cmpt 102 introduction to scientific computer programming

1 © Janice Regan, CMPT 102, Sept. 2006

CMPT 102Introduction to Scientific Computer Programming

Control Structures

while Loops

continue; and break; statements

© Janice Regan, CMPT 102, Sept. 2006 2

Control Structures Three methods of processing a program

In sequence Branching Looping

Branch: Altering the flow of program execution by making a selection or choice

Loop: Altering the flow of program execution by repetition of a particular block of statement(s)


Basic Loops When one action is to be repeated a number of

times a loop is used. Loops are repetition structures

There are two common types of loops while loop or do...while loop

Used to continue repetition while a condition holds Can also be used (along with a counter variable) to repeat a

particular number of times

for loop Specifically designed to repeat a particular number of times


A while Loop in C while ( condition )

{ /* Series of actions to be taken */ /* each time the loop is executed */ /* loop is executed when condition is True */

action 1;action 2;

}/* When condition is false execute following actions */actions;


Structure of while loop

T

Fcondition

Statement 1;

Statement n;


Counting while statement

T

loopIndex < end

loopIndex += step;

start = nend = m

loopIndex = start;

Initial statements

Loop condition

First Statement:…..

Last Statement;

F

Repeats Statements m-n times

Update statement


Example while Loop in C /* Sum the Integers from 1 to 25 inclusive */

X = 1;

Sum = 0;

while ( X <= 25 )

{

Sum += X;

X++;

}

printf(“%d\n”, Sum);


do…while Loop in C do

{/*Series of actions to be taken *//*each time the loop is executed *//*Always executed on first pass *//* Subsequently executed if condition is True */action 1;action 2;

}

while ( condition ); /* When condition is false execute following actions */actions;


Structure of do…while Loop

T

Fcondition

Statement 1;

Statement n;T

Loop condition


Counting do…while statement

Update statement

T loopIndex < end

start = nend = m

loopIndex = start;

Initial statement

Loop condition


Last Statement;

F


loopIndex += step;


Example do…while Loop /* Sum the Integers from 1 to 25 inclusive */ X = 1; sum = 0; do { sum += X;

X++; } while ( X <= 25 ); printf(“%d”, sum);


Differences do vs do…while /* Sum Integers from n to m */ n = 4: m = 3: sum = 0; do { sum += n;

n++; } while ( n <= m ); printf(“%d”, sum);

/* Sum Integers from n to m */ n = 4: m = 3: sum = 0; while ( n <= m ) { sum += n;

n++; } printf(“%d”, sum);

After code sum = 4Body of loop executes once

After code sum = 0Body of loop does not execute


Infinite Loops When using while or do..while loops it is possible that the

loop condition is always true. If the variable or expression tested in the loop condition

is not modified in the action statements within the loop, then if the condition is true for the first loop test it remains true for ever

Even if the variable or expression changes it may satisfy the test condition for ever

An infinite loop will cause your program to execute forever If your program is caught in an infinite loop you can

terminate it by typing <CNTRL>C into your linux or cygwin command window


Uses of an infinite loop To make an infinite loop useful, you must have a

way to exit from it It is possible to exit from a loop by using a break;

statement (discussed below) If you use an infinite loop intentionally it is usually

a loop you intend to execute many times until Some condition is met OR The loop has executed the maximum number of times

If you use an infinite loop intentionally you should always exit after a maximum number of times through the loop, just in case the condition you expect to take you out of the loop never occurs.


The Break Statement: Loops

Sometimes you wish to exit from a loop even if the loop condition is still true For example if your loop is reading and processing

data and an error occurs that can be recognized but cannot be corrected, you may wish to print an error message and exit the loop immediately

break; A C statement that causes you to exit the loop you

are in Program execution will continue at the statement

following the loop


break statement

T

F

ConditionLoop condition

…..Statement n;

break;T

F

Statement 1;…..

condition2


The Continue Statement: Sometimes you wish to execute only some of

the statements in the body of the loop for a particular value of the loop index For example you may wish to skip the last half of the

body of the loop if (loopindex/14)= = 1 continue;

A C statement that causes you to go from anywhere in the loop to the update statement and loop condition

Program execution will continue at the update statement of the loop


continue statement

T

F

Condition

…..Statement n;

continue;T

F

Statement 1;…..

condition2


continue in Counting while

Update statement

T

loopIndex < end

Start = nEnd = m

loopIndex = start;Initial statement

Loop condition

…..Last Statement;

F


loopIndex += step;

F

Tcondition2


continue;


Example infinite Loop in C /* The user inputs a positive integer */ /* the routine prints the square of the number, */ /* to terminate the user enters -999 */ while ( 1 ) { printf("enter a positive integer " ); scanf("%d", &positiveIntNum); if(positiveIntNum < 0) { if( positiveIntNum == -999) {break}; printf("error*** negative input ***\n"); continue; } printf("%d\n", positiveIntNum*positiveIntNum); }

cmpt 102 introduction to scientific computer programming

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