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Week 4Repetition Structures /Loops

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Outline Introduction

While loops

Three types of while loops

Do-while loops

For loops Nested loops

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Why Need Loops ? Suppose we want to add five numbers and

find the average.

From what you have learned so far, you couldproceed as followsscanf(%d %d %d %d %d, &num1, &num2,

&num3, &num4, &num5);

sum = num1 + num2 + num3 + num4 + num5;average = sum / 5;

If 100 numbers, 1000 numbers?

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Repetition (Loop) Used to control the flow of a program Loops are basically repetitions or iterations used to

repeat a segment of code

Three statements can be used to implement loops inC whilestatement do-whilestatement forstatement

whileand forstatements are similar inimplementation, but have different syntax

The do-whilestatement is different - the followingcode is executed at least once

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The whileloop structure The general form of the whilestatement is:

while (expression)statement;

To avoid an infinite loop, make sure thatthe loops body contains statement (s) thatassure that the exit condition i.e.theexpression in the while statement will

eventually be false.whileloop repeates until condition becomesfalse

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Flowchart of a whilestatement

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The whileloop structure-cont There are basically three types of while

loops:

Counter-controlled whileloop

Sentinel-controlled whileloop

Flag-controlled whileloop

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Counter-Controlled while

Loops Definite repetition: know how many times loop will execute

Control variableused to count repetitions

Example:int counter = 1; // declare and initializewhile ( counter

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Counter-Controlled while

LoopsAnother example:

int product = 2;

while ( product

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Sentinel-Controlled while

Loops Indefinite repetition

Used when number of repetitions not known

and loop needs to input value repeatedly foreach iteration

Sentinel valueindicates "end of data

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Sentinel-Controlled while

Loops-continuedExample:int number, count, sum;sum = 0;count = 0;

printf(To stop enter -999. Enter positive numbers : " );scanf(%d, &number);while (number != -999){

sum = sum + number; //find sum of numbers enteredcount++; //count how many numbers entered

printf(To stop enter -999. Enter positive numbers : " );

scanf(%d, &number);}

printf(\nThe sum of %d numbers is %d, count, sum);

Requirement:1. Readcontrolvariablevalue

before enter loop2. A condition thattestscontrolvariables validity(i.e., whetherlooping should

continue)3. Read againcontrol variablebefore end of loop

Sentinel value

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Sentinel-Controlled while

Loops-continued Another example:

char choice;

printf(Continue? y-yes n-no: );

scanf(%c, &choice);while ( choice == y)

{ printf(\nEnter grade point:);

scanf(%f, &grade_pt);

if(grade_pt > 2.0)

printf(\nPass);printf(Continue? y-yes n-no: );

scanf(%c, &choice);

}

read control variable

test condition

read again

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Flag-Controlled whileLoops Uses a boolean variableto control the loop.

Loop exit when expression is evaluated to false.

bool found = false;

while (!found){ printf(Enter number between 1 and 200:);

scanf(%d, &guess);if ((guess>= 88) &&(guess

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The do-whileRepetition

Structure Similar to thewhilestructure Condition for repetition is tested afterthe

body of the loop is performed All actions are performed at least once Expression can be written as count-controlled

or sentinel-controlled Format:

do {statement;

} while ( condition);

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Flowchart of a do-while

structure

true

false

action(s)

condition

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The do-whileRepetition

Structure

Example : prints the integers from 1to 10

int counter = 1;

do {

printf( "%d ", counter );

} while (++counter 2.0)printf(\nPass);

printf(Continue?y-yes n-no :);

scanf(%c, &choice);

}while ( choice == y)

count-controlled

sentinel-controlled

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The do-whileRepetitionStructure

To avoid an infinite loop, make sure that the loop body contains astatement that ultimately makes the expression false and assures thatit exits

Another example:

int i = 0;

do{

printf (%d ,i);i = i + 5;

} while (i

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The do-whileLoopingStructure (Example)

Example: Consider the following two loops

printf (%d ,i); printf (%d ,i);

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The do-whileLoopingStructure (Example)

In (a), the whileloop, produces nothing

In (b) the do...whileloop, outputs the

number 11

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The forRepetition Structure

Format when using forloopsfor(initialization ; loop continuation test ; incrementstatement)

Use forloops when already know howmany times to repeat

Example:

for(counter = 1;counter

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The forRepetition Structure

for loops can usually be rewritten as while loops:

initialization;while( loop continuation test)

{

statement;increment statement;

}

Initialization and increment

Can be comma-separated lists

Example: for(int i=0,j=0;j+i

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Flow of a forstatement

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Nested loop

Loop within a loop

Inner loop is performed first

e.g.

for(i=1;i

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Nested loop

Another example: Program finds and prints avg of three test scores, then asks

whether user wants to continue

do{

for(count=0; count

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End Week 4 - Loops

Q & A!