Jozef Goetz, 2015 1 2011 - 2015 Pearson Education, Inc. All rights reserved. 2002 Prentice Hall. All rights reserved. expanded by J. Goetz, 2014 credits:

Jozef Goetz, 2015

1

2011 - 2015 Pearson Education, Inc. All rights reserved.

2002 Prentice Hall. All rights reserved.

expanded by J. Goetz, 2014

credits:

Jozef Goetz, 2015

2

Chapter 5 – Control Structures Part 1

Outline5.1 Introduction5.2 Algorithms5.3 Pseudocode5.4 Control Structures5.5 if Selection Structure5.6 use if/else Selection Structure5.7 use while Repetition Structure5.8 Formulating Algorithms: Case Study 1

(Counter-Controlled Repetition)5.9 Formulating Algorithms with Top-Down, Stepwise Refinement:

Case Study 2 (Sentinel-Controlled Repetition)5.10 Formulating Algorithms with Top-Down, Stepwise Refinement:

Case Study 3 (Nested Control Structures)5.11 Assignment Operators5.12 Increment and Decrement Operators

5.13 Simple Types

Jozef Goetz, 2015

3

The wheel is come full circle. William Shakespeare

Let’s all move one place on. Lewis Carroll

How many apples fell on Newton’s head before he took the hint! Robert Frost

All the evolution we know of proceeds from the vague to the definite. Charles Sanders Peirce

Jozef Goetz, 2015

4Objectives

To understand basic problem-solving techniques of programming.

To develop algorithms through the process of top-down, stepwise refinement.

To use the if and if/else selection structures to choose among alternative actions.

To use the while repetition structure to execute statements in a program repeatedly.

To understand counter-controlled repetition and sentinel-controlled repetition.

To use the increment, decrement and assignment operators.

Jozef Goetz, 2015

55.1 Introduction

Before writing program Have thorough understanding of problem Carefully planned approach for solving it

While writing program Know what “building blocks” are available Use good programming principles

We learn about Control Structures Structured-programming principle Control structures help build and manipulate objects

(Chapter 5-8)

Jozef Goetz, 2015

65.2 Algorithms

Algorithm is a procedure for solving a problem in terms of Series of actions in specific order

The actions executed The order in which actions execute

Example: "Rise and Shine" algorithm Get out of bed, take off pajamas, take a shower, get dressed, eat breakfast, carpool to

work

• Program control Specifying the order in which actions execute

Control structures help specify this order

Algorithmic thinking is a formal and structured thinking type which helps us to design and understand modern software. Learning algorithmic thinking is not easy and needs a lot of practice and

endurance/patience. Experience has shown that the most difficult part of solving a problem on a computer is

developing the algorithm for the solution.

Jozef Goetz, 2015

75.2 Algorithms

Computational thinking means thinking algorithmically and with the ability to apply mathematical concepts such as induction to develop more efficient, fair, and secure solutions.

Center for Computational Thinking, Carnegie Mellon

Tim the Train: Binpacking

Problem: Given is a sequence of parts. Find a sequence of commands that fill the wagons of the

train with as many parts as possible. The wagons must not be overloaded.

Jozef Goetz, 2015

85.3 Pseudocode

Pseudocode Artificial, informal language

Helps develop algorithms

Similar to everyday English Not actually executed on computers “Think out” program before writing it

Easy to convert into corresponding C# program

Consists only of executable statements

• Declarations are not executable statements

– Actions: input, output, calculation

Jozef Goetz, 2015

95.4 Control Structures

Sequential execution Execute statements in the order they appear in the code

Transfer of control (flow of control) Changing the order in which statements execute

All programs can be written in terms of only three control structures:

sequence selection

– The if and if/else statements

– The goto statement• No longer used unless absolutely needed• Causes many readability and debuging problems

repetition

– The while and do/while loops (chapter 5)

– The for and foreach loops (chapter 6)

Jozef Goetz, 2015


Flowchart Graphical representation of an algorithm Drawn using symbols connected by arrows called flowlines

Rectangle symbol (action symbol)

– Indicates any type of action Diamond symbol (decision symbol)

– Indicates that a decision to be made Oval symbol:

– Indicates beginning or end of a program, or a section of code (circles)

add grade to total

add 1 to counter

total = total + grade;

counter = counter + 1;

Indicates a portion of an algorithm

Jozef Goetz, 2015


add grade to total

add 1 to counter

total = total + grade;

counter = counter + 1;

Fig. 5.1 Flowcharting C#’s sequence structure.

•Flowchart of sequence structure: Two actions performed in order

When drawing a portion, small circles is used

When flowcharting a complete algorithm

Oval containing "Begin" is first symbol

Oval containing "End" is last symbol

When drawing a portion, small circles usedFig. 6.20 C#’s single-entry/single-exit sequence, selection and repetition structures. (part 1)

Sequence

.

.

