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IntroductionIntroduction

to Loopsto Loops

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GoalsGoals

By the end of this unit, you shouldBy the end of this unit, you shouldunderstand understand

basic loop concepts, includingbasic loop concepts, including

pretest & posttest loops, looppretest & posttest loops, loopinitialization & updates, event &initialization & updates, event &start-controlled loops.start-controlled loops.

how to program a while loop.how to program a while loop.

how to program a do-while loop.how to program a do-while loop. how to program a for loops.how to program a for loops.

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What is a loop?What is a loop?

AA looploop is a programming structureis a programming structure

that allows an action to repeat untilthat allows an action to repeat until

the program meets a given condition.the program meets a given condition.

After eachAfter each iterationiteration of a loop, theof a loop, the

loop checks against aloop checks against a loop controlloop control

expressionexpression to see if the program metto see if the program met

the given condition. If it did, the loopthe given condition. If it did, the loopstops. If not, the loop moves on tostops. If not, the loop moves on to

the next iteration.the next iteration.

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Types of LoopsTypes of Loops

C supports two categories ofC supports two categories of

loops, based on where theloops, based on where the

program tests the condition:program tests the condition: Pretest LoopsPretest Loops Post-test LoopsPost-test Loops

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Pretest LoopsPretest Loops

With each iteration, the program tests theWith each iteration, the program tests thecondition first before executing the loopscondition first before executing the loopsblock.block.

If the condition results toIf the condition results to truetrue, the loop, the loopcontinues and executes the block; if thecontinues and executes the block; if thecondition results tocondition results to falsefalse, the loop, the loopterminates.terminates.

With a pretest loop, there is a chance theWith a pretest loop, there is a chance theloop may never execute once in the program.loop may never execute once in the program.

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Post-Test LoopsPost-Test Loops

With each iteration, the programWith each iteration, the programexecutes the loops block first and testsexecutes the loops block first and testsagainst a condition.against a condition.

If the condition tests toIf the condition tests to truetrue, the loop, the loopcontinues and executes anothercontinues and executes anotheriteration; if the condition tests toiteration; if the condition tests to falsefalse,,the loop terminates.the loop terminates.

With a post-test loop, the loop willWith a post-test loop, the loop willalways execute at least once!always execute at least once!

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Pretest vs. Post-Test LoopsPretest vs. Post-Test Loops

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Loop InitializationLoop Initialization

Programs typically require some type ofPrograms typically require some type ofpreparation before executing loops.preparation before executing loops.

Loop initializationLoop initialization, which happens before a, which happens before a

loops first iteration, is a way to prepare theloops first iteration, is a way to prepare theloop..loop..

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Updating a LoopUpdating a Loop

A loop update is what happens inside aA loop update is what happens inside aloops block that eventually causes theloops block that eventually causes theloop to satisfy the condition, thus endingloop to satisfy the condition, thus ending

the loop.the loop. Updating happens during each loopUpdating happens during each loop

iteration.iteration.

Without a loop update, the loop would beWithout a loop update, the loop would bean infinite loop.an infinite loop.

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Initialization & UpdatingInitialization & Updating

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Concept of aConcept of a

start-Controlled Loopstart-Controlled Loop

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Loop Comparison in aLoop Comparison in a

start-Controlled Loopstart-Controlled Loop

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TheThewhilewhile LoopLoop

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Example of while loopExample of while loop#include#include

#include#includeint main()int main()

{{

int start, end;int start, end;

scanf("%d",&end);scanf("%d",&end);

start = 0;start = 0;

while ( start < end)while ( start < end)

{{start++;start++;

printf("%d\n", start);printf("%d\n", start);

}}

return 0;return 0;

If we input value

of end as 10 then

output will be 1 to

10

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ExampleExample#include#include

int main()int main()

{ int i;{ int i;

i = 0;i = 0;

while(i++ < 5)while(i++ < 5){ printf("%d\n", i);{ printf("%d\n", i);

}}

printf("\n");printf("\n");

i = 0;i = 0;while(++i < 5)while(++i < 5)

{ printf("%d\n", i); }{ printf("%d\n", i); }

return 0; }return 0; }

The output of the postfix and

prefix increment example willlook like this:

1

2

3

4

5

1

2

3

4

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TheThe forfor LoopLoop

AA forfor loop is a pretest loop that includes threeloop is a pretest loop that includes threeexpressions in its header:expressions in its header: Loop initialization statementLoop initialization statement Limit test expressionLimit test expression

Loop update statementLoop update statement

The for loop is often used as a start-controlledThe for loop is often used as a start-controlledloop since we can accurately predict theloop since we can accurately predict themaximum number of iterations.maximum number of iterations.

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TheThe forfor LoopLoop

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ComparingComparingwhilewhile withwith forfor

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ComparingComparingwhilewhile withwith forfor

i = 1;i = 1;

sum =0;sum =0;

while (i

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Example of for loopExample of for loopmain ( )main ( )

{{

int p, n, count ;int p, n, count ;

float r, si ;float r, si ;

for ( count = 1 ; count

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ExampleExample

#include #include #include#include

void main()void main()

{{

int i = 0, j = 8;int i = 0, j = 8;

printf("Times 8 Table\n");printf("Times 8 Table\n");

for(i = 0; i

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NestedNested forfor LoopsLoops

We can nest any statement, evenWe can nest any statement, even

anotheranotherforfor loop, inside the body ofloop, inside the body of

a parenta parent forfor loop.loop. When we nest a childWhen we nest a child forfor loop, itloop, it

iterates all of its cycles for eachiterates all of its cycles for each

iteration of the parent.iteration of the parent.

