. You will confirm this when you compile and run this program.

In line 17, we tell the system to assign the value of 23 to the variable to which the pointer Where

points as an illustration, and figure 12-3 pictures the data space following this assignment. If you

understood the discussion in the previous paragraph, you will understand that we are actually assigning

the variable named Index the value of 23 because that is where the pointer named Where is pointing.

In line 18, we once again display the value of the variable Index 3 times just as we did in line 15. It

would be to your advantage to compile and run this program to see that the value of 17 is output three

times, then the value of 23 is output three times because in both cases, we are actually outputting the

same value three times.

In a program as simple as this, the value of pointers is not at all clear, but a simple program is required

in order to make the technique clear. Display the program named POINT.PAS on your monitor againbecause we are not yet finished with it.

A FEW MORE POINTERS

In line 4, we define a new type named Int_Pointwhich is a pointer type to an integer variable. We

use this new type in line 9 to define three more pointers and in line 20, we assign one of them the

address of the variable named Index. Since the pointers are of identical types, we can assign Pt2 the

value of Pt1, as illustrated in line 21. This is actually the address of the variable named Index.

Likewise, the pointer Pt3 is assigned the value of Pt2, and we have all three pointers pointing to the

variable named Index. TURBO Pascal will allow you to assign pointers like this only if they are of the

same type, which these three are. However, since the pointers named Where and Who are declared

individually, they are not of the same type according to the rules of Pascal and if line 14 were changed

in such a way that it read "Who := Where;", a compilation error would occur. The variables are

only assignment compatible if they are declared with the same type name.

Finally, we assign the only variable in this program which is named Index the value of 15 in line 23

and display the value 15 three times as we did above. Compile and run this program again to see that it

does indeed display the value 15 three times. Of course, you could write out the value 15 six times by

using the other two pointers and the variable name Index in addition to the three new pointers.

THIS IS FOR TURBO PASCAL ONLY



Display the program named POINT2.PAS on your monitor for an example of another new extension to

the Pascal programming language by Borland. This program is identical to the last example program

except in lines 13, 14 and 20, where the symbol @ is used to denote the address of the variable Index

rather than the function Addr. This was added to TURBO Pascal as a convenience for you. In ANSI

standard Pascal the @ symbol is used as a synonym for the ^ symbol but Borland chose to use it for a

completely different purpose. Use of this symbol will result in a program that will not compile properly

with any Pascal compiler other than TURBO Pascal.


program TURBO_4_Pointer_Example;

type Int_Point = Înteger;

var Index : Integer;

Where : Înteger;

Who : Înteger;

Pt1, Pt2, Pt3 : Int_Point;

begin

Index := 17;

Where := @Index;

Who := @Index;


Where^ := 23;


Pt1 := @Index;

Pt2 := Pt1;

Pt3 := Pt2;

Pt2^ := 15;

Writeln('The Pt values are',Pt1^:5,Pt2^:5,Pt3^:5);

end.




The Pt values are 15 15 15

}

OUR FIRST LOOK AT DYNAMIC ALLOCATION

If you examine the file named POINTERS.PAS, you will see a very trivial example of pointers and

how they are used with dynamically allocated variables. In the var declaration, you will see that the

two variables have a ^ in front of their respective types once again, defining two pointers. They will be

used to point to dynamically allocated variables that have not yet been defined.

The pointer My_Name is a pointer to a 20 character string. The pointer actually points to an address

somewhere within the computer memory, but we don't know where yet. Actually, there is nothing for it

to point at because we have not defined a variable. After we assign it something to point to, we can use

the pointer to access the data stored at that address.

Your computer has some amount of memory installed in it. If it is an IBM-PC or compatible, it can

have up to 640K of RAM which is addressable by various programs. The operating system requires

about 60K of the total, and the TURBO Pascal run time system requires about 4K to 8K depending on

which version you are using, and what functions you have called. The TURBO Pascal program can use

up to 64K. Adding those three numbers together results in about 128K or 132K. Any memory you have

installed in excess of that is available for the stack and the heap. The stack is a standard area defined

and controlled by DOS that can grow and shrink as needed. Many books are available to define the

stack and its use if you are interested in more information on it.




program Pointer_Use_Example;

type Name = string[20];

var My_Name : ^Name; (* My_Name is a pointer to a string[20] *)

My_Age : înteger; (* My_Age is a pointer to an integer *)

begin

New(My_Name);

New(My_Age);

My_Name^ := 'John Q Doe';

My_Age^ := 27;

Writeln('My name is ',My_Name^);

Writeln('My age is ',My_Age^:3);

Dispose(My_Name);

Dispose(My_Age);

end.


My name is John Q Doe

My age is 27

}

WHAT IS THE HEAP?

The heap is a Pascal defined entity that utilises otherwise unused memory to store data. It beginsimmediately following the program and grows as necessary upward toward the stack which is growing

downward. As long as they never meet, there is no problem. If they meet, a run-time error is generated.

The heap is therefore outside of any memory limitation imposed by TURBO Pascal or any other Pascalcompiler.

Newer versions of TURBO Pascal do not limit us to 64K for the entire program, but there are other

reasons for using the heap in addition to any memory limitation. These should be evident as we learn

how the heap works.

If you did not understand the last few paragraphs, don't worry. Simply remember that dynamically

allocated variables are stored on the heap and do not count in the 64K limitation placed upon you by

some compilers.

Back to our example program, POINTERS.PAS. When we actually begin executing the program, westill have not defined the variables we wish to use to store data in. The first executable statement in line

10 generates a variable for us with no name and stores it on the heap. Since it has no name, we cannot

do anything with it, except for the fact that we do have a pointer My_Name that is pointing to it. By

using the pointer, we can store up to 20 characters in it, because that is its type, and later go back and

retrieve the 20 characters.

WHAT IS DYNAMIC ALLOCATION?

The variable we have just described is a dynamically allocated variable because it was not defined in a

var declaration, but with a New procedure. The New procedure creates a variable of the type definedby the pointer, puts the variable on the heap, and finally assigns the address of the variable to the

pointer itself. Thus My_Name contains the address of the variable generated. The variable itself is



referenced by using the pointer to it followed by a ^, just like in the last program, and is read, "the

variable to which the pointer points".

The statement in line 11 assigns a place on the heap to an integer type variable and puts its address in

My_Age. The data space can now be pictured as in figure 12-5. Note that we have the data locations

defined but there is no data stored in the locations yet.

Following the New statements we have two assignment statements in which the two variables pointed

at are assigned values compatible with their respective types, and they are both written out to the video

display in much the same manner as we did in the program named POINT.PAS.

Following execution of lines 13 and 14, the data space is configured as illustrated in figure 12-6. Lines

16 and 17 illustrate that the dynamically allocated data can be used in the same manner as any data

provided the "carat" is used with the variable name.

GETTING RID OF DYNAMICALLY ALLOCATED DATA

The two statements in lines 19 and 20 are illustrations of the way the dynamically allocated variablesare removed from use. When they are no longer needed, they are disposed of with the Dispose

procedure which frees up their space on the heap so it can be reused.

In such a simple program, pointers cannot be appreciated, but it is necessary for a simple illustration. In

a large, very active program, it is possible to define many variables, dispose of some of them, define

more, and dispose of more, etc. Each time some variables are disposed of, their space is then made

available for additional variables defined with the New procedure.

The heap can be made up of any assortment of variables, they are not required to be of one type. One

point must be kept in mind however. Anytime a variable is defined, it will have a pointer pointing to it.

The pointer is the only means by which the variable can be accessed. If the pointer to the variable is

lost or changed, the data itself is lost for all practical purposes. This will be illustrated in a later

example program in this chapter.

Compile and run this program and examine the output. If you do not understand how this program

works, review it carefully before going on to the next example program.

DYNAMICALLY STORING RECORDS

The next example program, DYNREC.PAS, is a repeat of one we studied in an earlier chapter. For

your own edification, review the example program BIGREC.PAS before going ahead in this chapter.

Assuming that you are back in DYNREC.PAS, you will notice that this program looks very similar to

the earlier one, and in fact they do exactly the same thing. The only difference in the type declaration is

the addition of a pointer Person_Id, and in the var declaration, the first four variables are defined

as pointers here, and were defined as record variables in the last program.

A point should be made here. Pointers are not generally used in very small programs. This example

program is a good bit larger than the last few programs, and should be a clue to you as to why such a

trivial program was used to introduce pointers in this tutorial. A very small, concise program can

illustrate a topic much better that an large complex program, but we must go on to more useful

constructs of any new topic. This of course, requires more elaborate programs.




program A_Dynamic_Storage_Record;

const Number_Of_Friends = 50;

type Full_Name = record


Initial : char;


end;

Date = record

Day : byte;

Month : byte;

Year : integer;

end;

Person_Id = ^Person;

Person = record

Name : Full_Name;

City : string[15];

State : string[2];

Zipcode : string[5];

Birthday : Date;end;

var Friend : array[1..Number_Of_Friends] of Person_Id;

Self,Mother,Father : Person_Id;

Temp : Person;

Index : byte;


New(Self); (* create the dynamic variable *)

Self^.Name.First_Name := 'Charley';

Self^.Name.Initial := 'Z';

Self^.Name.Last_Name := 'Brown';

with Self^ do begin

City := 'Anywhere';

State := 'CA';

Zipcode := '97342';Birthday.Day := 17;

with Birthday do begin

Month := 7;

Year := 1938;

end;

end; (* all data for self now defined *)

New(Mother);

Mother := Self;

New(Father);

Father^ := Mother^;

for Index := 1 to Number_Of_Friends do begin

New(Friend[Index]);

Friend[Index]^ := Mother^;

end;

Temp := Friend[27]^;

Write(Temp.Name.First_Name,' ');

Temp := Friend[33]^;

Write(Temp.Name.Initial,' ');

Temp := Father^;

Write(Temp.Name.Last_Name);

Writeln;

Dispose(Self);

{ Dispose(Mother); } (* since Mother is lost, it cannot

be disposed of *)

Dispose(Father);

for Index := 1 to Number_Of_Friends do

Dispose(Friend[Index]);





Charley Z Brown

}

WE JUST BROKE THE GREAT RULE OF PASCAL

The observant student will notice that, in the type declaration we used the identifier Person in line 18

before we defined it in line 19, which is illegal in Pascal. Foreseeing the need to define a pointer prior

to the record, the designers of Pascal allow us to break the rule in this one place. The pointer could

have been defined after the record in this particular case, but it was more convenient to put it before,

and in the next example program, it will be required to put it before the record. We will get there soon.

Since Friend is an array of 50 pointers, we have now defined 53 different pointers to records, but so

far have defined no variables other than Temp and Index. We immediately use the New procedure to

dynamically allocate a record with Self pointing to it, and use the pointer so defined to fill the

dynamically allocated record. Compare this to the program named BIGREC.PAS and you will see thatit is identical except for the addition of the New and adding the ^ to each use of the pointer to designate

the data pointed to.

