C While Loop

The while loop in C is a control flow statement that repeatedly executes a block of code as long as a specified condition is true. It is one of the most commonly used loops in C programming for scenarios where the number of iterations is not known beforehand.

Syntax

while (condition) {
    // Code to be executed repeatedly
}

• condition: An expression that is evaluated before each iteration. If it evaluates to true (non-zero), the loop continues. If it evaluates to false (zero), the loop terminates.
• The code inside the loop will execute as long as the condition is true.

Example 1: Basic while Loop

#include <stdio.h>

int main() {
    int i = 1;

    while (i <= 5) {
        printf("Iteration %d\n", i);
        i++; // Increment i to avoid an infinite loop
    }

    return 0;
}

Output:

Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5

In this example, the loop starts with i = 1. As long as i is less than or equal to 5, the loop prints the value of i and then increments i by 1 after each iteration.

Example 2: Infinite while Loop (Avoiding Infinite Loops)

A while loop can potentially run infinitely if the condition is always true. Here’s an example of an infinite loop:

#include <stdio.h>

int main() {
    while (1) {  // Infinite loop (1 is always true)
        printf("This will run forever unless interrupted.\n");
    }

    return 0;
}

To stop an infinite loop, you would need to manually break it (e.g., using Ctrl+C in the terminal).

Example 3: Using break to Exit a while Loop

You can use the break statement to exit a while loop prematurely based on a condition.

#include <stdio.h>

int main() {
    int i = 1;

    while (i <= 10) {
        if (i == 5) {
            break;  // Exit the loop when i equals 5
        }
        printf("Iteration %d\n", i);
        i++;
    }

    return 0;
}

Output:

Iteration 1
Iteration 2
Iteration 3
Iteration 4

In this example, the loop will terminate when i becomes 5, thanks to the break statement.

Example 4: Using continue in a while Loop

The continue statement skips the remaining code in the current iteration and proceeds to the next iteration of the loop.

#include <stdio.h>

int main() {
    int i = 1;

    while (i <= 5) {
        if (i == 3) {
            i++;  // Increment i to avoid infinite loop
            continue;  // Skip printing when i equals 3
        }
        printf("Iteration %d\n", i);
        i++;
    }

    return 0;
}

Output:

Iteration 1
Iteration 2
Iteration 4
Iteration 5

Here, when i equals 3, the continue statement is executed, which skips the printf statement for that iteration.

Example 5: Using while with User Input

A while loop can be useful when you want to repeat an action based on user input.

#include <stdio.h>

int main() {
    int number;

    printf("Enter a positive number: ");
    scanf("%d", &number);

    while (number > 0) {
        printf("You entered: %d\n", number);
        printf("Enter another positive number (or a non-positive number to quit): ");
        scanf("%d", &number);
    }

    printf("You entered a non-positive number, exiting the loop.\n");

    return 0;
}

Output:

Enter a positive number: 5
You entered: 5
Enter another positive number (or a non-positive number to quit): 3
You entered: 3
Enter another positive number (or a non-positive number to quit): -1
You entered a non-positive number, exiting the loop.

In this example, the loop continues to prompt the user to enter positive numbers and prints them until a non-positive number is entered, at which point the loop terminates.

Summary

• The while loop repeats a block of code as long as a specified condition is true.
• The condition is evaluated before each iteration, meaning the loop may not execute at all if the condition is initially false.
• It is essential to modify the condition inside the loop (e.g., by updating variables) to avoid infinite loops.
• The break statement can be used to exit the loop prematurely, and continue skips the current iteration and moves to the next.