C while Loop

What You’ll Learn

The while loop repeats code while a condition stays true. Unlike for, you manage the setup and update yourself—ideal when you do not know how many iterations you need upfront, such as reading user input until they quit.

Definition and Usage

A while loop is a pre-test loop: C evaluates the condition before each iteration. If the condition is false, the body is skipped entirely—even on the first pass. When true, the body runs, then control returns to the condition check.

Use while when repetition depends on something that changes during the program—user typing numbers, reading characters until end-of-file, or waiting for a flag. Use for when counting with a clear start, stop, and step fits naturally in one line.

📝 Syntax

General form of the C while loop:

while (condition) {
    // statements to repeat
}

Equivalent counting pattern

This while loop does the same work as for (int i = 1; i <= 5; i++):

int i = 1;              /* initialization before loop */
while (i <= 5) {        /* condition */
    printf("%d\n", i);
    i++;                /* update inside body */
}

Compile

gcc program.c -std=c11 -o program

Examples Gallery

Compile with gcc example.c -std=c11 -o example. Each program shows a different while loop pattern.

#include <stdio.h>

int main(void) {
    int i = 1;

    while (i <= 5) {
        printf("%d\n", i);
        i++;
    }

    return 0;
}

1
2
3
4
5

#include <stdio.h>

int main(void) {
    int n = 5;

    while (n > 0) {
        printf("%d...\n", n);
        n--;
    }

    printf("Blastoff!\n");
    return 0;
}

5...
4...
3...
2...
1...
Blastoff!

#include <stdio.h>

int main(void) {
    int number;

    printf("Enter a number (0 to exit): ");
    if (scanf("%d", &number) != 1) {
        return 1;
    }

    while (number != 0) {
        printf("You entered: %d\n", number);
        printf("Enter a number (0 to exit): ");
        if (scanf("%d", &number) != 1) {
            break;
        }
    }

    printf("Exited the loop.\n");
    return 0;
}

Enter a number (0 to exit): 7
You entered: 7
Enter a number (0 to exit): 3
You entered: 3
Enter a number (0 to exit): 0
Exited the loop.

#include <stdio.h>

int main(void) {
    int i = 1;

    while (i <= 3) {
        int j = 1;

        while (j <= 2) {
            printf("i = %d, j = %d\n", i, j);
            j++;
        }

        i++;
    }

    return 0;
}

i = 1, j = 1
i = 1, j = 2
i = 2, j = 1
i = 2, j = 2
i = 3, j = 1
i = 3, j = 2

#include <stdio.h>

int main(void) {
    int choice;

    while (1) {
        printf("\n1) Say hello\n2) Double a number\n0) Quit\nChoice: ");
        if (scanf("%d", &choice) != 1) {
            break;
        }

        if (choice == 0) {
            break;
        } else if (choice == 1) {
            printf("Hello!\n");
        } else if (choice == 2) {
            int x;
            printf("Enter number: ");
            if (scanf("%d", &x) == 1) {
                printf("Double = %d\n", x * 2);
            }
        } else {
            printf("Unknown option.\n");
        }
    }

    printf("Goodbye.\n");
    return 0;
}

1) Say hello
2) Double a number
0) Quit
Choice: 1
Hello!

1) Say hello
2) Double a number
0) Quit
Choice: 0
Goodbye.

🚀 Common Use Cases

• User input — repeat until valid data or a quit command.
• File reading — while ((c = getchar()) != EOF) style loops.
• Game loops — update and render until the player quits.
• Waiting — poll a condition until hardware or network is ready.
• Parsing — consume tokens while more data remains.
• Menus — while (1) with break on exit option.

📝 Notes

• If the condition starts false, the body never runs—unlike do-while.
• Always update variables that affect the condition inside the loop.
• while (1) is intentional infinity; pair it with break or return.
• Check scanf return value when input errors or EOF should stop the loop.
• continue skips to the next condition test; break leaves the loop.
• Any non-zero value is true in C; only 0 is false.

⚡ Optimization

Hoist invariant work out of the loop—do not recompute values that never change on each iteration. For tight numeric loops, a for header can be clearer to the compiler and reader than an equivalent while. For I/O-bound loops (reading input or files), performance is dominated by I/O, not the loop form you choose.

💡 Best Practices