THIS IS A TRICK, BE CAREFUL

Now go down to line 48 where Mother is allocated a record and is, by definition, pointing to the

record. It seems an easy thing to do then to simply assign all of the values of Self to all the values of

Mother as shown in the next statement, but it doesn't work. The only thing the statement does is make

the pointer Mother point to the same place where Self is pointing because we did a pointer

assignment. The data that was allocated to the pointer Mother is now somewhere on the heap, but we

don't know where, and we cannot find it, use it, or deallocate it since we have lost the reference to it.This is an example of losing data on the heap.

The proper way to assign data from one record to another is given in lines 50 and 51 where all fields of

Father are defined by all fields of Mother which is pointing at the original Self record. Note that

since Mother and Self are both pointing at the same record, if we changed the data with either

pointer, the data appears to be changed in both because there is, in fact, only one record where this data

is stored.

In order to Write from or Read into a dynamically assigned record it is necessary to use a temporary

record since dynamically assigned records are not allowed to be used in I/O statements. This is

illustrated in lines 57 through 63 of the program where some data is written to the monitor.

Finally, the dynamically allocated variables are disposed of prior to ending the program. For a simpleprogram such as this, it is not necessary to dispose of them because all dynamic variables are disposed

of automatically when the program is terminated and we return to DOS or the TURBO Pascal

integrated environment. Notice that if the "Dispose(Mother);" statement was included in the

program, the data could not be found due to the lost pointer, and the program would be unpredictable,possibly even resulting in a system crash.

It would be a meaningful exercise for you to diagram the data space for this program at a few selected

points in its execution. This should be done in a manner similar to that done in figure 12-1 to figure 12-

5 of this chapter.



WHAT GOOD IS THIS ANYWAY?

Remember when you were initially studying BIGREC.PAS? I suggested that you see how big you

could make the constant Number_Of_Friendsbefore you ran out of memory. At that time we

found that it could be made slightly greater than 1000 before we got the memory overflow message at

compilation. Try the same thing with DYNREC.PAS to see how many records it can handle,

remembering that the records are created dynamically, so you will have to run the program to actually

run out of memory. The final result will depend on how much memory you have installed, and how

many memory resident programs you are using. If you have a full memory of 640K, I would suggestyou start somewhere in the neighbourhood of 8000 records of Friend.

Now you should have a good idea of why dynamic allocation can be used to greatly increase the

usefulness of your programs. There is, however, one more important topic we must cover on dynamic

allocation. That is the linked list.

WHAT IS A LINKED LIST?

Understanding and using a linked list is by far the most baffling topic you will confront in Pascal.Many people simply throw up their hands and never try to use a linked list. I will try to help you

understand it by use of an example and lots of explanation. Examine the program named

LINKLIST.PAS for an example of a linked list. I tried to keep it short so you could see the entireoperation and yet do something meaningful.

To begin with, notice that there are two types defined in lines 4 and 6, a pointer to the record and the

record itself. The record, however, has one thing about it that is new to us, the last entry, Next, is a

pointer to a record of this type. This record then, has the ability to point to itself, which would be trivial

and meaningless, or to another record of the same type which will be extremely useful in this case. Infact, this is the way a linked list is used. I must point out, that the pointer to another record, in this case

called Next, does not have to be last in the list, it can be anywhere in the list that is convenient for

you.

A couple of pages ago, we discussed the fact that we had to break the great rule of Pascal and use an

identifier before it was defined. This is the reason the exception to the rule was allowed. Since the

pointer points to the record, and the record contains a reference to the pointer, one has to be defined

after being used, and by rules of Pascal, the pointer can be defined first, provided that the record is

defined immediately following it. That is a mouthful but if you just use the syntax shown in the

example, you will not get into trouble with it.




program Linked_List_Example;

type Next_Pointer = ^Full_Name;

Full_Name = record


Initial : char;


Next : Next_Pointer;

end;

var Start_Of_List : Next_Pointer;

Place_In_List : Next_Pointer;

Temp_Place : Next_Pointer;

Index : integer;


(* generate the first name in the list *)

New(Place_In_List);

Start_Of_List := Place_In_List;

Place_In_List^.First_Name := 'John';

Place_In_List^.Initial := 'Q';

Place_In_List^.Last_Name := 'Doe';Place_In_List^.Next := nil;

(* generate another name in the list *)

Temp_Place := Place_In_List;

New(Place_In_List);

Temp_Place^.Next := Place_In_List;

Place_In_List^.First_Name := 'Mary';

Place_In_List^.Initial := 'R';

Place_In_List^.Last_Name := 'Johnson';

Place_In_List^.Next := nil;

(* add 10 more names to complete the list *)



New(Place_In_List);

Temp_Place^.Next := Place_In_List;

Place_In_List^.First_Name := 'William';

Place_In_List^.Initial := 'S';Place_In_List^.Last_Name := 'Jones';

Place_In_List^.Next := nil;

end;

(* display the list on the video monitor *)

Place_In_List := Start_Of_List;

repeat

Write(Place_In_List^.First_Name);

Write(' ',Place_In_List^.Initial);

Writeln(' ',Place_In_List^.Last_Name);


Place_In_List := Place_In_List^.Next;

until Temp_Place^.Next = nil;



John Q Doe

Mary R Johnson

William S Jones

William S Jones

William S Jones

William S Jones

William S Jones

William S Jones

William S Jones

William S Jones

William S Jones

William S Jones

}



STILL NO VARIABLES?

It may seem strange, but we still have no variables defined, except for our old friend Index. In fact,

for this example, we will only define 3 pointers. In the last example we defined 54 pointers, and had

lots of storage room. Before we are finished, we will have at least a dozen pointers but they will be

stored in our records, so they too will be dynamically allocated.

Let's look at the program itself now. In line 20, we create a dynamically allocated record and define it

by the pointer named Place_In_List. It is composed of the three data fields, and another pointer.

We define Start_Of_List to point to the first record created, and we will leave it unchanged

throughout the program. The pointer Start_Of_Listwill always point to the first record in the

linked list which we are building up. The data space is as depicted in figure 12-7.

WHAT IS "nil" AND WHAT IS IT USED FOR?

We define the three variables in the record to be any name we desire for illustrative purposes, and setthe pointer in the record to nil. The word nil is another reserved word that doesn't give the pointer an

address but defines it as empty. A pointer that is currently nil cannot be used to manipulate databecause it has no value, but it can be tested in a logical statement to see if it is nil. It is therefore a

dummy assignment. With all of that, the first record is completely defined.

DEFINING THE SECOND RECORD

When you were young you may have played a searching game in which you were given a clue telling

you where to find the next clue. The next clue had a clue to the location of the third clue. You kept

going from clue to clue until you found the prize. You simply exercised a linked list. We will now

build up the same kind of a list in which each record will tell us where the next record can be found.

In lines 27 through 33 we will define the second record. Our goal will be to store a pointer to thesecond record in the pointer field of the first record. In order to keep track of the last record, the one in

which we need to update the pointer, we will keep a pointer to it in Temp_Place. Now we can

dynamically allocate another new record and use Place_In_List to point to it. Since

Temp_Place is now pointing at the first record, we can use it to store the value of the pointer which

points to the new record which we do in line 29. The 3 data fields of the new record are assignednonsense data for our illustration, and the pointer field of the new record is assigned nil. We have

reached the point when the data space is as depicted in figure 12-8.

Let's review our progress to this point. We now have the first record with a person's name and a pointer

to the second record, and a second record containing a different person's name and a pointer assignednil. We also have three pointers, one pointing to the first record, one pointing to the last record, and one

we used just to get here since it is only a temporary pointer. If you understand what is happening so far,

let's go on to add some additional records to the list. If you are confused, go back over this materialagain.

TEN MORE RECORDS

The next section of code is contained within a for loop so the statements are simply repeated ten times.

If you observe carefully, you will notice that the statements are identical to the second group of

statements in the program (except of course for the name assigned). They operate in exactly the same

manner, and we end up with ten more names added to the list. You will now see why the temporary

pointer was necessary, but pointers use little memory and are therefore relatively cheap, so feel free to

use them at will. A pointer generally uses only 4 bytes of memory.



FINALLY, A COMPLETE LINKED LIST

We now have generated a linked list of twelve entries. We have a pointer pointing at the first entry, and

another pointer pointing at the last. The only data stored within the program itself are three pointers,

and one integer, all of the data is on the heap. This is one advantage to a linked list, it uses very little

local memory, but it is costly in terms of programming. (Keep in mind that all of the data must be

stored somewhere in memory, and in the case of the linked list, it is stored on the heap.) You should

never use a linked list simply to save memory, but only because a certain program lends itself well to it.

Some sorting routines are extremely fast because of using a linked list, and it could be advantageous to

use in a database.

HOW DO WE GET TO THE DATA NOW?

Since the data is in a list, how can we get a copy of the fourth entry for example? The only way is to

start at the beginning of the list and successively examine pointers until you reach the desired one.

Suppose you are at the fourth and then wish to examine the third. You cannot back up, because youdidn't define the list that way, you can only start at the beginning and count to the third. You could have

defined the record with two pointers, one pointing forward, and one pointing backward. This would bea doubly-linked list and you could then go directly from entry four to entry three.

Now that the list is defined, we will read the data from the list and display it on the video monitor. We

begin by defining the pointer, Place_In_List, as the start of the list. Now you see why it was

important to keep a copy of where the list started. In the same manner as filling the list, we go from

record to record until we find the record with the value nil stored in its pointer.

There are entire books on how to use linked lists, and most Pascal programmers will seldom, if ever,

use them. For this reason, additional detail is considered unnecessary, but to be a fully informed Pascalprogrammer, some insight is necessary.


1. Write a program to store a few names dynamically, then display the stored names on the monitor. As

your first exercise in dynamic allocation, keep it very simple.



Chapter 13 : UNITS IN TURBO PASCAL

When Nicklaus Wirth originally defined Pascal, it was intended to be a very small language to be used

primarily for teaching programming concepts to computer neophytes. A program would be contained in

a single file and compiled in its entirety each time it was compiled. There was no provision for splittinga program up into smaller parts, compiling each part separately, and linking all of the parts together

into a final completed package.

Since human beings make mistakes, and because the entire program must be recompiled each time any

mistake is discovered, pure Pascal is unsuitable for very large programs. Seeing this problem, manycompiler writers have defined some method by which a large program could be broken down into

smaller parts and separately compiled.

This chapter will define and illustrate the way Borland International has chosen to allow TURBO

Pascal to be broken up into smaller pieces to permit compilation of smaller portions of a program. This

allows you to write a much larger program since it does not have to be compiled all at once.

PART OF A PROGRAM

Load the program named AREAS.PAS and display it on your monitor. This is the first example of a

TURBO Pascal unit and although it is similar to a program in many ways, it has a few differences

which must be pointed out. We will start by pointing out the major sections, then get into the details of each section.