Jozef Goetz, 2015

125.4 Control Structures Sequential execution

Program statements execute one after the other

Transfer of control Next statement executed is not the next one in

sequence. Overuse of goto in 1960's led to many problems

structured programming became almost synonymous with “goto” elimination

C# has a goto statement, but it is used to transfer control out of a nested scope

Three control statements can specify order of statements Sequence structure Selection structure Repetition structure

Bohm and Jacopini All programs can be written in terms of 3 control

structures Sequence, Selection and Repetition

Jozef Goetz, 2015


8 control structures C# and Java has a sequence structure “built-in” C# provides three selection structures

if if/else (double selection) Switch

C# provides four (Java first three, VB.NET seven) repetition structures Repeatedly performs an action while its loop-continuation condition remains true while - performs the actions in its body zero or more times do/while - performs the actions in its body one or more times for - performs the actions in its body zero or more times foreach (specific for C#, VB.NET) - - performs the actions in its body zero

or more times

Each of these words is a C# keyword Words reserved for C# don’t use as identifiers or variable names

Jozef Goetz, 2015


Single-entry/single-exit control structures make it easy to build programs

Connect exit point of one control structure to entry point of the next (control-structure stacking)

Assemble program (connect control structures) by

Stacking– Placing one after another

Nesting– Inserting of one structure into another

Any these control structures can be combined in only 2 ways: control-structure stacking and nesting.

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15

6.9 Structured-Programming Summary

Fig. 6.20 | C#’s single-entry/single-exit sequence, selection and repetition statements.

Jozef Goetz, 2015

16 Structured Programming Summary

Stacked, nested and overlapped building blocks.

Stacked building blocks

Overlapping building blocks (illegal in structured programs)

Nested building blocks

Combination of control structures:

1. Stacking

Placing one after another

2. Nesting

Inserting of one structure into another

Jozef Goetz, 2015

175.4 Control Structures (Cont.)

UML activity diagram (www.uml.org) Models the workflow (or activity) of a part of a software system Action-state symbols (rectangles with their sides replaced with outward-curving arcs)

Represent action expressions specifying actions to perform

Diamonds Decision symbols (explained in section 5.5) Merge symbols (explained in section 5.7)

Small circles Solid circle represents the activity’s initial state Solid circle surrounded by a hollow circle represents the activity’s final state

Transition arrows Indicate the order in which actions are performed

Notes (rectangles with the upper-right corners folded over) Explain the purposes of symbols (like comments in C#) Arc connected to the symbols they describe by dotted lines

<= Guard conditions

Jozef Goetz, 2015

18C# keywords

C# Keywords

abstract as base bool break byte case catch char checked class const continue decimal default delegate do double else enum event explicit extern false finally fixed float for foreach get goto if implicit in int interface internal is lock long namespace new null object operator out override params private protected public readonly ref return sbyte sealed set short sizeof stackalloc static string struct switch this throw true try typeof uint ulong unchecked unsafe ushort using value virtual void volatile while Fig. C# keywords.

Jozef Goetz, 2015

195.5 if Selection Structure

The if structure Causes the program to make a selection

Chooses based on conditional Any expression that evaluates to a bool type True: perform an action False: skip the action

Single entry/exit point Require no semicolon in syntax

Jozef Goetz, 2015

205.5 The if Selection Structure

Selection structure Psuedocode statement

If student’s grade is greater than or equal to 60 Print “Passed”

code statement in C#if ( studentGrade >= 60 ) Console.WriteLine( "Passed" );

code statement in Javaif ( studentGrade >= 60 )

System.out.println( "Passed" );

Notice similarity

Jozef Goetz, 2015

21Fig. 5.2 | if single-selection statement UML activity diagram.

true

false

grade >= 60 print “Passed”

Flowchart example of the if selection structure

“if” is a single-entry/single-exit structure

Diamond symbol - decision symbol has two paths

Perform action only when condition is true

Important - indicates an decision is to be made

Decision can be made on anything that evaluates a value of data type boolean

true or false

Single-entry/single-exit structure

UML

Jozef Goetz, 2015

22Action/decision model of computing

1. Programmer assemble structures by stacking and nesting

2. Then defines actions and decisions

Jozef Goetz, 2015

235.6 The if/else Selection Structure

Selection structures if

Only performs an action if condition true

if/else Performs different action when condition true than when

condition false Psuedocode

If student’s grade is greater than or equal to 60Print “Passed”

elsePrint “Failed”

C# code: if ( studentGrade >= 60 )

Console.WriteLine( "Passed" ); else

Console.WriteLine( "Failed" );

Note spacing/indentation conventions

Indent both body statements of an if...else statement.

Jozef Goetz, 2015


Flowchart of if/else structure Symbols (besides circles and arrows)

Rectangles: actions Diamonds: decisions

truefalse

print “Failed” print “Passed”

grade >= 60

Fig. 5.3 Flowcharting a double-selection if/else structure.

Jozef Goetz, 2015


Ternary conditional operator (?:) Takes three arguments

(boolean value ? if true : if false) C#’s only ternary operator Similar to an if/else structure

Our pseudocode could be written:

Console.WriteLine( studentGrade >= 60 ? “Passed” : “Failed” );

Jozef Goetz, 2015


Nested if/else structures Test for multiple cases Place if/else structures inside if/else structures If first condition met, other statements skipped

Nested structures: if ( studentGrade >= 90 )

Console.WriteLine( "A" );else if ( studentGrade >= 80 ) Console.WriteLine( "B" ); else if ( studentGrade >= 70 ) Console.WriteLine( "C" ); else if ( studentGrade >= 60 ) Console.WriteLine( "D" ); else Console.WriteLine( "F" );

Avoid deep indentation

else only executes when the if condition fails.