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Example of nested for loopExample of nested for loop /* Demonstration of nested loops *//* Demonstration of nested loops */

main( )main( )

{{

int r, c, sum ;int r, c, sum ;

for ( r = 1 ; r

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TheThe do whiledo while LoopLoop

C implements a post-test loop using aC implements a post-test loop using astructure called astructure called a do whiledo while loop.loop.

In theIn the do whiledo while, the loop begins with, the loop begins with

the keywordthe keyword dodo, followed by the body,, followed by the body,followed by the keywordfollowed by the keywordwhilewhile and theand theloop expression.loop expression.

A semi-colon (A semi-colon (;;) follows the loop) follows the loopexpression.expression.

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TheThe do whiledo while LoopLoop

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Example of do-whileExample of do-while/* Execution of a loop an unknown number of time*//* Execution of a loop an unknown number of time*/

main( )main( )

{{

char another ;char another ;

int num ;int num ;

dodo

{{

printf ( "Enter a number " ) ;printf ( "Enter a number " ) ;

scanf ( "%d", &num ) ;scanf ( "%d", &num ) ;printf ( "square of %d is %d", num, num * num ) ;printf ( "square of %d is %d", num, num * num ) ;

printf ( "\nWant to enter another number y/n " ) ;printf ( "\nWant to enter another number y/n " ) ;

scanf ( " %c", &another ) ;scanf ( " %c", &another ) ;

} while ( another == 'y' ) ;}} while ( another == 'y' ) ;}

Output:

Enter a number 5

square of 5 is 25

Want to enter another number y/n yEnter a number 7

square of 7 is 49

Want to enter another number y/n n

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Same using for loopSame using for loop

/* odd loop using a for loop *//* odd loop using a for loop */main( )main( )

{{

char another = 'y' ;char another = 'y' ;

int num ;int num ;

for ( ; another == 'y' ; )for ( ; another == 'y' ; )

{{


scanf ( "%d", &num ) ;scanf ( "%d", &num ) ;

printf ( "square of %d is %d", num, num * num ) ;printf ( "square of %d is %d", num, num * num ) ;



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Same using while loopSame using while loop/* odd loop using a while loop *//* odd loop using a while loop */

main( )main( ){{

char another = 'y' ;char another = 'y' ;

int num ;int num ;

while ( another == 'y' )while ( another == 'y' )

{{


scanf ( "%d", &num ) ;scanf ( "%d", &num ) ;printf ( "square of %d is %d", num, num * num ) ;printf ( "square of %d is %d", num, num * num ) ;



}}}}

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ComparingComparing

do whiledo while withwithwhilewhile

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TheThebreakbreak Statement in LoopsStatement in Loops

We often come across situations where we wantWe often come across situations where we want

to jump out of a loop instantly, without waiting toto jump out of a loop instantly, without waiting to

get back to the conditional test.get back to the conditional test.

The keywordThe keyword break allows us to do this.break allows us to do this. When break isWhen break is encountered inside any loop,encountered inside any loop,

control automatically passes to the firstcontrol automatically passes to the first

statement after the loop.statement after the loop.

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Example of breakExample of break Write a program to determine whether a number isWrite a program to determine whether a number is

prime or not. A prime number is one, which isprime or not. A prime number is one, which is

divisible only by 1 or itself.divisible only by 1 or itself.

All we have to do to test whether a number is prime orAll we have to do to test whether a number is prime or

not, is to divide it successively by all numbers from 2 tonot, is to divide it successively by all numbers from 2 to

one less than itself.one less than itself.

If remainder of any of these divisions is zero, the numberIf remainder of any of these divisions is zero, the number

is not a prime. If no division yields a zero then theis not a prime. If no division yields a zero then the

number is a prime number.number is a prime number.

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ExampleExamplemain( )main( )

{int num, i ;{int num, i ;


scanf ( "%d", &num ) ;scanf ( "%d", &num ) ;

i = 2 ;i = 2 ;

while ( i

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ExampleExample

The keywordThe keyword breakbreakbreaks thebreaks the

control onlycontrol only

from the while infrom the while in

which it is placed.which it is placed.

main( )

{int i = 1 , j = 1 ;

while ( i++

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TheThe continuecontinue StatementStatement

In some programming situations we want to takeIn some programming situations we want to take

the control to the beginning of the loop,the control to the beginning of the loop,

bypassing the statements inside the loop,bypassing the statements inside the loop,

which have not yet been executed. The keywordwhich have not yet been executed. The keywordcontinue allowscontinue allows us to do this.us to do this.

WhenWhen continue is encountered inside anycontinue is encountered inside any

loop,loop, control automatically passes to thecontrol automatically passes to the

beginning of the loop.beginning of the loop.

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Example of continue statementExample of continue statement

main( )main( )

{{

int i, j ;int i, j ;

for ( i = 1 ; i

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