You will first notice that this program begins with the reserved word unit instead of our usual program,

followed by the unit name, Areas. In line 10, the reserved word interface is used and all of the

statements following it down to the next reserved word implementation, are part of the interface with

any program outside of this unit. The reserved word, implementation, defines the beginning of the

definitions and executable parts of the private portion of the unit.

Finally, in lines 48 through 50, we find what appears to be a program block just like we have been

using all through this tutorial, but actually is not. We will see in a few paragraphs that this is the

initialisation section and does a very specific job for us even though somewhat different than what we

have become used to.




unit Areas;

(*****************************************************************)

(* *)

(* This unit includes a collection of functions to calculate the *)

(* areas of four different geometric shapes. *)

(* *)

(*****************************************************************)

interface

function Area_Of_Circle(Radius : real ) : real;

function Area_Of_Square(Length_Of_Side : real) : real;

function Area_Of_Rectangle(Length,Width : real) : real;

function Area_Of_Triangle(Base, Height : real) : real;

implementation

var My_Pi : real;

procedure Mult_Two_Numbers(Number1, Number2 : real;

var Result : real);

begin

Result := Number1 * Number2;

end;

function Area_Of_Circle;

var Rad_Squared : real;

begin

Mult_Two_Numbers(Radius,Radius,Rad_Squared);

Area_Of_Circle := Rad_Squared * My_Pi;

end;

function Area_Of_Square;

begin

Area_Of_Square := Length_Of_Side * Length_Of_Side;

end;

function Area_Of_Rectangle;

begin

Area_Of_Rectangle := Length * Width;end;

function Area_Of_Triangle;

begin

Area_Of_Triangle := Base * Height / 2.0;

end;

begin (* This is the initialization code *)

My_Pi := 3.14159267;

end. (* of unit Areas *)


(A unit cannot be executed)

}

THE INTERFACE PART

Following the unit name we have a section of code in lines 10 through 15 that define the interface of

this module to the outside world. Anything defined here is available to the outside world and can be

used by any other program provided it has a "uses Areas;" statement in it. Constants, types, and

variables could also be defined here, and if they were, they too would be available to any user program,

but in this case, only the four functions are made available. It should be fairly obvious that the

functions calculate the areas of four different geometric shapes.



These four functions are available for use in any program in much the same way that any of the

standard Pascal functions are available for use. The only difference is that a uses clause must beincluded in order to use these functions.

THE IMPLEMENTATION PART

From line 16 through line 47 we have the implementation part as delineated by the reserved word

implementation and the beginning of the initialisation block. The implementation part is the actual

workhorse of the unit since it contains all of the executable code for the four functions defined above.

Lines 26 through 31 contain the code needed to generate the area of a circle, and this code is no

different than the code that would be used if this function were placed in the declaration part of any

Pascal program. There is a difference in the function header since the formal parameters are not

repeated here, but are defined only in the interface part in this example. TURBO Pascal allows you to

either drop the formal parameters here or include them if you think the code would be more readable. If

you include them, they must be exactly as shown in the interface part or you will get a compile error.

A LOCAL PROCEDURE

In lines 20 through 24, we have a procedure that is used within one of the four functions, namely the

first. It is really a stupid procedure since it really wastes time setting up linkage for the procedure call

and does nothing that couldn't be done just as easy with a simple multiply, but it does illustrate that you

can use another procedure within the unit body. The procedure Mult_Two_Numbers cannot be used

outside of this unit because it is not included in the interface part of the unit. It is, in effect, invisible to

the outside world.

The variable My_Pi would be more correctly represented as a constant but it is defined as a variable to

illustrate the use of the body of the unit later. Since My_Pi is not defined in the interface part of the

unit, it also is invisible to the outside world and in fact protected from accidental corruption by a

misplaced statement in another program. The procedure Mult_Two_Numbers and the variable

My_Pi for all practical purposes have an impenetrable barrier around them protecting them from

unauthorised use or modification by the outside world, but the functions internal to this unit have free

access to them just as in any other program.

WHAT IS THE BODY USED FOR?

Lines 48 through 50 constitute the body of the unit and although they appear to consist of another

executable program that can be called and used, they actually perform another very specific and useful

purpose. This is actually an initialisation section and all of the statements in this part of the unit areexecuted once and only once, and they are executed when the main program is loaded. This is done

automatically for you by the system. There is no way provided for you to call the statements in thebody after the program has begun execution. This is why the variable My_Pi was defined as a variable,

so we could use this section to initialise it to a useful value as an illustration.

The body can actually have function and procedure calls that are executed when the program is loaded,

as well as loops or conditional statements.

If you would like to execute some statements during initialisation and again during the execution of the

program one or more times, you can write a procedure or function to accomplish your desires and call

it at the appropriate times in the main program.

SELECTIVE NAMING OF FUNCTIONS AND PROCEDURES



If you will study the interface part of this unit you will find that everything you need to use this unit is

contained within it, provided that you know enough about plane geometry to understand the functions.You should strive for this understanding in all of your interfaces so that the implementation doesn't

even require consultation. Keep in mind, that if you need to, you can include comments to further

define the functions in the interface part of the unit.

At this time, you should compile this unit. You will have to compile it to disk rather than only tomemory so it will be available for use later in this chapter. You do this by using the menus to changethe Compile/Destination to the Disk option. Note that it will not generate an .EXE file but instead a

.TPU file. This is Borland's filename extension for a unit.

ANOTHER UNIT

Load the file named PERIMS.PAS for another example of a unit. This is similar to the last except that

it does not contain an internal procedure, and it is composed of three procedures that calculate the

perimeters of geometric shapes, all of which are visible to the outside world because they are included

in the interface part of the unit. Once again, we have a private variable named My_Pi and a block of

code (actually a single statement) to initialise the value of My_Pi when the unit is loaded.


unit Perims;

(*****************************************************************)

(* *)

(* This unit contains three procedures to calculate perimieters *)

(* of three geometric shapes. *)

(* *)

(*****************************************************************)

interface

procedure Perimeter_Of_Circle(Radius : real;

var Perimeter : real );

procedure Perimeter_Of_Square(Length_Of_Side : real;

var Perimeter : real);

procedure Perimeter_Of_Rectangle(Length,Width : real;var Perimeter : real);

implementation

var My_Pi : real;

procedure Perimeter_Of_Circle;

begin

Perimeter := My_Pi * 2.0 * Radius;

end;

procedure Perimeter_Of_Square;

begin

Perimeter := 4.0 * Length_Of_Side;

end;

procedure Perimeter_Of_Rectangle;

begin

Perimeter := 2.0 * (Length + Width);

end;

begin (* This is the initialization code *)

My_Pi := 3.14159267;

end. (* of unit Perimeters *)


(A unit cannot be executed)

}



Be sure you compile this unit to disk in the same manner as the last and they will be ready for use. Note

that it is not a requirement that a unit be composed of only functions or only procedures. They can be

freely mixed in a unit. It was only done this way in this example because it was convenient.

Now that we have several functions and procedures that can be used to calculate the areas or perimetersof several different shapes, we need a program to illustrate their use, so if you load and display theprogram named GARDEN.PAS you will have an example of their use.

HOW DO WE USE OUR DEFINED UNITS?

GARDEN.PAS is a very simple program that uses one of the functions and one of the procedures. The

only thing you must do is add the names of the units prior to using the external functions or procedures.

Lines 16 and 17 each use one of our newly defined routines. As you can see, there is nothing magic

about the new routines, and once you include the unit names in a uses statement, the new routines are

in a sense, an extension to the Pascal language. Compile and run this program and see that it really does

what you expect it to do.


program Garden;

(*****************************************************************)

(* *)

(* This program calculates how much area there is to plow in a *)

(* circular garden, and how long the wall will be around it. *)

(* *)

(*****************************************************************)

uses Areas, Perims;

var Radius, Area, Circumference : real;

begin

Radius := 12.0;

Area := Area_Of_Circle(Radius);

Perimeter_Of_Circle(Radius,Circumference);

(* Calculations complete, output results *)

Writeln('The radius of the garden is ',Radius:6:1,' feet.');

Writeln('The area to plow is ',Area:6:1,' square feet.');

Writeln('A wall around the garden will be ',Circumference:6:1,

' feet long.');

Writeln;

end.


The radius of the garden is 12.0 feet.

The area to plow is 452.4 square feet.

A wall around the garden will be 75.4 feet long.

}

ONE MORE EXAMPLE OF UNIT USE

Load and display the program named SHAPES.PAS for another example of using a predefined unit. In

line 3, this program includes our new unit named Areas so all four of the area functions are available,

and in fact, all four are used within the body of the program. This program should not be difficult for

you to understand and you will be left to study it on your own. The supplied unit named Crt is also

included in the uses clause to allow the use of the keyboard subprograms in lines 15 and 16.




program Calculate_Area_Of_Shapes;

uses Areas,Crt;

var In_Char : char;

Length,Width,Height,Base,Radius : real;


repeat

Writeln;

Writeln('Please input the first letter of the selection');

Writeln('Select shape; Square Rectangle Triangle Circle Quit');

Write('Requested shape is ');

Repeat until Keypressed;

In_Char := ReadKey;

case UpCase(In_Char) of

'S' : begin

Write('Square Enter length of side ');

Readln(Length);

Writeln('The area is ',Area_Of_Square(Length):12:4);

end;

'R' : beginWrite('Rectangle Enter width ');

Readln(Width);

Write('Enter height ');

Read(Height);

Writeln(' The area is ',

Area_Of_Rectangle(Width,Height):12:4);

end;

'T' : begin

Write('Triangle Enter base ');

Readln(Base);

Write('Enter height ');

Read(Height);

Writeln(' The area is ',

Area_Of_Triangle(Base,Height):12:3);

end;

'C' : begin

Write('Circle Enter radius ');

Readln(Radius);

Writeln('The area is ',Area_Of_Circle(Radius):12:3);

end;

'Q' : Writeln('Quit');

else Writeln(' undefined entry');

end;

until (In_Char = 'Q') or (In_Char = 'q');



(The output depends on the data entered at the keyboard)

}

MULTIPLE USES OF AN IDENTIFIER

Suppose we wanted to move the variable named My_Pi to the interface section in both of the units we

defined earlier. Then in the program named GARDEN.PAS when we included both of the units in the

uses statement, both variables named My_Pi would be available for use so we would have a bit of a

problem defining which one we really meant to use. TURBO Pascal has a way to tell the system whichone you wish to use by using a qualifier in much the same way that you use a field of a record. The



variable name Areas.My_Piwould refer to that variable from the unit named Areas, and the name

Perims.My_Pi would refer to the variable from the unit named Perims.

You could even define a new variable of the same name in your main program and refer to it by the

qualified name Garden.My_Pi if you chose to. This is not recommended as it would get very

confusing to you. The compiler would be very happy to compile and run such a program, because it

would not get confused.