Once condition is met, rest of statements skipped.

Jozef Goetz, 2015


Alternate form of nested structures Avoids deep indentation Preferred to previous format

if ( grade >= 90 ) Console.WriteLine( "A" );else if ( grade >= 80 ) Console.WriteLine( "B" );else if ( grade >= 70 ) Console.WriteLine( "C" );else if ( grade >= 60 ) Console.WriteLine( "D" );else Console.WriteLine( "F" );

As before, else only executes when the if condition fails.

Jozef Goetz, 2015


Important note C# associates else with previous if unless

braces ({}) present

Dangling-else problemx = 2; y = 2;

if ( x > 5 ) if ( y > 5 ) Console.WriteLine( "x and y are > 5" );else Console.WriteLine( "x is <= 5" );

Does not execute as it appears, executes as if ( x > 5 )

if ( y > 5 ) Console.WriteLine("x and y are > 5" ); else Console.WriteLine("x is <= 5" );

If x <= 5, nothing is output,

Is logic OK?

Jozef Goetz, 2015


Important note Must force structure to execute as intended

Use braces to indicate that second if in body of first if ( x > 5 )

{ if ( y > 5 ) Console.WriteLine("x and y are > 5" );}

else Console.WriteLine( "x is <= 5" );

Block statement (sometimes called a compound statement) Set of statements within braces

Can be used wherever a single statement can

if expects one statement in its body enclose multiple statements in braces to have one equivalent one

statement

Jozef Goetz, 2015


Block Example:

if (grade >= 60) Console.WriteLine( "Passed" );else

{

Console.WriteLine( "Failed" ); Console.WriteLine( "You must take this course again." );}

Without braces, third WriteLine always executes

Block can contain declarations Example: body of Main

Jozef Goetz, 2015


Errors

Syntax errors Caught by compiler Example: forgetting a brace in a block

Logic errors Have effect at execution time Non-fatal: program runs, but has incorrect output Fatal: program exits prematurely

Jozef Goetz, 2015

325.7 The while Repetition Structure while repetition structure

A Repeat action while some condition remains true while loop repeated until condition becomes false

Psuedocode:

While there are more items on my shopping list Purchase next item and cross it off my list

First statement after repetition structure executed

Body may be a single or block statement If condition initially false, body never executed

If there is no body what happens?

product <= 1000 product = 2 * producttrue

false

Find the first power of 2 larger than 1000

Jozef Goetz, 2015

33

Fig. 5.4 | while repetition statement UML activity diagram.

product <= 100 product = 3 * producttrue

false

Find the first power of 3 larger than 100

int product = 3;

while ( product <= 100 ) product = 3 * product;

Jozef Goetz, 2015

34Common Programming Error 5.3

Not providing in the body of a while statement an action that eventually causes the condition in the while to become false normally results in a logic error called an

infinite loop, in which the loop never terminates.

Jozef Goetz, 2015

355.8 Formulating Algorithms: Case

Study 1 (Counter Controlled Repetition- CCR)

Counter Controlled Repetition Used to enter data one at a time

Constant amount A counter is used to determine when the loop should break

When counter hits certain value, the statement terminates

Number of repetitions known

Jozef Goetz, 2015

365.8 Formulating Algorithms: Case

Study 1 (Counter Controlled Repetition)

•Illustrate how algorithms are developed - Consider class averaging problem

•Problem statement:

A class of 10 students took a quiz. The grades (integers in the range 0 to 100) for this quiz are available to you.

Determine the class average on the quiz.

Jozef Goetz, 2015

375.8 Formulating Algorithms: Case Study 1 (Counter Controlled Repetition)

Set total to zero

Set grade counter to one

While grade counter is less than or equal to tenInput the next gradeAdd the grade into the totalAdd one to the grade counter

Set the class average to the total divided by tenPrint the class average

Total - used to accumulate sum of a series of values Initialize to zero to clear contents

Counter - variable used to count

Fig. 5.5 Pseudocode algorithm that uses counter-controlled repetition to solve the class-average problem.

A total is a variable in which a script accumulates the sum of a series of values

•Variables that store totals should normally be initialized to zero before they are used in a program

2002 Prentice Hall.All rights reserved.

Outline38

1 // Fig. 4.7 ed1: Average1.cs2 // Class average with counter-controlled repetition.3 4 using System;5 6 class Average17 {8 static void Main( string[] args )9 {10 int total, // sum of grades11 gradeCounter, // number of grades entered12 gradeValue, // grade value13 average; // average of all grades14 15 // 1. initialization phase16 total = 0; // clear total17 gradeCounter = 1; // prepare to loop18 19 // 2. processing phase20 while ( gradeCounter <= 10 ) // loop 10 times21 {22 // prompt for input and read grade from user23 Console.Write( "Enter integer grade: " );24 25 // read input and convert to integer26 gradeValue = Int32.Parse( Console.ReadLine() );27 28 // add gradeValue to total29 total = total + gradeValue;30 31 // add 1 to gradeCounter32 gradeCounter = gradeCounter + 1;33 }

The while loop will loop through 10 times to get the grades of the 10 students

Initialize gradeCounter to 1

Accumulate the total of the 10 grades

Add 1 to the counter so the loop will eventually end

Initialize total to 0

Prompt the user to enter a grade

Set total to zeroSet grade counter to one

While grade counter is less than or equal to tenInput the next gradeAdd the grade into the totalAdd one to the grade counter



