Recursion versus iteration

Recursion solves a problem by a function calling itself on smaller inputs, whereas iteration repeats a block of statements using loops (for, while). Recursion uses more memory (a stack frame per call) and is usually slower, but is shorter for naturally recursive problems; iteration uses less memory and is faster. Both compute the same results:

#include <stdio.h>
int sum_rec(int n) { return n == 0 ? 0 : n + sum_rec(n - 1); }
int sum_iter(int n)
{
    int i, s = 0;
    for (i = 1; i <= n; i++)
        s += i;
    return s;
}
int main(void)
{
    printf("%d %d\n", sum_rec(5), sum_iter(5));
    return 0;
}

Output:

15 15