It is not illustrated in the example program, but this technique applies to procedure and function names

as well. If you used the same procedure name in two different units, you could specify which procedure

you intend to use by using the dot notation with the unit name and the procedure name.

Unit_Name.Procedure_Name would therefore refer to the procedure named Procedure_Name

that is a part of the unit named Unit_Name.

WHY USE UNITS?

There are basically three reasons to use units in your programming. First, some programs are so large

that they should be broken up into smaller chunks for ease of handling and reasonable compilation size.In fact some are so large that they cannot be compiled all at one time since TURBO Pascal has an

upper limit of 64K of code which can be compiled at once. Most other Pascal compilers have a similarlimit also.

Secondly, once you complete the code to perform a certain job, you may wish to use the same code in

another program to do the same job. If you put the code in a unit, it is ready to simply call and use

again in the same manner that we reused Areas in SHAPES.PAS. This is becoming a rather important

topic in software engineering usually referred to as "Reusable Software".

THIS IS INFORMATION HIDING

Finally, it is sometimes important to hide a portion of code from the rest of the program to assure that it

cannot be unduly modified by an error somewhere else in the program. This too is becoming an

important area of software engineering and is usually referred to as information hiding.


1. Move My_Pi to the interface in both units and change one of the values slightly to see if you can

read in the right one at the right time. Define another variable of the same name in your main

program and see if you can differentiate between all three values. Note that, due to the nature of

this exercise, no answer is given for it on the distribution disk.



Chapter 14 : ENCAPSULATION & INHERITANCE

Encapsulation is the cornerstone upon which object oriented programming is built, and without which

it would not exist. We will cover the topic of encapsulation in this chapter in enough depth to illustrate

its use and what it can do for you in software development. Because there are many new terms in thischapter, you could very easily become intimidated, and wish to simply give up on this new topic. You

can be assured that the time spent studying encapsulation will be greatly rewarded as you apply this

new technique in your software development efforts.

Object oriented programming is not a panacea to solve all of your software problems, but it is a newand improved way of programming. In fact it is really more of a software packaging technology than a

new method of programming. You will find that your software will be easier to write and debug as you

gain experience using this new packaging method. Like any new endeavour however, it will require

some effort on your part to master these concepts.

If you are using an older version of TURBO Pascal you will not be able to compile and execute theexample programs in this chapter. Versions 5.5 and newer can be used, and as mentioned earlier, it

would be worth your effort to upgrade to a newer version so you can learn these techniques to improvethe quality of your programs.

OUR FIRST ENCAPSULATION

The example program named ENCAP1.PAS contains our first example of encapsulation. In order to

keep it easy to understand, it was kept very short. This results in a program that does not illustrate the

advantage of using object oriented programming, but it does give us a start in the right direction. With

this in mind, load ENCAP1.PAS and we will study the code contained in it.

Line 5 has our first new reserved word, object. This is used in much the same way that the reservedword record is used, but it has a much different meaning. An object is permitted to have not only data

embedded within it, but also procedures, functions, and constructors. Constructors will be described in

detail later. Since data plus procedures and functions can be grouped together in this fashion, the object

is said to be encapsulated. An object is therefore a group of related data and the subprograms that

operate on that data, all entities being very closely coupled together.




program Encapsulation_1;

type

Box = object

length : integer;

width : integer;

constructor Init(len, wid : integer);

procedure Set_Data(len, wid : integer);

function Get_Area : integer;

end;

constructor Box.Init(len, wid : integer);

begin

length := len;

width := wid;

end;

procedure Box.Set_Data(len, wid : integer);

begin

length := len;

width := wid;

end;

function Box.Get_Area : integer;

begin

Get_Area := length * width;

end;

var Small, Medium, Large : Box;

begin

Small.Init(8,8);

Medium.Init(10,12);

Large.Init(15,20);

WriteLn('The area of the small box is ',Small.Get_Area);

WriteLn('The area of the medium box is ',Medium.Get_Area);

WriteLn('The area of the large box is ',Large.Get_Area);

end.


The area of the small box is 64

The area of the medium box is 120

The area of the large box is 300

}

WHAT IS A METHOD?

A method is a term used with object oriented programming, and for the time being we will simply say

that a method is either a function or a procedure (including a constructor). A method is therefore a

method for doing an operation on some data. Lines 8 through 10 are method headers and give thepattern for all calls to these methods which can be used by the compiler to check for the correct number

and types of parameters.

Once again, we promise to discuss the constructor soon. For the time being, simply think of it as

another procedure.

The entire object type definition is given in lines 5 through 11. This object contains two variables

named length and width, each of type integer, and three methods which can be used to operate on the

two variables. In the same manner that the definition of a type in Pascal does not actually give you a



variable to use, only a pattern, the definition of an object type does not give you an object. We will

declare the objects when we get to line 30 of this program.

You will note that we are already using new terminology, but this is necessary. The field of object

oriented programming has its own vocabulary and in order for you to understand technical articles in

this field, you must begin now to learn the new terminology. It won't be too long until you feel

somewhat comfortable with it.

THE METHOD IMPLEMENTATION

The object type definition describes in detail what we can do with the object but we must now describe

what actions will take place when each of the methods is called. The implementation for each method

will define the operations for that method and are defined in lines 13 through 28 of this example

program. The only thing that is really different about these methods is the way their headers are

defined, namely the inclusion, in the header, of the object type name Box dotted to the method name.

This is required, but we will wait until the next example program to define why it is needed.

The observant student will also notice that we are referring to the object variables within the methods

of that object without the object name dotted to the variable name. This is because the implied object

name is automatically "with"ed to the variable names within the method implementations, allowing

the variables to be directly referred to within the objects because of the definition of object oriented

programming. It should be obvious that any mathematics or logical operations can be done within the

implementations of the methods. In fact, you can perform any legal Pascal operations within the

methods, just like you can in any

Pascal function or procedure. Very short operations were selected here because we wish to illustrate the

interfaces to the methods at this point in the tutorial.

AN INSTANCE OF AN OBJECT

We need another new term at this point. When we use the object type to declare variables of that type

as we do in line 30, we are creating instances of that object type. An instance is like a variable in

conventional Pascal (non object oriented), except that it can do more and has some very interestingproperties that a simple variable does not have. In line 30 we have created three instances of the object

type named Box and each has two simple variables associated with it. Three methods are available

which can be called to operate on these variables. We therefore have three objects named Small,

Medium, and Large. In order to initialise the values stored within the objects we call the three objects

in lines 34 through 36 to store values in their internal variables by dotting the name of the object to the

name of the method we wish to call. You will note that this looks like the same technique we use torefer to the fields of a record. We display the area of the three boxes in lines 38 through 40 using the

same technique used to initialise the values stored, and the program is complete.

We seem to have accomplished very little with this program that we could not have more easily

accomplished with an even shorter standard Pascal program, and that is true. This program is only

meant to introduce some of the mechanics of object oriented programming. Additional programs will

be used to illustrate some of the uses of this new technique.

NEW TERMINOLOGY

You may note that we switched terminology halfway through the above paragraphs. We began by

referring to the object types as object types and calling the variables declared in line 30 instances. Laterwe began calling the instances objects. In this tutorial we will refer to the types as object types and the

variables either as objects or instances. This terminology is consistent with current practice and should

help you learn the new terminology.



Another very important point is the fact that we pass a message to a method rather than call a

subprogram as in conventional Pascal. The difference is rather subtle, but there really is a difference aswe will see a little later in this tutorial.

WHAT DID WE ACCOMPLISH?

In this program we defined an object type, then declared several instances of that type, one of which

was named Small. The object named Small has two internal variables that should only be accessed via

its methods, so we will refer to them as private data points. Actually, they should be unavailable to any

user outside of the method implementations but Borland chose not to make them private in version 5.5.

It is up to you to discipline yourself to not refer to them directly. (TURBO Pascal version 6.0 has a way

to make the data private. We will study this in the next example program.) The proper way to use the

object is to send a message to the object telling it to do something to itself. In the case of the Init

method, we are telling it to store the two values in its private variables named length and width, and in

the case of the Get_Area method, we are telling it to give us the product of its own internally stored

width and length which we can then print out.

Remember that in the beginning of this chapter we said that object oriented programming is a codepackaging technique. That should help to explain some of the strange things we did in this program. As

we continue through the example programs, we will see that everything here was done for a reason andyou will eventually learn to use and prefer object oriented programming methods over the old familiar

procedural programming method you have been using.

Be sure to compile and execute this program to see if it does what the comments say it will do.

DATA & CODE PROTECTION

The data and methods are protected from outside influence because they are packaged together within

an object. Of even more importance is the fact that because they were to be packaged together, they

were probably carefully planned and designed together during the design stage. This would probablyresult in a more understandable program. The object keeps the data and methods together and keeps

them working in close synchronisation.

An object type is sometimes referred to as an abstract data type in the technical literature discussing

object oriented programming.

MORE ENCAPSULATION

The example program named ENCAP2.PAS uses most of the same techniques as the last program but

this is much more meaningful since it illustrates one of the simplest advantages of using object oriented

programming.

In this program, we define two object types in lines 5 through 21, Box and Pole. Each has its own

unique kinds of variables associated with it, and each has three methods that can be used with these

kinds of data. The method definitions in lines 35 and 46 clearly illustrate why the object name must be

associated with the method name in the method implementation. This allows you to use the same

method name in more than one object definition. In addition to the two method definitions named

Set_Data given here, we could also define and use another procedure with the name Set_Data that

was a normal Pascal procedure just like any others we have used in prior chapters of this tutorial. Using

the same method name in several places is often referred to as name overloading in object oriented

programming terminology.

You will note that in lines 5 through 21 we define the object types which define what each object will

do. In lines 23 through 55 we define the method implementations which define how we do it. It is



assumed that you know enough Pascal at this point to understand what each method does, so nothing

more will be said about the details of this program except for the private types.


program Encapsulation_2;

type

Box = objectconstructor Init(len, wid : integer);

procedure Set_Data(len, wid : integer);

function Get_Area : integer;

private (* Remove this if you are using TURBO Pascal 5.5, *)

length : integer; (* and move both variables *)

width : integer; (* ahead of the methods. *)

end;

Pole = object

constructor Init(hei, dep : integer);

procedure Set_Data(hei, dep : integer);

function Get_Total_Length : integer;

private (* Remove this if you are using TURBO Pascal 5.5, *)

height : integer; (* and move both variables *)

depth : integer; (* ahead of the methods. *)

end;

constructor Box.Init(len, wid : integer);

begin

length := len;

width := wid;

end;

constructor Pole.Init(hei, dep : integer);

begin

height := hei;

depth := dep;

end;

procedure Box.Set_Data(len, wid : integer);

begin

length := len;

width := wid;end;

function Box.Get_Area : integer;

begin

Get_Area := length * width;

end;

procedure Pole.Set_Data(hei, dep : integer);

begin

height := hei;

depth := dep;

end;

function Pole.Get_Total_length : integer;

begin

Get_Total_length := height + depth;end;

var Small, Medium, Large : Box;

Short, Average, Tall : Pole;

begin

Small.Init(8,8);

Medium.Init(10,12);

Large.Init(15,20);

Short.Init(8,2);

Average.Init(12,3);

Tall.Init(17,4);

(* Small.length := 7; *)






WriteLn('The overall length of the short pole is ',

Short.Get_Total_Length);

WriteLn('The overall length of the average pole is ',

Average.Get_Total_Length);

WriteLn('The overall length of the tall pole is ',

Tall.Get_Total_Length);

WriteLn;

Small.Set_Data(6,7);Short.Set_Data(6,1);

Average.Set_Data(11,2);




WriteLn('The overall length of the short pole is ',

Short.Get_Total_Length);

WriteLn('The overall length of the average pole is ',

Average.Get_Total_Length);

WriteLn('The overall length of the tall pole is ',

Tall.Get_Total_Length);

end.