Outline39

Average1.cs

Program Output

34 35 // 3. termination phase36 average = total / 10; // integer division37 38 // display average of exam grades39 Console.WriteLine( "\nClass average is {0}", average );40 41 } // end Main42 43 } // end class Average1

Enter integer grade: 100Enter integer grade: 88Enter integer grade: 93Enter integer grade: 55Enter integer grade: 68Enter integer grade: 77Enter integer grade: 83Enter integer grade: 95Enter integer grade: 73Enter integer grade: 62 Class average is 79

Divide the total by ten to get the average of the ten grades

Display the results

Jozef Goetz, 2015

40 1 // Fig. 5.6: GradeBookTest.cs

2 // Create GradeBook object and invoke its DetermineClassAverage method.

3 public class GradeBookTest

4 {

5 public static void Main( string[] args )

6 {

7 // create GradeBook object myGradeBook and

8 // pass course name to constructor

9 GradeBook myGradeBook = new GradeBook(

10 "CS101 Introduction to C# Programming" );

11

12 myGradeBook.DisplayMessage(); // display welcome message

13 myGradeBook.DetermineClassAverage(); // find average of 10 grades

14 } // end Main

15 } // end class GradeBookTest

Welcome to the grade book for CS101 Introduction to C# Programming! Enter grade: 88 Enter grade: 79 Enter grade: 95 Enter grade: 100 Enter grade: 48 Enter grade: 88 Enter grade: 92 Enter grade: 83 Enter grade: 90 Enter grade: 85

Total of all 10 grades is 848 Class average is 84

Outline

GradeBookTest.cs

Jozef Goetz, 2015

1 // Fig. 5.6: GradeBook.cs - class and driver approach (expanded Fig. 4.12)

2 // GradeBook class that solves class-average problem using

3 // counter-controlled repetition.

4 using System;

5

6 public class GradeBook

7 {

8 //private string courseName; // name of course this GradeBook represents

9

10 // constructor initializes courseName

11 public GradeBook( string name )

12 {

13 CourseName = name; // initializes courseName by using property

14 } // end constructor

15

16 // property to get and set the course name 17 public string CourseName { get; set; } 18 // end property CourseName

GradeBook.cs (1 of 3)

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4228

29 // display a welcome message to the GradeBook user

30 public void DisplayMessage()

31 {

32 // property CourseName gets the name of the course

33 Console.WriteLine( "Welcome to the grade book for\n{0}!\n",

34 CourseName );

35 } // end method DisplayMessage

36

37 // determine class average based on 10 grades entered by user

38 public void DetermineClassAverage()

39 {

40 int total; // sum of the grades entered by user

41 int gradeCounter; // number of the grade to be entered next

42 int grade; // grade value entered by the user

43 int average; // average of the grades

44

45 // initialization phase

46 total = 0; // initialize the total

47 gradeCounter = 1; // initialize the loop counter

GradeBook.cs (2 of 3)


While grade counter is less than or equal to ten

Input the next gradeAdd the grade into the totalAdd one to the grade counter


Welcome to the grade book forCS101 Introduction to C# Programming!Enter grade: 88Enter grade: 79Enter grade: 95Enter grade: 100Enter grade: 48Enter grade: 88Enter grade: 92Enter grade: 83Enter grade: 90Enter grade: 85Total of all 10 grades is 848Class average is 84

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4348

49 // processing phase

50 while ( gradeCounter <= 10 ) // loop 10 times

51 {

52 Console.Write( "Enter grade: " ); // prompt the user

53 grade = Convert.ToInt32( Console.ReadLine() ); // read grade

54 total = total + grade; // add the grade to total

55 gradeCounter = gradeCounter + 1; // increment the counter by 1

56 } // end while

57

58 // termination phase

59 average = total / 10; // integer division yields integer result

60

61 // display total and average of grades

62 Console.WriteLine( "\nTotal of all 10 grades is {0}", total );

63 Console.WriteLine( "Class average is {0}", average );

64 } // end method DetermineClassAverage

65 } // end class GradeBook

Outline GradeBook.cs

(3 of 3)

Welcome to the grade book forCS101 Introduction to C# Programming!Enter grade: 88Enter grade: 79Enter grade: 95Enter grade: 100Enter grade: 48Enter grade: 88Enter grade: 92Enter grade: 83Enter grade: 90Enter grade: 85Total of all 10 grades is 848Class average is 84





Exc. Replace hard coded values 10 by variable

Jozef Goetz, 2015




4 {


6 {





11


13 myGradeBook.DetermineClassAverage(); // find average of 10 grades

14 } // end Main

15 } // end class GradeBookTest

Welcome to the grade book for CS101 Introduction to C# Programming! Enter grade: 88 Enter grade: 79 Enter grade: 95 Enter grade: 100 Enter grade: 48 Enter grade: 88 Enter grade: 92 Enter grade: 83 Enter grade: 90 Enter grade: 85

Total of all 10 grades is 848 Class average is 84

Outline

GradeBookTest.cs

Jozef Goetz, 2015

45Good Programming Practice

Separate declarations from other statements in methods with a blank line for readability.

All local variables must be definitely assigned before their values are used in expressions.

