#include <stdio.h>

int is_prime (int);
/* Function returns 1 if the number is prime and 0 otherwise */

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.*/
{

	int i;
	if (num < 2)		// There are no primes less than two
		return 0;
	for (i=2; i <= num/2; i++) {
		if (num %i == 0)  {
			return 0;
		}
	}
	return 1;
}