The overall length of the short pole is 10

The overall length of the average pole is 15

The overall length of the tall pole is 21




The overall length of the short pole is 7

The overall length of the average pole is 13

The overall length of the tall pole is 21

}

THE PRIVATE TYPE

Borland added the private type to version 6.0, but it is not as flexible as it could be. You will notice

that the order of declarations within the objects is significantly different in this program. Within an

object declaration, you are permitted to have public variables and methods and private variables and

methods, but you are required to list all public entities first. The reserved word private is then used with

all private entities following its use. Within each section, the variables must come first, followed by themethods.

Even though the variables are listed as private in these classes, they are still available to the main

program because they are in the same Pascal unit. If the objects were placed in a different unit as

discussed in chapter 13, the variables defined as private would be unavailable in the calling program. If

you are using TURBO Pascal 5.5, you will have to remove the word private and rearrange the variables

to put them ahead of the methods in the two object definitions.

In lines 57 and 58, we declare several objects of the defined object types and use some of them in the

main program. We can finally illustrate one of the biggest advantages of object oriented programming.

We should all agree that it would be silly and meaningless to multiply the height of one of the poles by

the width of a box. If we were using standard procedural programming with all variables definedglobally, it would be a simple matter to accidentally write height*width and print out the result

thinking we had a meaningful answer. By encapsulating the data within the objects, we would have to



really work at it to get that meaningless answer because the system itself would prevent us from

accidentally using the wrong data. This is true only if we have agreed not to use any of the data directlybut to do all data access through the available methods.

Encapsulation is a form of information hiding, but TURBO Pascal has a rather weak form of it because,

as mentioned earlier, Borland chose not to make the variables within the object private in version 5.5.

In TURBO Pascal 6.0 the variables can be defined as private variables as mentioned earlier, and aretherefore inaccessible outside of the unit in which they are declared. The client would be forced to useonly the methods provided by the author of the object to access the contained data. This is true

information hiding and adds some degree of protection to the internal data.

If you are using TURBO Pascal 5.5, it is up to you to never refer to the data within the object directly

as stated by Borland in the OOP Guide included with the compiler. Even if you are using version 6.0,

you must still discipline yourself to not refer to the private data within the defining unit.

The careful student will notice that since all data is carefully tied up within the objects, inadvertent

mixing of the wrong data is impossible provided a few simple rules are followed as discussed above.

Once again, this is such a small program that it is difficult to see the advantage of going to all of this

trouble. In a larger program, once the objects are completed, it is a simple matter to use them knowing

that they are debugged and working.

After the data are all printed out, some of the variables are changed in lines 80 through 82, and the

same output statements are used to reprint the same data so you can observe the changes.

A FEW RULES ARE NEEDED

As with any new topic, there are a few rules we must follow to use this new technique. The public

variables must all be declared first in the object followed by the public method definitions. The

reserved word private is then declared followed by the private variables and finally the private

methods. The names of all variables within an object must be unique and may not be repeated as the

names of any of the formal variables in any of the methods. Thus length, width, len, and widmust be unique as used in lines 6, 7, 10, and 11. The names of formal variables may be reused in other

methods however, as illustrated in lines 6 and 7, and all names may be reused in another object. It

should be obvious that all object type names must be unique within a given file and all objects must

have unique names.

All of the above rules are obvious if you spend a little time thinking about them. They should therefore

not be a stumbling block to anyone with some procedural programming experience.

WHAT IS A CONSTRUCTOR?

It is time to keep our promise and define just what a constructor is. In this present context, that of

simple objects, the constructor does very little for us, but we will include one for nearly every object to

illustrate its use. The constructor can be named anything desired but it would be best to stick with the

convention and name every constructor Init as suggested by Borland. The constructor is used to

initialise all values within an object and do any other setup that must be done to use an object. The

constructor should be called once for every declared object. When we get to the topic of virtual

functions, constructors will be absolutely required for every object, but for the simple objects we are

using here, they are optional.

It would be best to include a constructor in every object type, define the constructor to initialise all

variables within the object, and call the constructor once for each instance of the object type. Until we

get to virtual methods, none of this is required, but it would be good practice to get in the habit of doing

it.



WHAT IS A DESTRUCTOR?

A destructor is another method that can be used for cleanup when you are finished with an object. It is

usually used in conjunction with dynamic allocation to assure that all dynamically allocated fields

associated with the object are deallocated prior to leaving the scope of the object. A destructor is not

illustrated in this tutorial but it should be easy for you to define and use one when you have a need for

one.

OUR FIRST INHERITANCE

Load the example program named INHERIT1.PAS for our first example of a program with inheritance.

As always, our first encounter with this new topic will be very simple.

In lines 7 through 14 we define a simple object type defining a vehicle and a few characteristics about

the vehicle. We have the ability to store a few values and read them out in several ways. Of course,

most of the interest is in the interfaces, so the implementations are purposely kept very small.


program Inheritance_1;

{ ******************** class definitions **********************}

type

Vehicle = object

Wheels : integer;

Weight : real;

constructor Init(In_Wheels : integer; In_Weight : real);

function Get_Wheels : integer;

function Get_Weight : real;

function Wheel_loading : real;

end;

Car = object(Vehicle)

Passenger_Load : integer;

constructor Init(In_Wheels : integer;

In_Weight : real;

People : integer);

function Passengers : integer;

end;

Truck = Object(Vehicle)


Payload : real;

constructor Init(People : integer;

Max_Load : real;

In_Wheels : integer;In_Weight : real);

function Efficiency : real;

function Wheel_Loading :real;

end;

{ ********************* class implementations ******************** }

constructor Vehicle.Init(In_Wheels : integer; In_Weight : real);

begin

Wheels := In_Wheels;

Weight := In_Weight;

end;

function Vehicle.Get_Wheels : integer;

begin

Get_Wheels := Wheels;

end;

function Vehicle.Get_Weight : real;



begin

Get_Weight := Weight;

end;

function Vehicle.Wheel_loading : real;

begin

Wheel_Loading := Weight/Wheels;

end;

constructor Car.Init(In_Wheels : integer;

In_Weight : real;

People : integer);

begin



Passenger_Load := People;

end;

function Car.Passengers : integer;

begin

Passengers := Passenger_Load;

end;

constructor Truck.Init(People : integer;

Max_Load : real;In_Wheels : integer;

In_Weight : real);

begin


Payload := Max_Load;

Vehicle.Init(In_Wheels, In_Weight);

end;

function Truck.Efficiency : real;

begin

Efficiency := 100.0 * Payload / (Payload + Weight);

end;

function Truck.Wheel_Loading : real;

begin

Wheel_Loading := (Weight + Payload)/Wheels;

end;

{ ************************ main program ************************** }

var Unicycle : Vehicle;

Sedan : Car;

Semi : Truck;

begin

Unicycle.Init(1, 12.0);

Sedan.Init(4, 2100.0, 5);

Semi.Init(1, 25000.0, 18, 5000.0);

WriteLn('The unicycle weighs ', Unicycle.Get_Weight:5:1,

' pounds, and has ', Unicycle.Get_Wheels, ' wheel.');

WriteLn('The car weighs ', Sedan.Get_Weight:7:1,

' pounds, and carries ', Sedan.Passengers,

' passengers.');

WriteLn('The semi has a wheel loading of ', Semi.Wheel_Loading:8:1,

' pounds per tire,');

WriteLn(' and has an efficiency of ', Semi.Efficiency:5:1,

' percent.');

with Semi do

begin

WriteLn('The semi has a wheel loading of ', Wheel_Loading:8:1,


WriteLn(' and has an efficiency of ', Efficiency:5:1,

' percent.');

end;end.




The unicycle weighs 12.0 pounds, and has 1 wheel.

The car weighs 2100.0 pounds, and carries 5 passengers.

The semi has a wheel loading of 1666.7 pounds per tire,

and has an efficiency of 83.3 percent.The semi has a wheel loading of 1666.7 pounds per tire,

and has an efficiency of 83.3 percent.

}

In lines 17 through 35, we declare two additional object types that use the Vehicle type as a base for

the new types as indicated by the previously defined name Vehicle in parentheses in the object

definitions in lines 17 and 26. The Vehicle object type is said to be the ancestor type and the two new

object types are called descendant types. The descendant types inherit some information from the

ancestor types according to well defined rules. The variables in the ancestor type are all included withinthe descendant types and are available in objects of the descendant types just as if they had been

defined within the descendant types. For that reason, all variable names must be unique within theancestor type and within each of the descendant types. A name can be reused in one or more

descendants however, as is illustrated in lines 18 and 27 where the variable name Passenger_Load

is used in both object types.

The method names from the ancestor object types can be repeated in the descendant object types but

this has the effect of overriding the method of the same name in the ancestor making the ancestor

method unavailable for use in objects of the descendant types. Objects instantiated of the type Car

therefore, have the three methods available in lines 11 through 13 of the ancestor type, the constructor

in line 19 which overrides the constructor in line 10 of the ancestor type, and the function given in line22. This object therefore has five different methods to perform its required operations.

Objects of type Truck have five methods available also, the two in lines 11 and 12 and the one in line

33. The two in lines 29 and 34 of the descendant overrides the two in lines 10 and 13 of the ancestor

object type.

You should note that even though some of the methods were overridden in the descendant object type,

they do not affect the ancestor, and instances of the Vehicle type have two variables and four

methods available.

In effect we have an object hierarchy which can be extended to as many levels as necessary to complete

the task at hand. The most important part of object oriented programming is the definition of theobjects in a meaningful manner, but it is not something you will learn to do overnight. It will take a

great deal of practice until you can see the objects in any given project in such a way that a clear

solution can be found. I was somewhat intimidated by the clever examples found in a classic text on

object oriented programming until I talked to a man that had shared an office with the author at thetime he was writing that particular book. I learned that what was finally put in the book was at least the

fourth iteration of each problem and in some cases the seventh before he finally arrived at a good

solution. We will have more to say about this topic as we progress through this tutorial.