Initialize each counter and total, either in its declaration or in an assignment statement. Totals are normally initialized to 0. Counters are normally

initialized to 0 or 1, depending on how they are used (we will show examples of each).

Jozef Goetz, 2015

465.9 Formulating Algorithms with Top-

Down, Stepwise Refinement: Case Study 2 (Sentinel-Controlled

Repetition - SCR) Let us generalize the class-average problemDevelop a class-averaging program that will process an arbitrary number of grades each time the program is run Unknown number of students - how will the program end?

Sentinel value Also called signal dummy value or flag value Indicates “end of data entry” Loop ends when sentinel is entered Sentinel chosen so it cannot be a regular input (an acceptable input

value) -1 in this case

Average2.cs has indefinite repetition User enters sentinel value (-1) to end repetition

Jozef Goetz, 2015

475.9 Sentinel-Controlled Repetition

Top-down, stepwise refinement – technique essential to the development of well-structured algorithms Begin with pseudocode representation of the top

Determine the class average for the quiz Complete representation of program

Top usually too general - must be refined Divide top into a series of smaller tasks and list them in the

order in which they need to be performed

First refinementInitialize variablesInput, sum and count the quiz gradesCalculate and print the class average

Jozef Goetz, 2015

48Software Engineering Observation 5.2

Each refinement, as well as the top itself, is a complete specification of the algorithm — only the level of detail varies.

Jozef Goetz, 2015


Refine "Initialize variables" to Initialize total to zero

Initialize counter to zero

Refine "Input, sum and count the quiz grades" to Input the first grade (possibly the sentinel)

While the user has not as yet entered the sentinelAdd this grade into the running totalAdd one to the grade counterInput the next grade (possibly the sentinel)

Refine "Calculate and print the class average" toIf the counter is not equal to zero

Set the average to the total divided by the counterPrint the average

elsePrint “No grades were entered”

Jozef Goetz, 2015

50

Initialize total to zeroInitialize counter to zero

Input the first grade (possibly the sentinel)

While the user has not as yet entered the sentinel Add this grade into the running totalAdd one to the grade counter

Input the next grade (possibly the sentinel)

If the counter is not equal to zeroSet the average to the total divided by the counterPrint the average

elsePrint “No grades were entered”

Fig. 5.8 Pseudocode algorithm that uses sentinel-controlled repetition to solve the class-average problem.

Set total to zero

Set grade counter to 1




Fig. 5.5 Pseudocode algorithm that uses counter-controlled repetition to solve the class-average problem.

Total - used to accumulate sum of a series of values

Initialize to zero to clear contents Counter - variable used to count

Jozef Goetz, 2015


Terminate the top-down:

stepwise refinement process terminates when you have specified the pseudocode algorithm in sufficient detail for you to convert the pseudocode to C#.

Normally, implementing the C# application is then straightforward.

Jozef Goetz, 2015


Many applications (programs) can be divided into three phases1. Initialization

Initializes the program variables

2. Processing Inputs data values, adjusts variables accordingly and calculates

3. Termination Prints the final results

Helps breakup of programs for top-down refinement

Upcoming program Use sentinel-controlled loop to calculate average Show program, then discuss


Outline53

Average2.cs

1 // Fig. 4.9 ed1: Average2.cs2 // Class average with sentinel-controlled repetition.3 4 using System;5 6 class Average27 {8 static void Main( string[] args )9 {10 int total, // sum of grades11 gradeCounter, // number of grades entered12 gradeValue; // grade value13 14 double average; // average of all grades15 // note – double value may be only an approximation 16 // 1. initialization phase17 total = 0; // clear total18 gradeCounter = 0; // prepare to loop19 20 // 2. processing phase21 // prompt for input and convert to integer22 Console.Write( "Enter Integer Grade, -1 to Quit: " );23 gradeValue = Int32.Parse( Console.ReadLine() );24

The variable average is set to a double so that it can be more exact and have an answer with decimals

Variables gradeCounter and total are set to zero at the beginning

Get a value from the user and store it in gradeValue

Initialize total to zeroInitialize counter to zero

Input the first grade (possibly the sentinel)


Outline54

25 // loop until a -1 is entered by user26 while ( gradeValue != -1 )27 {28 // add gradeValue to total29 total = total + gradeValue;30 31 // add 1 to gradeCounter32 gradeCounter = gradeCounter + 1;33 34 // prompt for input and read grade from user35 // convert grade from string to integer36 Console.Write( "Enter Integer Grade, -1 to Quit: " );37 gradeValue = Int32.Parse( Console.ReadLine() );38 39 } // end while40 41 // 3. termination phase42 // Make sure the total amount of entered grades was not 0 to prevent any errors42 if ( gradeCounter != 0 ) 43 {44 average = ( double ) total / gradeCounter; //double version of total is divided by the int

gradeCounter. gradeCounter is promoted to double; the calculation is performed and the result of the floating-point division is assigned to average

45 46 // display average of exam grades47 Console.WriteLine( "\nClass average is {0}", average );48 }49 else50 {51 Console.WriteLine( "\nNo grades were entered" );52 }53 54 } // end method Main55 56 } // end class Average2

Have the program loop as long as gradeValue is not -1

Prompt the user for another grade, this time it is in the loop so it can happen repeatedly

