C'

Since we're talking 'bout C... can anyone spot the error in the following 
program.  It takes one command line argument, n, a positive integer, and 
reports if it is prime or not.  Works fine for 1,2,3,4 but seems to stop 
working after that.  As far as I can tell this is a straight forward 
translation of Aaron's SML is_prime function into C.

gcc -o prime prime.c ; prime 5
5 is not prime.

-d

#include <stdio.h>

int is_factor(int y, int x) {
   ((y == 0) || ((x % y) == 0));
}

int is_prime (int x) {
   int test = 2;
  prime_test:
   if (test > x) return 0;
   else if ((test == x) || ((2 * test) > x)) return 1;
   else if (is_factor(test, x)) return 0;
   else {
     test++;
     goto prime_test;
   }
}

void usage () {
   printf("Usage: prime n, where n is a positive integer");
}

/* checks if string *str is a number */
int isnum (char *str) {
   int result = 1,
       i = 0;

   for (; str [i]; i++)
     if (!strchr ("-0123456789", str [i]))
       result = 0;

   return result;
}

int main(int argc, char *argv[]) {
   if ((argc != 2) || !isnum(argv[1])){
     usage();
     return 0;
   }

   int x = atoi(argv[1]);
   printf("%d is %sprime.\n", x, (is_prime(x) ? "" : "not "));
   return 0;
}