"Translating a simple iterative loop into a recursive version is about the quickest possible way of filtering out the non-programmers - even a rather inexperienced programmer can usually do it in 15 minutes, and it's a very language-agnostic problem, so the candidate can pick a language of their choice instead of stumbling over idiosyncracies."

iter_recur.c

#include <stdio.h>
/*
http://programmers.stackexchange.com/questions/182314/recursion-or-while-loops/182334#182334
Translating a simple iterative loop into 
a recursive version is about the quickest possible 
way of filtering out the non-programmers - 
even a rather inexperienced programmer can 
usually do it in 15 minutes, 
*/

int sum_iterative(int n)
{
    int i;
    int sum = 0;

    for (i = 0; i < n; i++) {
        sum += i;
    }

    return sum;
}

int sum_recursive(int n)
{
    if (n == 0)
        return n;

    return n + sum_recursive(--n);
}

int main(int argc, char *argv[])
{
    printf("%d\n", sum_iterative(20));

    printf("%d\n", sum_recursive(20));

    return 0;
}