Divide the total by the number of times the program looped to find the average

Display the average

Inform user if no grades were entered

Enter Integer Grade, -1 to Quit: 97Enter Integer Grade, -1 to Quit: 88Enter Integer Grade, -1 to Quit: 72Enter Integer Grade, -1 to Quit: -1 Class average is 85.6666666666667

While the user has not as yet entered the sentinel Add this grade into the running total Add one to the grade counter Input the next grade (possibly the sentinel)

If the counter is not equal to zero Set the average to the total divided by the counter Print the averageelse Print “No grades were entered”

Jozef Goetz, 2015


Arithmetic Can only be performed between variables of same type Promotion (implicit conversion) - promote a data type to

another type

if ( gradeCounter != 0 ) { average = ( double ) total / gradeCounter; }

Cast operator (double) Creates a temporary floating-point copy of its

operand (total)– Using a cast operator this manner is called

explicit conversion– Value in total still an integer

Jozef Goetz, 2015

565.9 Sentinel-Controlled Repetitionif ( gradeCounter != 0 ) { average = ( double ) total / gradeCounter;

A temporary copy of total is a double gradeCounter (an int) is promoted to a double

Cast operators Available for all data types

– Parenthesis around type name: (type) Unary operator (one operand) - associates right to left

Floating point numbers are represented approximately by computers (#s are not precisely represented)

Not 100% accurate– Good approximation, fine for most applications

Floating number 10/3 = 3.333333... with the sequence of 3’s repeating infinitely– The computer allocates a fixed amount of space - finite precision, so

• double value may be only an approximation – Don’t compare floating point values for equality and

inequality,

– test the absolute value of the difference

Jozef Goetz, 2015

57Common Programming Error 5.4

Assuming that integer division rounds down (rather than truncates) can lead to incorrect results. For example, 7 / 4, which yields 1.75 in

conventional arithmetic, truncates to 1 in integer arithmetic, rather than rounding to 2.

Jozef Goetz, 2015




4 {


6 {





11


13 myGradeBook.DetermineClassAverage(); // find average of grades

14 } // end Main

15 } // end class GradeBookTest Welcome to the grade book for CS101 Introduction to C# Programming!

Enter grade or -1 to quit: 96 Enter grade or -1 to quit: 88 Enter grade or -1 to quit: 79 Enter grade or -1 to quit: -1

Total of the 3 grades entered is 263 Class average is 87.67

Outline

GradeBookTest.cs

Good Programming Practice 5.7 In a sentinel-controlled loop,

the prompts requesting data entry should explicitly remind the user of the sentinel value.

Exc. Expand method DetermineClassAverage() in the way that the user is prompted if the number of grades is known or is unknown. Provide the piece of code to process both cases.

Jozef Goetz, 2015

59

1 // Fig. 5.9: GradeBook.cs

2 // GradeBook class tlhat solves class-average problem using

3 // sentinel-controled repetition. 4 using System; 5 6 public class GradeBook 7 { 8 // autoimplemented property CourseName 9 public string CourseName { get; set; } 10 11 // constructor initializes the CourseName property 12 public GradeBook( string name ) 13 { 14 CourseName = name; // set CourseName to name 15 } // end constructor 17 27

Outline

GradeBook.cs

( 1 of 3 )

• Class GradeBook now implements the algorithm for sentinel-controlled repetition (Fig. 5.9)

Fig. 5.9 | GradeBook class that solves the class-average problemusing sentinel-controlled repetition. (Part 1 of 3.)

Jozef Goetz, 2015

6028

29 // display a welcome message to the GradeBook user

30 public void DisplayMessage()

31 {

32 Console.WriteLine( "Welcome to the grade book for\n{0}!\n",

33 CourseName );

34 } // end method DisplayMessage

35

36 // determine the average of an arbitrary number of grades

37 public void DetermineClassAverage()

38 {

39 int total; // sum of grades

40 int gradeCounter; // number of grades entered

41 int grade; // grade value

42 double average; // number with decimal point for average

43

44 // initialization phase

45 total = 0; // initialize total

46 gradeCounter = 0; // initialize loop counter

47

48 // processing phase

49 // prompt for input and read grade from user

50 Console.Write( "Enter grade or -1 to quit: " );

51 grade = Convert.ToInt32( Console.ReadLine() );

52

53 // loop until sentinel value read from user

54 while ( grade != -1 )

55 {

56 total = total + grade; // add grade to total

57 gradeCounter = gradeCounter + 1; // increment counter

Outline

GradeBook.cs

(2 of 3)Welcome to the grade book forCS101 Introduction to C# Programming!Enter grade or -1 to quit: 96Enter grade or -1 to quit: 88Enter grade or -1 to quit: 79Enter grade or -1 to quit: -1Total of the 3 grades entered is 263Class average is 87.67

Input the first grade (possibly the sentinel)While the user has not as yet entered the sentinel Add this grade into the running total Add one to the grade counter Input the next grade (possibly the sentinel)


Jozef Goetz, 2015

6158

59 // prompt for input and read next grade from user

60 Console.Write( "Enter grade or -1 to quit: " );

61 grade = Convert.ToInt32( Console.ReadLine() );

62 } // end while

63

64 // termination phase

65 // if user entered at least one grade...

