#include <stdio.h>

int is_prime (int);
/* Prototype for a function taking an integer and returning an integer */

int main(int argc, char **argv)
{
    int i;
    for (i= 1; i < 21; i++) {
        if (is_prime(i)) {
            printf ("%d is a prime number.\n",i);
        } else {
            printf ("%d is not a prime number.\n",i);
        }
    }
    return 0;
}

int is_prime (int num)
/* This function takes one argument "num" it returns 1 if num is a prime
number and 0 if num is NOT a prime number.  Remember that in C, 0 means
FALSE and 1 means TRUE.*/
{
    /* Write your function here */
}