While & Do-While Loops in C

A while loop in C repeats a block of code as long as a condition stays true, and it checks that condition before each pass. A do-while loop does the same job but checks the condition after the body runs, so the body always executes at least once. That single difference — test-first versus test-last — is the whole story.

The while loop

The while loop is the simplest counting and condition-driven loop in C. Its shape is:

while (condition) {
    /* body runs while condition is true */
}

C evaluates condition. If it is non-zero (true), the body runs, then C re-checks the condition. The moment the condition becomes zero (false), the loop stops.

#include <stdio.h>

int main(void) {
    int count = 1;            /* start the counter */

    while (count <= 5) {      /* condition checked BEFORE each pass */
        printf("%d ", count); /* prints the current value */
        count++;              /* move toward the stopping condition */
    }

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

Output: 1 2 3 4 5 then Done. If you forget count++, the condition never becomes false and you get an infinite loop.

The do-while loop

A do-while loop puts the body first and the test at the bottom:

do {
    /* body runs at least once */
} while (condition);   /* note the semicolon here */

Because the test comes last, the body runs once before the condition is ever checked. This is exactly what you want for input validation or menus — you must show the menu at least once.

#include <stdio.h>

int main(void) {
    int number;

    do {
        printf("Enter a positive number: ");
        if (scanf("%d", &number) != 1) {  /* guard against bad input */
            return 1;
        }
    } while (number <= 0);   /* repeat until a positive value is given */

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

Even if the very first value is valid, the prompt still appeared once — that is the do-while guarantee.

When to choose which

• Use while when the body might need to run zero times (e.g. processing a list that could be empty).
• Use do-while when the body must run at least once (menus, retry-until-valid input).

Both are interchangeable in many cases; pick the one that makes your intent obvious. If you are counting a known number of times, a for loop is usually cleaner.

Common mistakes

• Putting a semicolon after while (...) in a normal while loop: while (x < 5); creates an empty body and loops forever. The semicolon belongs only at the end of a do-while.
• Forgetting to update the loop variable, so the condition never changes — an infinite loop.
• Off-by-one errors: <= versus < changes how many times the loop runs. Trace the first and last passes by hand.
• Confusing = and ==: while (x = 5) assigns 5 (always true) instead of comparing. Use == for comparison.
• Reading input without checking scanf's return value, which lets bad input slip through.

FAQ

Is do-while used much in C? Less than while and for, but it shines for input validation and menu loops where one guaranteed pass is natural.

Can I break out of a while loop early? Yes — break; exits immediately, and continue; skips to the next condition check.

Related reading: the hub at C Programming, For Loops in C, and Operators in C.

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