while and do-while loops

Repeat code in C with while and do-while loops — including condition testing, loop body execution, and the difference between the two forms.

Loops are what give programs their power to do large amounts of work. Without them, every line of code executes exactly once. With loops, a five-line program can process a million inputs. C has three loop constructs: while, do-while, and for. This lesson covers the first two.

The while loop

while (condition) {
    /* body executes repeatedly while condition is non-zero */
}

A concrete countdown:

#include <stdio.h>

int main(void) {
    int count = 5;

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

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

Output:

5
4
3
2
1
Liftoff!

The structure is:

Evaluate the condition: is count > 0?
If true, execute the body.
After the body, go back to step 1.
If false, exit the loop.

The condition is tested before the body executes. If the condition is false from the start, the body never runs:

int x = -1;
while (x > 0) {
    printf("This never prints\n"); /* condition false immediately */
}

Updating the loop variable

The most common mistake with while loops is forgetting to update the variable that the condition tests:

int i = 0;
while (i < 10) {
    printf("%d\n", i);
    /* forgot: i++; */
}
/* This loop runs forever */

Every while loop should have three things: an initialiser (before the loop), a condition (in the while), and an update (inside the body that eventually makes the condition false).

Summing with while

#include <stdio.h>

int main(void) {
    int sum = 0;
    int i = 1;

    while (i <= 100) {
        sum += i;
        i++;
    }

    printf("Sum 1 to 100 = %d\n", sum); /* 5050 */
    return 0;
}

The do-while loop

do {
    /* body executes at least once */
} while (condition);

The difference: the condition is tested after the body, so the body always executes at least once.

#include <stdio.h>

int main(void) {
    int n;

    do {
        printf("Enter a positive number: ");
        /* In real code you'd use scanf here -- using a fixed value for the example */
        n = 5; /* pretend the user typed 5 */
    } while (n <= 0);

    printf("You entered: %d\n", n);
    return 0;
}

The classic use for do-while is input validation: you always want to read input at least once before checking whether it is valid.

do-while is less common than while and for. Most loops should be written with while or for. Reach for do-while specifically when the loop body must run at least once — typically for reading user input or processing a stream where you need to read before you can test.

Infinite loops

A while (1) loop runs forever — the condition 1 is always non-zero:

while (1) {
    /* infinite loop -- useful for event loops and servers */
    /* must use break, return, or exit() to escape */
}

Infinite loops are legitimate in systems programming: event loops, server request handlers, and embedded firmware often run forever by design. You exit them with break, return, or the exit() function from <stdlib.h>.

A practical example: digit sum

#include <stdio.h>

int main(void) {
    int n = 12345;
    int digit_sum = 0;

    while (n > 0) {
        digit_sum += n % 10; /* extract last digit */
        n /= 10;             /* remove last digit */
    }

    printf("Digit sum: %d\n", digit_sum); /* 15 */
    return 0;
}

This pattern — extract the last digit with % 10, then remove it with / 10 — comes up repeatedly in programming exercises.

Where to go next

Next: for loops — a loop form that keeps the initialiser, condition, and update in one place, which is cleaner for loops that count through a range.

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