C - While Loop: A Comprehensive Guide

Learn about the while loop in C, an entry-verified loop that allows for repeated execution of code based on a specified condition. This section covers the syntax of the while loop and provides examples to enhance your understanding of its use.

C - While Loop

In C, while is a keyword to form loops. It's often called the entry verified loop. The other loops are for and do-while.

Syntax of C while Loop

while(expression){
   statement(s);
}

The while keyword is followed by a parenthesis with a Boolean expression and a block of statements inside curly brackets.

How while Loop Works in C?

The compiler evaluates the expression. If true, it executes the block. If false, it skips the block and moves to the next statement. The while loop is tested before entering the loop, so it might not execute at all if the condition is false initially.

Example: Printing "Hello World" 5 Times

#include <stdio.h>

int main() {
    int a = 1;
    while(a <= 5) {
        printf("Hello World \n");
        a++;
    }
    printf("End of loop");
    return 0;
}

Hello World
Hello World
Hello World
Hello World
Hello World
End of loop

The variable a controls the repetitions. It starts at 1, increments by 1 each loop, and stops when it reaches 6. Changing a to 10 initially skips the loop entirely as the condition is false.

Using while as a Conditional Loop

A while loop can continue till a condition is satisfied.

#include <stdio.h>

int main() {
    int x = 0;
    while(x >= 0) {
        (x % 2 == 0) ? printf("%d is Even \n", x) : printf("%d is Odd \n", x);
        printf("\n Enter a positive number: ");
        scanf("%d", &x);
    }
    printf("\n End of loop");
    return 0;
}

0 is Even

Enter a positive number: 12
12 is Even

Enter a positive number: 25
25 is Odd

Enter a positive number: -1

End of loop

While Loop with break and continue

break terminates a loop:

while (expr) {
   if (condition)
      break;
}

continue makes a loop repeat from the beginning:

while (expr) {
   if (condition)
      continue;
}

More Examples of C while Loop

Example: Printing Lowercase Alphabets

#include <stdio.h>

int main() {
    char a = 'a';
    while(a <= 'z') {
        printf("%c", a);
        a++;
    }
    printf("\n End of loop");
    return 0;
}

abcdefghijklmnopqrstuvwxyz
End of loop

Example: Equate Two Variables

#include <stdio.h>

int main() {
    int a = 10, b = 0;
    while(a != b) {
        a--;
        b++;
        printf("a: %d b: %d\n", a, b);
    }
    printf("\n End of loop");
    return 0;
}

a: 9 b: 1
a: 8 b: 2
a: 7 b: 3
a: 6 b: 4
a: 5 b: 5

End of loop

while Vs. do while Loops

The do-while loop is called the exit verified loop and has a different syntax. Differences are explained in the do-while chapter.