commands/factor/factor.c

   1 /* factor - print the prime factors of a number      Author: Andy Tanenbaum */
   2
   3 #include <stdlib.h>
   4 #include <stdio.h>
   5
   6 int main(int argc, char **argv);
   7 long first(long k);
   8
   9 int main(argc, argv)
  10 int argc;
  11 char *argv[];
  12 {
  13 /* Factor a number */
  14
  15   long i, n, flag = 0;
  16
  17   if (argc != 2 || (n = atol(argv[1])) < 2) {
  18         printf("Usage: factor n   (2 <= n < 2**31)\n");
  19         exit(1);
  20   }
  21   if (n == 2) {
  22         printf("2 is a prime\n");
  23         exit(0);
  24   }
  25   while (1) {
  26         i = first(n);
  27         if (i == 0) {
  28                 if (flag == 0)
  29                         printf("%ld is a prime\n", n);
  30                 else
  31                         printf("%ld\n", n);
  32                 exit(0);
  33         }
  34         printf("%ld ", i);
  35         n = n / i;
  36         flag = 1;
  37   }
  38 }
  39
  40
  41 long first(k)
  42 long k;
  43 {
  44 /* Return the first factor of k.  If it is a prime, return 0; */
  45
  46   long i;
  47
  48   if (k == 2) return(0);
  49   if (k % 2 == 0) return (2);
  50   for (i = 3; i <= k / 3; i += 2)
  51         if (k % i == 0) return(i);
  52   return(0);
  53 }