/**
*   Requires math.h
*   Returns 1 if num is prime else returns 0.
*/
int is_prime(int num) {
    if (num <= 2) return 0;
    if (num == 2) return 1;
    if (num%2 == 0) return 0;
    for (int i=3; i<=sqrt(num); i+=2) {
        if (num%i == 0) return 0;
    }
    return 1;
}