CS241 PASCAL I - Control Structures1 PASCAL Control Structures Modified Slides of Philip Fees

CS241 PASCAL I - Control Structures

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PASCAL Control StructuresPASCAL Control Structures

Modified Slides of Philip Fees


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Reserved WordsReserved Words

Cannot be used for identifiers (names) of

constant,

type,

variable,

function,

procedure


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Basic Data TypesBasic Data Types

integer (-maxint <= integer <= maxint) real (4567.123, 4.567123 x 103,

4.567123E3) char (‘a’, ‘7’, ‘W‘, . . .) string constant (const name = ‘Cyprus’;)


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Basic OutputBasic Output

write - output without newline terminator writeln - output with newline terminator Writeln (expr1[:n[:n]], expr2[:n[:n]], … ,

exprN[:n[:n]]);


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TopicsTopics

Arithmetic Operators Basic Debugging (tracing and print) Basic Input Constants Standard Functions


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Arithmetic OperatorsArithmetic Operators

( ), *, MOD, DIV, /, +, - What is an operator? Precedence - evaluation of sequence of

multiple operator expressions Typically left to right, but can be inside

to outside or right to left.


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Basic InputBasic Input

Variables - name vs. memory location Data pointer

– location of next data item to be read from input stream

read - input next data item readln - input next data item, skip to

next line


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Constants and variablesConstants and variables

CONSTpi = 3.14;

VARarea, radius : real;

. . .area := 3.14 * radius * radius;

. . .area := pi * radius * radius;


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Standard FunctionsStandard Functions

What is a function– perform some operation on argument(s)– returns value(s) as determined by

operation Example

area := pi * sqr(radius);


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TopicsTopics

Boolean Logic– Relational Operators– Boolean Expressions

IF . . . THEN . . . ELSE Statements CASE Statements


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Boolean ExpressionBoolean Expression

Relational Operators

(=, <, >, <=, >=, <>)

used to build Boolean Expressions

Logical Operators

(AND, OR, NOT)

used to build Compound Boolean Expressions


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IF . . . THEN StatementsIF . . . THEN Statements

A decision-making statement

Syntax:IF <boolean expression> THEN

<statement>


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Compound StatementsCompound Statements

Treats a set of statements as one Syntax:

IF <boolean expression> THEN BEGIN

<statement1>;<statement2>;...<statementN>;

END;


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IF . . . THEN . . . ELSE IF . . . THEN . . . ELSE StatementsStatements

Action to occur only if a boolean statement is false


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Nested IF StatementsNested IF Statements

When the <statement> part of IF or ELSE is an IF THEN ELSE statement

Nested IF ELSE is considered as a single statement and doesn’t require BEGIN END


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CASE StatementCASE Statement

Abbreviated IF THEN, ELSE IF THEN, etc statement

Protect case with IF THEN ELSE or use of OTHERWISE


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ExercisesExercises

Modify LAB #2 to improve the display New features

– If the coefficient is 1 don’t display the 1:1x + 2y = 5 becomes x + 2y = 5

– if the coefficient is 0 don’t display:0x + 5y = 5 becomes 5y = 5

– display subtraction2x + -1y becomes 2x - y = 0

– give error if division by 0 would occur


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TopicsTopics

Nested Selection

No additional information available


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Week 5 TopicsWeek 5 Topics

Repetition Problem Conditional Branch Pre-testing (Top-testing) Loops Post-testing (Bottom-testing) Loops Comparing Loops


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Repetition ProblemRepetition Problem

A class of problems that can be solved by repeatedly performing some task until some condition is no longer valid.

Example 1: Monopoly, “Go to Jail” until you roll doubles, 3 rolls, or pay $50.

Example 2: N ! = 1 * 2 * 3 * ... (N-1) * N Iterations could be written as sequence of

steps - # of iterations may vary.


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Conditional BranchConditional Branch

Predates looping constructslabel:

statement1;

statement2;

...

statementN;

if ( boolean expression is true)goto label;

Exercise: Write N! using conditional branch logic


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Loop TerminologyLoop Terminology

initialization - assign values before evaluation evaluate - determine if loop should continue increment - modify or increase loop controls iteration - one pass over loop (evaluate, body,

increment) loop body - syntax that is executed with each

iteration of loop


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Pre-testing (Top-testing) LoopsPre-testing (Top-testing) Loops

Evaluates looping condition before executing body of loop

Body of loop executes a minimum of 0 times Pascal has FOR and WHILE loops FOR loop (pg. 239)

– based on FORTRAN FOR loop– used for fixed increment looping

WHILE loop (pg. 252)– General purpose top-testing loop


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For LoopFor Loop

Syntax:FOR <var> := <init> [TO | DOWNTO] <value> DO

Good practice to have maximum iteration value defined as a CONST or VAR (i.e., don’t hard code).

Loop control variable may or may not be defined as VAR

Exercise: pg. 251 7-10


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While LoopWhile Loop

Syntax:WHILE <Boolean expr> DO

Exercise: pg. 264 #3, 4, 5


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Post-testing (Bottom-testing) Post-testing (Bottom-testing) LoopsLoops

Evaluates looping condition after executing body of loop

Body of loop executes a minimum of 1 times Syntax:

REPEAT

UNTIL <Boolean expr>

Exercise: pg. 272 #3, 4, 5.


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Comparing LoopsComparing Loops

Each loop can be rewritten in another loop’s syntax

Easier to use similar testing loops (i.e. for and while)

Review Example 6.22, and 6.23 on pg 275


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ExercisesExercises

Write one of the following:– pg. 312 # 12 (easier) NOTE: read term from

keyboard instead of file– pg. 310 # 6a (moderate)– pg. 311 #8 (harder)


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Week 6 TopicsWeek 6 Topics

Nested Loops


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ExamplesExamples

pg. 285 Example 6.26 pg. 285 Example 6.30 pg. 291 Program Problem 6 b pg. 310


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Week 7 & 8 TopicsWeek 7 & 8 Topics

Subprogramming Procedures Parameters Side Effects


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ProceduresProcedures

Design to perform small discreet task Thought of as a small program - test as such Program is a collection/series of procedure

(function) calls Discuss procedure format slide (pg. 321)


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Procedures (cont.)Procedures (cont.)

Placed in declaration section of program/procedure/function

use {-------------} to denote procedure boundary

procedure name unique to program (scope)


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Procedure ExecutionProcedure Execution

a procedure is called or invoked by name flow of control jumps to first line in procedure on completing procedure, flow of control

returns to first line after procedure call walk through exercise 11 pg. 335


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ParametersParameters

PROCEDURE <name> ( <parameter list> ); parameter list format: similar to VAR format Discuss parameter notes slide


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Parameter TypesParameter Types

formal - variables in procedure heading (parameters)

actual - values in procedure call (arguments) call by value (value parameters) arguments

are copies to parameters call by reference (variable parameters)

parameters refer to arguments


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Side EffectsSide Effects

Unintended change in a variable Typically associated with call by reference

parameter passing Considered “bad” programming. Why? Sometimes desirable. When/Example?


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ExerciseExercise

Walk through program on pg. 349 Do prime program in class #2, pg. 372 Lab: #3 pg. 372, #8 pg. 373, prime program

above.

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CS241 PASCAL I - Control Structures1 PASCAL Control Structures Modified Slides of Philip Fees