HOW DO WE USE THE OBJECTS?

The implementations of the methods are given in lines 39 through 92 and should be self explanatory,

except for a few notable exceptions. You will note that in line 81 we send a message to the

Vehicle.Init method to initialise some data. A change to the Vehicle type will be reflected in

the Truck type also because of this call. In lines 64 and 65 we are using some inherited variables just

as if they had been defined as part of the descendant object types.



In lines 96 through 98 we instantiate one of each and send a message to their constructors in lines 102

through 104 then print out a few of the stored values.

Lines 113 through 116 are repeated in lines 120 through 123 where they are placed within a with

section to illustrate that the with can be used for the calls to the methods in the same manner that it is

used for accessing the fields of a record. Any other details of this program can be gleaned by the

diligent student. Be sure to compile and execute this program so you can verify the given result.

WHY USE INHERITANCE?

Once you have an object that is completely debugged and working, it is possible that it can be reused

on your next project. If you cannot use it exactly as is, but are required to make a change to it, you have

two choices. You can modify the existing code and hope you don't introduce any analysis or coding

errors, or you can inherit it into a new object and add code around the completely debugged object

without actually modifying the original code. Several studies have shown that modifying existing code

lead to a high probability of introducing errors into the modified code that are difficult to find because

you are not as familiar with the code as you should be. Adding a few methods to an existing object,

however, is not nearly as error prone and is the preferred method.

AN OBJECT IN A UNIT

Load the example program named VEHICLES.PAS for an example of the proper way to package the

object so it can be conveniently reused for another project.

The object type definition is given in the public part of the unit so it is available to any Pascal program

which needs to use it. The implementation of the methods are hidden in the implementation part of the

unit where they are not directly available to any calling program. Note that it is also possible to define a

few local methods within the implementation for use only within the implementation but none areillustrated here. There is no body to this unit, only the end statement in line 39 so there is no

initialisation code to be executed during loading. It would be perfectly legal to include an initialisation

body, but even if you do, you should be sure to include a constructor to be called once for each object.

This is to prepare you for the use of virtual functions which we will study in the next chapter.

Referring back to the discussion of the second example program in this chapter, you will find the

means necessary to make the variables truly private in this program. If you move the two variables to a

location just after the end of the methods, and add the reserved word private just before the two

variables, they will be truly private and unavailable for direct modification outside of this unit. If you

are using TURBO Pascal 6.0, you should make this modification and try to access the variables directly

in the calling program which will be discussed next. You must compile this unit to disk so it can be

used with the rest of the example programs in this chapter.




unit Vehicles;

interface

type

Vehicle = object

Wheels : integer;

Weight : real;


function Get_Wheels : integer;

function Get_Weight : real;

function Wheel_loading : real;

end;

implementation


begin



end;

function Vehicle.Get_Wheels : integer;begin

Get_Wheels := Wheels;

end;

function Vehicle.Get_Weight : real;

begin

Get_Weight := Weight;

end;

function Vehicle.Wheel_loading : real;

begin

Wheel_Loading := Weight/Wheels;

end;

end.


This file cannot be executed.)

}

ANOTHER OBJECT IN A UNIT

The example program named CARTRUCK.PAS continues the new packaging scheme by including

two descendant object types in its interface after telling the system that it uses the Vehicles unit in line

6.

The remainder of this unit is constructed just like the last one so nothing more needs to be said about it.Be sure to compile this unit to disk so it will be available for use with the next example program.




unit CarTruck;

interface

uses Vehicles;

type




In_Weight : real;

People : integer);

function Passengers : integer;

end;



Payload : real;


Max_Load : real;

In_Wheels : integer;

In_Weight : real);function Efficiency : real;

function Wheel_Loading :real;

end;

implementation


In_Weight : real;

People : integer);

begin




end;

function Car.Passengers : integer;begin

Passengers := Passenger_Load;

end;


Max_Load : real;


In_Weight : real);

begin



Vehicle.Init(In_Wheels, In_Weight);

end;

function Truck.Efficiency : real;

begin

Efficiency := 100.0 * Payload / (Payload + Weight);

end;

function Truck.Wheel_Loading : real;

begin

Wheel_Loading := (Weight + Payload)/Wheels;

end;

end.


(This file cannot be executed)

}



Note that this unit could have been further divided into two separate units, one for each object type, but

it was felt that it was important to illustrate that several can be combined in this manner if desired. In

like manner, the last unit could have been combined with this unit, but once again, it was desired to

illustrate the generality of program decomposition and packaging.

USING THE OBJECTS DEFINED IN UNITS

Load the program named INHERIT2.PAS for a program that uses the units of the last two example

programs and is identical to the program named INHERIT1.PAS.

The only difference in these two programs is in the way the code was packaged. The second way is

much more general and more conducive to good software engineering practices because it allows

separate development of each of the three program units. Each can be refined independently of the

other two and the overall package can be simpler to debug and maintain. It should be clear that any

changes to the Car object, for example, will be localised to that single unit and not scattered all over the

software terrain.



uses vehicles, CarTruck;

{ ************************ main program ************************** }


Sedan : Car;

Semi : Truck;

begin


Sedan.Init(4, 2100.0, 5);

Semi.Init(1, 25000.0, 18, 5000.0);



WriteLn('The car weighs ', Sedan.Get_Weight:7:1,

' pounds, and carries ', Sedan.Passengers,

' passengers.');

WriteLn('The semi has a wheel loading of ', Semi.Wheel_Loading:8:1,


WriteLn(' and has an efficiency of ', Semi.Efficiency:5:1,

' percent.');

with Semi do

begin

WriteLn('The semi has a wheel loading of ', Wheel_Loading:8:1,' pounds per tire,');


' percent.');

end;

end.







and has an efficiency of 83.3 percent.}



AN ARRAY AND A POINTER

Examine the example program named INHERIT3.PAS for an example of the use of a pointer to an

object and the use of an array of objects.



uses vehicles, CarTruck;

{ ************************ main program ************************** }


Sedan : array[1..3] of Car;

Semi_Point : ^Truck;

Index : integer;

begin



Sedan[Index].Init(4, 2100.0, 5 + Index);

New(Semi_Point);Semi_Point^.Init(1, 25000.0, 18, 5000.0);




WriteLn('The car weighs ', Sedan[Index].Get_Weight:7:1,

' pounds, and carries ', Sedan[Index].Passengers,

' passengers.');

WriteLn('The semi has a wheel loading of ',

Semi_Point^.Wheel_Loading:8:1,


WriteLn(' and has an efficiency of ', Semi_Point^.Efficiency:5:1,

' percent.');

with Semi_Point^ do

begin

WriteLn('The semi has a wheel loading of ', Wheel_Loading:8:1,



' percent.');

end;

Dispose(Semi_Point);

end.




The car weighs 2100.0 pounds, and carries 7 passengers.The car weighs 2100.0 pounds, and carries 8 passengers.





}

This program is nearly identical to INHERIT2.PAS except for the addition of an array of Car type

objects named Sedan[1] to Sedan[3], and the definition of a pointer to the Truck type object named

Semi_Point. Lines 16 and 17 illustrate the initialisation of the array of Sedan, and lines 23 through

26 illustrates its use when the data is printed out. An object is dynamically allocated in line 18 and it isthen initialised in the next line. Its use is illustrated in lines 28 through 40 and it is deallocated in line

41.



TURBO Pascal 5.5 and newer, have an extension to the New procedure allowing the dynamicallocation and the initialisation to take place in the same procedure call. The line;

New(Semi_Point, Init(1, 25000.0, 18, 5000.0));

can be used to replace lines 18 and 19 in this program if you desire to do so.

This program should illustrate that objects can be used with arrays and pointers in the same manner as a

record. Be sure to compile and execute this program.

WHAT IS MULTIPLE INHERITANCE?

Multiple inheritance allows the programmer to inherit data and methods from two or more ancestor

objects. When this is done however, there is a real problem if there are two variables or methods of the

same name and it is up to the programmer to somehow define which will be used by the descendent.

Some object oriented programming languages allow multiple inheritance, but most do not. TURBO

Pascal has no provision for multiple inheritance, and Borland has made no indication at this time

whether future versions will permit it.

WHAT SHOULD YOU DO NOW?

You have reached a major point in your excursion of object oriented programming, because you now

have most of the knowledge you need to do some serious object oriented programming. The best thing

for you to do at this point is stop studying and get busy programming, using some of these techniquesfor your projects. The only topic left is the use of virtual methods and you can easily defer its use for a

long time.

One point should be made before you begin a serious programming project. Your first program could

have too many objects and be nearly unreadable unless you strive to use only a few objects. After yougain experience, you can confidently use more objects with each programming project. For your first

project, define only a few objects and write the majority of the program in standard procedural

programming methods. Add a few more objects to your next project and, as you gain experience, you

will feel very comfortable with the use of objects and your programming methods will be very clear.

Now is the time to begin using this new knowledge but be sure you enter the water slowly the first

time.


1. Modify ENCAP2.PAS in such a way to multiply the height of the short pole times the length of themedium box and print the result out. Even though this is possible to do, it requires you to expend a

bit of effort to accomplish. Remember that you should not use the components of an objectdirectly, only through use of the available methods.

2. Add an object named Pick_Up to INHERIT2.PAS of type Truck and initialize it to some

reasonable values. Print out its loading and efficiency in a manner similar to the Semi.



Chapter 15 : VIRTUAL METHODS

Since we covered encapsulation and inheritance in the last chapter, we are left with only virtual

methods to complete the major topics of object oriented programming. Virtual methods, as they are

called in TURBO Pascal, have several other names in the literature to describe the same technique.This technique is sometimes called run-time binding or late binding referring to when the decision is

made as to what method will respond to the message. The use of virtual methods moves the

responsibility of selection from the client (the logic sending the message) to the supplier (the methods

responding to the message). We will begin with a skeleton of a program without a virtual method and

add one to show the effect of adding a virtual method.

WITHOUT A VIRTUAL METHOD

The example program named VIRTUAL1.PAS will be used as the starting point for the study of virtual

methods. We must state that this program does not contain a virtual method, it is only the starting point

for studying them.

The objects included here are very similar to the objects describing vehicles which we were working

with in the last chapter. You will notice that all three objects contain a method named Message in lines

11, 20, and 31. The Message method will be the center of our study in the first three example programs.

