Chapter 3 · Lesson 2

Loops

Loops in C

C provides three loop constructs.

for loop All three parts (init; condition; update) are optional:

for (int i = 0; i < n; i++) { … }

while loop Condition tested before the body:

while (condition) { … }

do-while loop Body executes at least once:

do { … } while (condition);

Nested loops Common for multi-dimensional iteration. Outer loop rows, inner loop columns.

break and continue

break exits the innermost loop (or switch).
continue skips to the next iteration of the innermost loop.

Key points:

Omitting the for-loop condition creates an infinite loop: for (;;) { }.
Loop variables declared inside for (int i = …) require C99 or later.
Be careful of off-by-one errors: i < n vs i <= n.

Code Examples

for, while, do-whilec

#include <stdio.h>

int main(void) {
    // for loop — sum 1 to 5
    int sum = 0;
    for (int i = 1; i <= 5; i++) {
        sum += i;
    }
    printf("Sum: %d\n", sum);

    // while — find smallest power of 2 >= 50
    int n = 1;
    while (n < 50) n *= 2;
    printf("Power of 2 >= 50: %d\n", n);

    // do-while
    int x = 1;
    do {
        printf("%d ", x);
        x *= 2;
    } while (x <= 16);
    printf("\n");
    return 0;
}

do-while always runs the body once. The while loop condition is checked before each iteration.

Nested loops and break/continuec

#include <stdio.h>

int main(void) {
    // Multiplication table (3x3)
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            printf("%2d ", i * j);
        }
        printf("\n");
    }

    // continue: skip even numbers
    printf("Odd 1-10: ");
    for (int i = 1; i <= 10; i++) {
        if (i % 2 == 0) continue;
        printf("%d ", i);
    }
    printf("\n");
    return 0;
}

Nested loops iterate over a 2D grid. continue skips even numbers — execution jumps to i++ and the next iteration.

Quick Quiz

1. What does `for (;;) { }` represent in C?

2. Which loop guarantees at least one execution of the body?

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