66 if ( gradeCounter != 0 )

67 {

68 // calculate average of all grades entered

69 average = ( double ) total / gradeCounter;

70

71 // display total and average (with two digits of precision)

72 Console.WriteLine( "\nTotal of the {0} grades entered is {1}",

73 gradeCounter, total );

74 Console.WriteLine( "Class average is {0:F2}", average );

75 } // end if F2 – floating point with 2 decimal decimal places

76 else // no grades were entered, so output error message

77 Console.WriteLine( "No grades were entered" );

78 } // end method DetermineClassAverage

79 } // end class GradeBook

Outline

GradeBook.cs

(3 of 3)

Welcome to the grade book forCS101 Introduction to C# Programming!Enter grade or -1 to quit: 96Enter grade or -1 to quit: 88Enter grade or -1 to quit: 79Enter grade or -1 to quit: -1Total of the 3 grades entered is 263Class average is 87.67

While the user has not as yet entered the sentinel Add this grade into the running total Add one to the grade counter Input the next grade (possibly the sentinel)


Exc. Draw a flowchart for method DetermineClassAverage()

Jozef Goetz, 2015


Some experienced programmers write applications

without ever using application-development

tools like pseudocode or flowcharts.

They feel that their ultimate goal is to solve the problem on a computer and that writing pseudocode merely delays the production of final outputs.

Although this method may work for simple and familiar problems, it can lead to serious errors and delays in large, complex projects.

Jozef Goetz, 2015

635.10 Formulating Algorithms with Top-Down, Stepwise Refinement: Case

Study 3 (Nested Control Structures)

Nesting The insertion of one control structure inside another

Multiple loops Loops with if statements

Jozef Goetz, 2015

645.10Nested Control Structures Problem statement

A college has a list of test results (1 = pass, 2 = fail) for a licensing exam for 10 students. Write a program that analyzes the results and print the number of passes and failures. If more than 8 students pass, print "Raise Tuition".

Program should analyze results as follows: Input each test result (1 or 2). Display message

"Enter result" on screen each time program requests another result.

Count number of results of each type Display summary of test results - indicate how

many students passed and how many failed If more than 8 passed, print "Raise tuition"

Jozef Goetz, 2015

655.10Nested Control Structures

Observations Program must process 10 test results

Use counter-controlled loop 3 counters can be used

One for passes, one for fails Student counter for # of students

Each test result is either a 1 (pass) or a 2 (fail) If not a 1, assume it is a 2

Must decide if more than 8 passes

Psuedocode representation of top Analyze exam results and decide if tuition should be raised Top complete representation of problem

May need several refinements

Jozef Goetz, 2015


First refinement1. Initialize variables

2. Input the ten quiz grades and count passes and failures

3. Print a summary of the exam results and decide if tuition should be raised

1. Refine "Initialize variables" to

Initialize passes to zeroInitialize failures to zeroInitialize student counter to 1

Jozef Goetz, 2015


2. Refine "Input the ten quiz grades and count passes and failures" to

While student counter is less than or equal to ten Input the next exam result

if the student passed

Add one to passeselse Add one to failures

Add one to student counter

Jozef Goetz, 2015


3. Refine "Print a summary of the exam results and decide if tuition should be raised" toPrint the number of passesPrint the number of failures

If more than eight students passed Print “Raise tuition”

Psuedocode sufficiently refined Can be converted to a C# program

Jozef Goetz, 2015

69Initialize passes to zeroInitialize failures to zeroInitialize student counter to 1


If the student passed Add one to passes

else Add one to failures


Print the number of passesPrint the number of failures


Fig 5.11 Pseudocode for examination-results problem.

Jozef Goetz, 2015

70 1 // Fig. 5.12: Analysis.cs

2 // Analysis of examination results, using nested control statements.

3 using System;

4

5 public class Analysis

6 {

7 public void ProcessExamResults()

8 {

9 // initializing variables in declarations

10 int passes = 0; // number of passes

11 int failures = 0; // number of failures

12 int studentCounter = 1; // student counter

13 int result; // one exam result from user

14

15 // process 10 students using counter-controlled repetition

16 while ( studentCounter <= 10 )

17 {

18 // prompt user for input and obtain value from user

19 Console.Write( "Enter result (1 = pass, 2 = fail): " );

20 result = Convert.ToInt32( Console.ReadLine() );

21

22 // if...else nested in while

23 if ( result == 1 ) // if result 1,

24 passes = passes + 1;

25 else // else result is not 1, so

26 failures = failures + 1; // increment failures

27

28 // increment studentCounter so loop eventually terminates

29 studentCounter = studentCounter + 1;

30 } // end while 31 32 // termination phase; prepare and display results

33 Console.WriteLine( "Passed: {0}\nFailed: {1}", passes, failures );

34

35 // determine whether more than 8 students passed

36 if ( passes > 8 )

37 Console.WriteLine( "Raise Tuition" );

38 } // end method ProcessExamResults

39 } // end class Analysis

Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 2Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 1Passed: 9Failed: 1Raise Tuition

Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 2Enter result (1 = pass, 2 = fail): 2Enter result (1 = pass, 2 = fail): 2Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 1Enter result (1 = pass, 2 = fail): 2Enter result (1 = pass, 2 = fail): 2Passed: 5Failed: 5

Initialize passes to zeroInitialize failures to zeroInitialize student counter to one


If the student passed Add one to passes

else Add one to failures


Print the number of passesPrint the number of failures


Jozef Goetz, 2015

71 1 // Fig. 5.13: AnalysisTest.cs

2 // Test application for class Analysis.