It should be pointed out that the constructors for the three objects are called in lines 98 through 100

even though a constructor call is still not absolutely necessary in this case. We will have more to sayabout the constructor calls during the next example program.


program Virtual_1;


type

Vehicle = objectWheels : integer;

Weight : real;


procedure Message;

end;




In_Weight : real;

People : integer);

procedure Message;

end;



Payload : real;


Max_Load : real;


In_Weight : real);

procedure Message;

end;



begin



end;

procedure Vehicle.Message;



begin

WriteLn('This message is from the vehicle.');

end;


In_Weight : real;

People : integer);begin




end;

procedure Car.Message;

begin

WriteLn('This message is from the car.');

end;


Max_Load : real;


begin





end;

procedure Truck.Message;

begin

WriteLn('This message is from the truck.');

end;

procedure Output_A_Message(VAR Machine : Vehicle);

begin

Write('This is from Output_A_message; ');

Machine.Message;

end;

{ ************************ main program ************************** }


Sedan : Car;

Semi : Truck;

begin


Sedan.Init(4, 2100.0, 5);

Semi.Init(1, 25000.0, 18, 5000.0);

WriteLn;

Unicycle.Message;

Sedan.Message;

Semi.Message;

WriteLn;

Output_A_Message(Unicycle);

Output_A_Message(Sedan);

Output_A_Message(Semi);

end.




This message is from the vehicle.

This message is from the car.

This message is from the truck.

This is from Output_A_Message; This message is from the vehicle.



}

Compile and execute the program and you will find that even though it is legal to pass the objects of

type Car and Truck to the method named Output_A_Message in lines 109 and 110, the method

that is called from line 86 is the method named Message in the parent type Vehicle. This is

probably no surprise to you since we defined an object of type Vehicle as a formal parameter of the

method Output_A_Message . We need only one small change and we will have a virtual procedure

call.

Even though this program seems to do very little, it will be the basis of our study of virtual methods so

you should study the code in detail.

NOW TO MAKE IT A VIRTUAL METHOD

Examine the example program named VIRTUAL2.PAS, and you will find only one small change in the

code but a world of difference in the way it executes.

The careful student will notice the addition of the reserved word virtual in lines 13, 22, and 33. This

makes the method named Message a virtual method which operates a little differently from the way it

did in the last program. Once again, we call the three constructors in lines 100 through 102 and this

time the constructor calls are absolutely essential. We will discuss why in a couple of paragraphs.

(* Chapter 15 - Program 2 *)program Virtual_2;


{$R+}

type

Vehicle = object

Wheels : integer;

Weight : real;


procedure Message; virtual;

end;




In_Weight : real;

People : integer);


end;



Payload : real;


Max_Load : real;


In_Weight : real);


end;





begin



end;


beginWriteLn('This message is from the vehicle.');

end;


In_Weight : real;

People : integer);

begin




end;


beginWriteLn('This message is from the car.');

end;


Max_Load : real;


In_Weight : real);

begin





end;


begin


end;

procedure Output_A_Message(VAR Machine : Vehicle);

begin


Machine.Message;

end;

{ ************************ main program ************************** }

var Unicycle : Vehicle;Sedan : Car;

Semi : Truck;

begin


Sedan.Init(4, 2100.0, 5);

Semi.Init(1, 25000.0, 18, 5000.0);

WriteLn;

Unicycle.Message;

Sedan.Message;

Semi.Message;

WriteLn;

Output_A_Message(Unicycle);Output_A_Message(Sedan);

Output_A_Message(Semi);



end.


This message is from the vehicle.This message is from the car.



This is from Output_A_Message; This message is from the car.

This is from Output_A_Message; This message is from the truck.

}

Once again we send a message to Output_A_Message three times in lines 110 through 112 and line

88 is used to send a message to the Message method. When we compile and execute this program, we

find that even though the method Output_A_Message only uses the parent type Vehicle, the

system calls the correct procedure based on the type of the actual object passed to this method. The

system sends a message to the objects of the correct type instead of to the parent type as may be

expected. It should be clear to you that the object that is to receive the message is not known at compile

time but must be selected at run time when the object arrives at the method Output_A_Message .

This is known as late binding since the type is not known until run time as opposed to early binding

where the type is known at compile time. Every subprogram call in this entire tutorial, up to this point,

has been early binding.

You will note that even though the method Output_A_Message only knows about the objects of

type Vehicle, it has the ability to pass through other types, provided of course that they are

descendant types of Vehicle. The method Output_A_Message only passes the message through,

it does not do the selection. The selection is done by the objects themselves which answer the messages

passed to them. This means that the sender does not know where the message will be answered from,and it is up to the receiver to find that a message is being sent its way and to respond to it. It is oftensaid that the supplier (the method doing the work) must make the decision to answer the message,

rather than the client (the user of the work done). The burden is placed on the supplier to do the right

thing.

If a method is declared virtual, all methods of that name must also be virtual including all ancestors andall descendants. It is not possible to declare part of the methods of the same name virtual and part

standard. All parameter lists for all virtual methods of the same name must also be identical since they

must all be capable of being called by the same method call.

ASSIGNING DESCENDANTS TO ANCESTORS?

It is legal in any object oriented language to assign a descendant object to an ancestor variable but thereverse is not true. A vehicle, for example, can be used to define a car, a truck, a bus, or any number of

other kinds of vehicles so it can be assigned any of those values. A car on the other hand, is too specific

to be used for the definition of anything but a car, so it cannot have any other value assigned to it. A

vehicle is very general and can cover a wide range of values, but a car is very specific and can therefore

only define a car.

WHY USE A CONSTRUCTOR?

The constructor is absolutely required in this case because of the way the authors of TURBO Pascal

defined the use of virtual functions. The constructor sets up a pointer to a virtual method table (VMT)which is used to find the virtual methods. If there is no pointer, the system jumps off to some unknown



location and tries to execute whatever happens to be there and could do almost anything at that

unknown and undefined point in the code. So it is important to call a constructor once for each objectas is done here so the pointer to the VMT can be initialised to the proper value. If you make several

objects of one type, it is not enough to call a constructor for one object and copy that object into each of

the other objects. Each object must have its own constructor call in order to prevent a system crash.

The strange looking code in line 6 tells the system to check each call to a virtual function to see if theconstructor has been called. This slows the program down slightly but will result in an error message if a virtual method is called prior to its VMT being properly set up with a constructor call. After a

program is thoroughly tested, the code can be removed from line 6 to speed up the program slightly by

eliminating the checks. Be warned however, that a call to a virtual method without A VMT will

probably result in the computer hanging up.

VIRTUALS AND POINTERS

The example program named VIRTUAL3.PAS is nearly identical to the last program except that this

program uses pointers to objects instead of using the objects directly.


program Virtual_3;


type

Vehicle = object

Wheels : integer;

Weight : real;



end;

Vehicle_Pointer = ^Vehicle;

Car = object(Vehicle)Passenger_Load : integer;


In_Weight : real;

People : integer);


end;

Car_Pointer = ^Car;



Payload : real;


Max_Load : real;



end;

Truck_Pointer = ^Truck;



begin



end;


begin

WriteLn('This message is from the vehicle.');

end;




In_Weight : real;

People : integer);

begin


Weight := In_Weight;Passenger_Load := People;

end;


begin

WriteLn('This message is from the car.');

end;


Max_Load : real;


In_Weight : real);

begin

Passenger_Load := People;Payload := Max_Load;



end;


begin


end;

procedure Output_A_Message(VAR Machine : Vehicle_Pointer);

begin


Machine^.Message;

end;

{ ************************ main program ************************** }

var Unicycle : Vehicle_Pointer;

Sedan : Car_Pointer;

Semi : Truck_Pointer;

begin

New(Unicycle);

New(Sedan);

New(Semi);

Unicycle^.Init(1, 12.0);

Sedan^.Init(4, 2100.0, 5);

Semi^.Init(1, 25000.0, 18, 5000.0);

WriteLn;

Unicycle^.Message;

Sedan^.Message;

Semi^.Message;

WriteLn;


Dispose(Unicycle);

Unicycle := Sedan;


Unicycle := Semi;


Dispose(Sedan);

Dispose(Semi);

end.




This message is from the vehicle.

This message is from the car.



This is from Output_A_Message; This message is from the car.

This is from Output_A_Message; This message is from the truck.

}

You will notice that once again, the methods named Message are all defined as virtual and a pointer

type is defined for each object type. In lines 99 through 101, three pointers are declared and memory is

dynamically allocated on the heap for the objects themselves. The objects are all sent a constructormessage to initialise the stored data within the objects and to set up the VMT for each. The rest of the

program is nearly identical to the last program except that Dispose procedures are called for each of the

dynamically allocated objects. The code used in line 6 of the last program to force a check of each

virtual method call has been removed to illustrate that it doesn't have to be there if you are sure amessage is sent to a constructor once for each object with a virtual method.

Compiling and executing this program will give the same result as the last program indicating that it is

perfectly legal to use pointers to objects as well as the objects themselves.

AN ANCESTOR OBJECT

The example program PERSON.PAS is not a complete program at all but only an object definitionwithin a unit. This unit should pose no problem for you to understand so we will not say much except

to point out that the method named Display is a virtual method.

This example program, as well as the next two example programs, have been carefully selected to

illustrate the proper way to package objects for use in a clear understandable manner.

Compile this unit to disk in order to make it available for use in the remainder of this chapter.


unit Person;

interface

type

Person_ID = object

Name : string[30];

Salary : integer;

constructor Init;procedure Display; virtual;

end;

implementation

constructor Person_ID.Init;

begin

end;

procedure Person_ID.Display;

begin

WriteLn('Error - this procedure should never be called.');

end;

end.



SOME DESCENDENT OBJECTS

The example program named SUPERVSR.PAS is another unit which contains three descendants of the

previously defined object named Person_ID. You will notice that each of the objects have a method

named Display which is virtual just as the same method in the ancestor object was.

The interface for each object has been purposely kept very simple in order to illustrate the use of

objects. The implementation has also been kept as simple as possible for the same reason so the diligent

student should have no trouble in understanding this unit completely.

Once again, be sure to compile this unit to disk in order to make it available for use in the next few

example programs.


unit Supervsr;

interface

uses Person;

typeSupervisor = object(Person_ID)

Title : string[15];

constructor Init(In_Name : string;

In_Salary : integer;

In_Title : string);

procedure Display; virtual;

end;

Programmer = object(Person_ID)

Language : string[25];



In_Language : string);


end;

Secretary = object(Person_ID)

Shorthand : boolean;

typing_Speed : integer;



In_Shorthand : boolean;

In_TYping_Speed : integer);


end;

implementation

constructor Supervisor.Init(In_Name : string;


In_Title : string);

begin

Name := In_Name;Salary := In_Salary;

Title := In_Title;

end;

procedure Supervisor.Display;

begin

WriteLn(Name,' is the ',Title, ' and makes $', Salary,

' per month.');

end;

constructor Programmer.Init(In_Name : string;


In_Language : string);

begin

Name := In_Name;Salary := In_Salary;

Language := In_Language;



end;

procedure Programmer.Display;

begin

WriteLn(Name,' specializes in ',Language, ' and makes $', Salary,

' per month.');

end;

constructor Secretary.Init(In_Name : string;


In_Shorthand : boolean;

In_Typing_Speed : integer);

begin

Name := In_Name;

Salary := In_Salary;

Shorthand := In_Shorthand;

Typing_Speed := In_Typing_Speed;

end;

procedure Secretary.Display;

begin

WriteLn(Name,' can type ',Typing_Speed, ' words per minute.');

end;

end.

A COMPLETE EMPLOYEE PROGRAM

Although the program named EMPLOYEE.PAS is a very short program that does very little, it is a

complete program to handle a very small amount of data about your employees.

You will notice that we declare an array of ten pointers to the Person_ID object and one pointer to

each of the three descendant objects. In the main program we send a message to the constructor for

each of the array elements. Inspection of the Person_ID.Init code will reveal that this

initialisation does nothing. It is used to initialise the pointer to the VMT for each object, so the messagemust be sent. We then dynamically allocate six objects of assorted descendant objects being careful to

send a message to the constructor for each object. This is done to generate a VMT for each object as it

is allocated. Finally, we send a message to the first six objects pointed to by the array of pointers

instructing them to display their values.

When the program is compiled and executed, we find that the virtual methods were called as explained

in the last example program. Even though only one kind of pointer was passed to the Display

method, three different messages were actually displayed, each message being of the proper kind based

on the type of pointer used.

You will notice how clean and neat the main program is. It is extremely easy to follow because all of

the implementation details have been moved to the objects themselves. Once the objects are carefully

defined and debugged, the main program is usually a snap to write and debug.

Object oriented programming requires a whole new mindset over the procedural methods you have

been using but after you catch on to the technique, you will find your programs much easier to debug

and maintain. The one thing you should avoid is the use of too many objects in your first program. It is

best to define a few simple objects for your first attempt at object oriented programming and write the

rest of the program using standard procedural methods. Then as you gain experience, you can begin

using more and more objects until you finally write a program that is essentially all objects. Of course,

you will find that you will always write at least part of your program in a standard procedural format as

was done in EMPLOYEE.PAS in this chapter.




program Employee;

{$R+}

uses Person, Supervsr;

var staff : array[1..10] of ^Person_ID;

Sup : ^Supervisor;

Prog : ^Programmer;

Sec : ^Secretary;

Index : integer;

begin


staff[Index]^.Init;

WriteLn('XYZ Staff assignments.');

WriteLn;

new(Sup);

staff[1] := Sup;

Sup^.Init('Big John', 5100, 'President');

new(Prog);

staff[2] := Prog;

Prog^.Init('Joe Hacker', 3500, 'Pascal');

new(Prog);

staff[3] := Prog;

Prog^.Init('OOP Wizard', 7700, 'OOP Pascal');

new(Sec);

staff[4] := Sec;

Sec^.Init('Tillie Typer', 2200, True, 85);

new(Sup);

staff[5] := Sup;

Sup^.Init('Tom Talker', 5430,'Sales Manager');

new(Prog);

staff[6] := Prog;

Prog^.Init('Dave Debug', 5725, 'Assembly Language');


staff[Index]^.Display;

end.

(* Result of execution

XYZ Staff assignments.

Big John is the president and makes $5100 per month.

Joe Hacker specializes in Pascal and makes $3500 per month.

OOP Wizard specializes in OOP Pascal and makes $7700 per month.

Tillie TYper can type 85 words per minute.

Tom Talker is the Sales Manager and makes $5430 per month.

Dave Debug specializes in Assembly Language and makes $5725 per month.

*)


3. Add a new object type to SUPERVSR.PAS to define a Consultant defining appropriate data fieldsfor him, then add a couple of Consultant type objects to EMPLOYEE.PAS to use the new object

type.



Chapter 16 : COMPLETE SAMPLE PROGRAMS

Prior to this point, this tutorial has given you many example programs illustrating a point of some

kind, but these have all been "nonsense" programs as far as being useful. It would be a disservice to

you to simply quit with only tiny programs to study, so the following programs are offered to you as

examples of good Pascal programming practice. They are useful programs, but they are still short

enough to easily grasp their meaning. We will discuss them one at a time.

AMORTIZATION TABLE GENERATOR

This is not one program, but five. Each one is an improvement on the previous one, and the series is

intended to give you an idea of program development.

AMORT1.PAS

This is the bare outline of the amortisation program. Although it is an operating program, it doesn't do

very much. After some thought and planning, the main program was written to allow for an

initialisation, then an annual repeating loop. The annual loop would require a header, a monthly

calculation, and an annual balance. Finally, a procedure was outlined for each of these functions with a

minimum of calculations in each procedure. This program can be compiled and run to see that it does

do something for each month and for each year. It has a major problem because it does not stop whenthe loan is paid off but keeps going to the end of that year. The primary structure is complete.

AMORT2.PAS

This is an improvement over AMORT1. The monthly calculations are correct but the final payment is

still incorrectly done. Notice that for ease of testing, the loan variables are simply defined as constants

in the initialise procedure. To make the procedures easier to find, comments with asterisks were added.This program is nearly usable. Compile and run it.

AMORT3.PAS

Now we calculate the final payment correctly and we have a correct annual header with column

headings. We have introduced a new variable to be used for an annual interest accumulation. This isneat to have at income tax time. This program can also be compiled and run.

AMORT4.PAS

This program does nearly everything we would like it to do. All of the information needed to build the

table for any loan is now read in from the keyboard, greatly adding to the flexibility. After the

information is available, the monthly payment is calculated in the newly added procedure

Calculate_Payment. The annual header has a new line added to include the original loan amount

and the interest rate in the information. Compile and run this program to see its operation.

AMORT5.PAS - The only additional feature in this program is the addition of a printout of the results.

Examining the program, you will notice that many of the output statements are duplicated with the Lst

included for the device selection. Compile and run this program, but be sure to turn your printer on to

get a printout of the amortisation table you ask for. If you are using TURBO Pascal version 3.0, youwill need to either comment out line 3 or remove it altogether.

TOP DOWN PROGRAMMING

The preceding example is an example of a top-down approach to programming. This is where the

overall task is outlined, and the details are added in whatever fashion makes sense to the designer. The

opposite is a bottom-up programming effort, in which the heart of the problem is defined and the rest of

the program is built up around it. In this case, the monthly payment schedule would probably be a

starting point and the remainder of the program slowly built up around it. Use whichever method works

best for you.

The final program AMORT5.PAS is by no means a program which can never be improved upon. Many

improvements can be thought of. These will be exercises for you if you so desire.



1. In the data input section, ask if a printout is desired, and only print if it was requested. This wouldinvolve defining a new variable and if statements controlling all write statements with Lst as a

device selector.

2. Format the printout with a form-feed every three years to cause a neater printout. The program

presently prints data right across the paper folds with no regard to the top of page.

3. Modify the program to include semi-monthly payments. Payments twice a month are becoming

popular, but this program cannot handle them.

4. Instead of listing the months as numbers, put in a case statement to cause the months to be printed

out as three letter names. You could also include the day of the month when the payment is due.

5. Any other modification you can think up. The more you modify this and other programs, the more

experience and confidence you will gain.

LIST.PAS, to list your Pascal programs

LIST.PAS is a very useful program that you can use to list your Pascal programs on the printer. It can

only be compiled with TURBO Pascal because it uses a TURBO extension, the string type variable.

The method used in the Initialize procedure to read the command line parameter should be no

problem for you to understand at this point. To use this program to print out the last program, for

example, you would enter the following at the DOS prompt LIST AMORT5.PAS. This program reads

in the AMORT5.PAS from the command line and uses it to define the input file. It should be pointed

out that this program cannot be run from a "compiled in memory" compilation with the TURBO Pascal

compiler. It must be compiled to a Disk file, and you must quit TURBO Pascal in order to run it from

the DOS command level.

The parameter read from the command line, AMORT5.PAS, is stored at computer memory location80(hexadecimal) referred to the present code segment. If you didn't understand that, don't worry, you

can still find the input parameter in any program using the method given in the initialise procedure for

your version of TURBO Pascal.

If you are not using a TURBO Pascal compiler, but you are using MS-DOS or PC-DOS, you can still

use this program because it is provided on your disk already compiled as LIST.EXE, and can be run

like any other .COM or .EXE program.

TIMEDATE.PAS, to get today's time and date

This is a very useful program as an example of using some of the extensions of TURBO Pascal. It

interrogates the inner workings of DOS and gets the present time and date for you, provided you

entered them correctly when you turned your computer on. The procedure Time_And_Date can be

included in any TURBO Pascal program you write to give you the time and date for your listings. Asan exercise in programming, add the time and date to the program LIST to improve on its usefulness. It

turns out to be an almost trivial program but is still a good illustration of how to use some of the newer

Borland extensions to Pascal. The observant student will notice that the time and date procedures have

already been added to LIST.PAS.

SETTIME.PAS, a useful utility program

This program is very interesting in that it changes the date and time stamp on any file in the current

directory. It is the program used to set the time and date on all of the files on the distribution disk

included with this tutorial. It sets the time to 12:00:00 and the date to Feb 4, 1991 but you can use it to

set any desired time.



OT.PAS, The OAKTREE directory program

This program should be very useful to you, especially if you have a hard disk. It will list the entire

contents of your hard disk (or floppy) in a very easy to read and easy to use form. The program is

documented in the file named OT.DOC. It uses many of the TURBO Pascal extensions and willprobably not compile with any other Pascal compiler without extensive modifications.

This is a very useful program, so you should spend the time necessary to both understand it and modify

it for your own needs.

You will find this program to be a good example of linked lists because it includes a sort routine using

a dynamically allocated B-TREE and another sorting routine that uses a dynamically allocated linkedlist with a bubble sort. These methods are completely defined in Niklaus Wirth's book, "Algorithms +

Data Structures = Programs", a highly recommended book if you are interested in advanced

programming techniques.

It might also be pointed out that OT.PAS makes use of recursive methods for both sorting and handling

subdirectories. It is definitely an example of advanced programming methods, and it would be a goodvehicle for your personal study.

MOST IMPORTANT - Your own programs

Having completed this tutorial on Pascal, you are well on your way to becoming a proficient Pascal

programmer. The best way you can improve your skills now is to actually write Pascal programs.Another way to aid in your building of skill and confidence is to study other Pascal programs. Many

programming examples can be found in computing magazines and books. There are many books

available devoted entirely to TURBO Pascal and you would do well to visit your local bookstore and

review a few of them.

You already own one of the best books available for reference if you are using TURBO Pascal.Although the TURBO Pascal reference manual is worth very little as a learning tool, it is excellent as a

language reference manual. Now that you have completed all 16 chapters of this tutorial, you have a

good grasp of the terminology of Pascal and should have little trouble reading and understanding your

reference manual. Your only limitation at this point is your own perseverance and imagination.

Whatever your programming level or needs may be, Pascal can fulfil them and do so in a very elegantway,

Happy Programming.

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Pascal Tutorial v2.6c