Control Structures: while

Notes

While Loop in C

Definition: The while loop is an entry-controlled loop that repeatedly executes a block of statements as long as the given condition remains true.

Syntax

while (condition)
{
    statements;
}

How while Loop Works

Example

To print numbers from 1 to 5, initialize i to 1 and continue the loop while i <= 5.

#include <stdio.h>

int main()
{
    int i = 1;

    while (i <= 5)
    {
        printf("%d ", i);
        i++;                // move to the next number
    }

    return 0;
}

Output

1 2 3 4 5

Why is while Called an Entry-Controlled Loop?

A while loop checks its condition before executing the loop body. Therefore, if the condition is false at the beginning, the loop body will not execute.

Real-World Example

A program may continue asking for information while a particular condition is true. For example, a system can keep processing items while items are still available.

Practical Example — Printing Numbers from 1 to N

Problem Statement

Write a C program to print all integers from 1 to N using a while loop.

Learning Outcomes

• Understand the use of the while loop.
• Initialize and update a loop counter correctly.
• Understand entry-controlled loop execution.

Hint

Initialize the counter to 1, print it, and increment it until it becomes greater than N.

Theory

A while loop checks its condition before each iteration. If the condition is true, the loop body executes; otherwise, the loop ends.

Program

#include <stdio.h>

int main()
{
    int n, i = 1;          // i is the loop counter, starting at 1

    printf("Enter N: ");
    scanf("%d", &n);

    while (i <= n)       // repeat while i has not exceeded n
    {
        printf("%d ", i);
        i++;               // move to the next number
    }

    printf("
");

    return 0;
}

Expected Output

Enter N: 10
1 2 3 4 5 6 7 8 9 10

Note

A common mistake is forgetting to update the loop variable. For example, forgetting i++ can create an infinite loop because the condition may never become false.

while Loop vs if Statement