While Loop in C

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The while loop represents the simplest loop structure in C programming. A while loop is a control structure that allows you to repeat a task a certain number of times. It is also called the entry controlled loop statement.

Syntax of while loop

while(condition) { // while loop scope }

• First of all loop condition is checked, if the condition is true then the scope of the loop will run otherwise not.
• The condition of while loop can be a relational expression or any expression which will return true ( any nonzero value) or false.
• After executing the loop scope, the condition will be checked again, and if it is true then the scope will run again, This process will continue until the condition becomes false.
• Keyword while is case-sensitive.
• The while loop scope may not execute a single time if the condition is already false.

Block Diagram of while Loop

See the block diagram of the while loop, later we will learn more with the help of examples.

Initialization & Increment/Decrement

There is no fixed rule for initialization, increment or decrement for the while loop but it is best practice to initialize variables before while loop and perform increment/decrement operation in the scope of while loop. See the below example

#include<stdio.h> int main() { int i=1; /* initialization */ while(i<=5) /* testing condition */ { printf("%d\n", i); i = i + 1; /* Incrementment */ } printf("It is an example of while loop"); return 0; }

Output

1 2 3 4 It is an example of while loop

Explanation

• Variable i get Initialized by 1, Now the while condition will be checked, the while condition is i<=5
• In first-round i=1 and condition is i<=5, the condition is true, so loop scope will run.
• Loop scope prints the value of i which is 1 and then i=i+1 calculated, so now the new value of i is 2
• Again while condition i<=5 checked, currently the value of i is 2, so the condition is again true, the scope will run. This will continue until the condition becomes false.
• After false condition, the next line after while loop runs, which prints a message.

Example

What is the output of below program...?

#include<stdio.h> int main() { int sum=0; int i=1; while(i<=5) { sum = sum + i * i; i = i + 1; } printf("Result = %d", sum); return 0; }

Output

Result = 55

Example

Write a program in C using while loop to find out the sum of all numbers from 1 to 100

#include<stdio.h> int main() { int sum=0; int a=1; while(a<=100) { sum = sum+a; a++; } printf("Sum Result = %d", sum); return 0; }

Output

Sum Result = 5050

Example

Write a program to check whether the entered number is palindrome or not, use while loop to solve the problem.

#include<stdio.h> int main() { int n, i, num, revno=0; printf("Enter the number\n"); scanf("%d", &n); i=n; while(n!=0) { num=n%10; revno=revno*10+num; n=n/10; } if(revno==i) { printf("The number is palindrome"); } else { printf("The number is not palindrome"); } return 0; }

Output

Enter the number 121 The number is palindrome

Nested while loop

Just like for loop, C programming also allows the nested form of while loop. Nested while loop means while loop within another while loop.

• C allows multiple levels nested form of while loop.
• In the below, Syntax of nested while loop:

while(condition_1) { // outer loop body while(condition_2) { // inner loop body } }

Working

• For one true outer while condition, inner while loop executes for all true conditions.
• Nested while loop executes for all true conditions of the outer while loop.
• If no condition of the outer while loop is true then, inner while loop condition will not execute.
• Nesting of a while loop can be at any level.
• In a single program, there can be many while loop or nested while loop.
• Let's take a few examples to understand the concept of the nested while loop.

Example

Write a program to print the table of number 1 to 5:

#include<stdio.h> int main() { int i=1, j=1, c=0, num=0; while(i<=5) { printf("Table of %d \n",i) j=1; c=i; while(j<=10) { num=c*j; printf("%d ", num); j=j+1; } i=i+1; printf("\n"); } }

Output

Table of 1 1 2 3 4 5 6 7 8 9 10 Table of 2 2 4 6 8 10 12 14 16 18 20 Table of 3 3 6 9 12 15 18 21 24 27 30 Table of 4 4 8 12 16 20 24 28 32 36 40 Table of 5 5 10 15 20 25 30 35 40 45 50

Example

#include<stdio.h> int main() { int i=1, j=1; while( i<=5 ) { j=1; while(j<=i) { printf("*"); j=j+1; } i=i+1; printf("\n"); } return 0; }

Output

* * * * * * * * * * * * * * *

Infinite while loop

If the condition of a while loop never becomes false then the while loop never gets terminated, this kind of while loop is called an infinite while loop.

Infinite while Loop Syntax

while(condition) /* interminable condition */ { // loop body }

Example

#include<stdio.h> int main() { int 1; while(i<3) { printf("Good Moning \n"); } return 0; }

Output

Good Moning Good Moning Good Moning Good Moning Good Moning Good Moning . . .

The above loop is the infinite while loop, this loop will never get terminated. In programming, we rarely use an infinite while loop but in case of requirement, we are free to use an infinite while loop.

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