3 public class AnalysisTest

4 {


6 {

7 Analysis application = new Analysis(); // create Analysis object

8 application.ProcessExamResults(); // call method to process results

9 } // end Main

10 } // end class AnalysisTest Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 2 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Passed: 9 Failed: 1 Raise Tuition Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 2 Enter result (1 = pass, 2 = fail): 2 Enter result (1 = pass, 2 = fail): 2 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 1 Enter result (1 = pass, 2 = fail): 2 Enter result (1 = pass, 2 = fail): 2 Passed: 5 Failed: 5

Outline

AnalysisTest.cs

Jozef Goetz, 2015

725.11 Compound Assignment Operators

Compound assignment operators Can reduce code

x += 2 is the same as x = x + 2 Can be done with all the math operators

+=, -=, *=, /=, and %=

Statements of the formvariable = variable operator expression;

can be rewritten asvariable operator= expression;

Other examples d -= 4 (d = d - 4)

e *= 5 (e = e * 5)f /= 3 (f = f / 3)g %= 9 (g = g % 9)

Compiler runs a bit faster

Jozef Goetz, 2015

735.11 Assignment Operators

Assignment operator Sample expression Explanation Assigns Assume: int c = 3, d = 5, e = 4, f = 6, g = 12;

+= c += 7 c = c + 7 10 to c

-= d -= 4 d = d - 4 1 to d

*= e *= 5 e = e * 5 20 to e

/= f /= 3 f = f / 3 2 to f

%= g %= 9 g = g % 9 3 to g

Fig. 5.13 Arithmetic assignment operators.

Jozef Goetz, 2015

745.12 Increment and Decrement Operators Unary (unry) increment operator (++)

Increment variable’s value by 1 Increment operator (++) i.e. i++

Can be used instead of c += 1

Unary decrement operator (--) Decrement variable’s value by 1 Decrement operator (--) i.e. i--

Can be used instead of c -= 1

Pre-increment vs. post-incrementc++ or c--

Will perform an action in which c resides and then add to or subtract one from the value

++c or --c Will add to or subtract one from the value and then perform an action in which c

resides

++(c + 1) – a syntax error, b. c + 1 is not a variable reference

Jozef Goetz, 2015

755.12 Increment and Decrement Operators

Operator Called Sample expression Explanation

++ preincrement ++a Increment a by 1, then use the new value of a in the expression in which a resides.

++ postincrement a++ Use the current value of a in the expression in which a resides, then increment a by 1.

-- predecrement --b Decrement b by 1, then use the new value of b in the expression in which b resides.

-- postdecrement b-- Use the current value of b in the expression in which b resides, then decrement b by 1.

Fig. 5.14 The increment and decrement operators.

Good Programming Practice 5.7 Unlike binary operators, the unary increment and decrement operators should (by convention) be placed next to their operands, with no intervening spaces.


Outline76

Increment.cs

Program Output

1 // Fig. 5.15: Increment.cs2 // Preincrementing and postincrementing3 4 using System;5 6 class Increment7 {8 static void Main(string[] args)9 { 10 int c;11 12 c = 5;13 Console.WriteLine( c ); // print 514 Console.WriteLine( c++ ); // print 5 then postincrement

// (then action in which c resides)15 Console.WriteLine( c ); // print 616 17 Console.WriteLine(); // skip a line18 19 c = 5;20 Console.WriteLine( c ); // print 521 Console.WriteLine( ++c ); // preincrement(first action in which c resides then print 622 Console.WriteLine( c ); // print 623 24 } // end of method Main25 26 } // end of class Increment

556 566

Declare variable cSet c equal to 5

Jozef Goetz, 2015

77Fig. 5.17 | Precedence and associativity of the operators discussed so far - see appendix A

p.951.

Operators Associativity Type

. new ++(postfix) --(postfix) left to right Highest precedence

++ -- + - (type) right to left unary prefix * / % left to right multiplicative

+ - left to right additive

< <= > >= left to right relational

== != left to right equality

?: right to left conditional

= += -= *= /= %= right to left assignment

Jozef Goetz, 2015

78

• The table in Appendix B p. 953 , Simple Types, lists the 13 simple types in C#.

• C# requires all variables to have a type.

• Instance variables of types char, byte, sbyte, short, ushort, int, uint, long, ulong, float (4

bytes), double (8 bytes), and decimal (16 bytes), are all given the value 0 by default.

• Instance variables of type bool are given the value false by default.

• Reference type instance variables are initialized by default to the value null

5.13 Simple Types

Jozef Goetz, 2015

Jozef Goetz, 2015

80Flowchart of the calculation of sum = 1 + x/ 1! + /2! + /3! + … + …+ x^n/n!

Values n (= number of terms without the first one)and x should be prompted to enter.

x 2 x 3

Convert the previous flowchart => to a new one

Modify the above flowchart for x = 1

Documents

Jozef Goetz, 2015 1 2011 - 2015 Pearson Education, Inc. All rights reserved. 2002 Prentice Hall. All rights reserved. expanded by J. Goetz, 2014 credits: