for Loop in C: Syntax, Examples, Counting Down and Looping Over Arrays

The for loop is the most common loop in C. It keeps the start, the condition and the step together on one line.

Syntax#

for (int i = 1; i <= 5; i++) {
    printf("%d ", i);
}
printf("\n");

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1. Initialisation (int i = 1): runs once, before the loop.
2. Condition (i <= 5): checked before each repetition; the loop stops when it is false.
3. Update (i++): runs after each repetition.

A variable declared in the header (int i) exists only inside the loop (C99 and later).

Different steps#

for (int i = 10; i >= 0; i -= 2) printf("%d ", i);
printf("\n");
for (int i = 1; i <= 1000; i *= 10) printf("%d ", i);
printf("\n");

10 8 6 4 2 0 
1 10 100 1000 

Looping over an array#

Array indexes run from 0 to length - 1. The number of elements can be calculated with sizeof:

#include <stdio.h>

int main(void) {
    int marks[] = {72, 95, 58, 88, 64};
    size_t count = sizeof marks / sizeof marks[0];

    int total = 0, best = marks[0];
    for (size_t i = 0; i < count; i++) {
        total += marks[i];
        if (marks[i] > best) best = marks[i];
    }
    printf("Count: %zu, total: %d, best: %d\n", count, total, best);
    printf("Average: %.1f\n", (double)total / count);
    return 0;
}

Count: 5, total: 377, best: 95
Average: 75.4

sizeof marks / sizeof marks[0] (total bytes divided by bytes per element) only works where the array itself is visible. Once an array is passed to a function it becomes a pointer, and you must pass its length separately (Pointers and Arrays).

Off-by-one errors#

The most common loop bug is running one time too many or too few:

int a[5];
for (int i = 0; i <= 5; i++) a[i] = 0;   /* bug: a[5] does not exist */

a[5] is past the end of the array. C does not check this: it writes into memory that belongs to something else, which is undefined behaviour. For an array of n elements, the condition is i < n.

Several variables#

The comma lets a for loop manage two variables, for example to walk from both ends towards the middle:

int a[] = {1, 2, 3, 4, 5, 6};
int n = 6;
for (int i = 0, j = n - 1; i < j; i++, j--) {
    int tmp = a[i];
    a[i] = a[j];
    a[j] = tmp;
}
for (int i = 0; i < n; i++) printf("%d ", a[i]);
printf("\n");

6 5 4 3 2 1 

Optional parts#

All three parts are optional. for (;;) is an infinite loop, a common idiom alongside while (1).

Practice#

#include <stdio.h>

int main(void) {
    for (int n = 2; n < 30; n++) {
        int is_prime = 1;
        for (int d = 2; d * d <= n; d++) {
            if (n % d == 0) {
                is_prime = 0;
                break;
            }
        }
        if (is_prime) printf("%d ", n);
    }
    printf("\n");
    return 0;
}

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Key takeaway#