/* test_factorial.c  the simplest example of a recursive function         */
/*                   a recursive function is a function that calls itself */

static int factorial(int n)  /* n! is n factorial = 1*2*3*...*(n-1)*n  */
{
  if( n <= 1 ) return 1;     /* must have a way to stop recursion      */
  return n * factorial(n-1); /* factorial calls factorial with n-1     */
}                            /* n * (n-1) * (n-2) * ... * (1)          */

#include <stdio.h>

int main()
{
  printf(" 0!=%d \n", factorial(0)); /* Yes, 0! is one */
  printf(" 1!=%d \n", factorial(1));
  printf(" 2!=%d \n", factorial(2));
  printf(" 3!=%d \n", factorial(3));
  printf(" 4!=%d \n", factorial(4));
  printf(" 5!=%d \n", factorial(5));
  printf(" 6!=%d \n", factorial(6));
  printf(" 7!=%d \n", factorial(7));
  printf(" 8!=%d \n", factorial(8));
  printf(" 9!=%d \n", factorial(9));
  printf("10!=%d \n", factorial(10));
  printf("11!=%d \n", factorial(11));
  printf("12!=%d \n", factorial(12));
  printf("13!=%d \n", factorial(13)); /* expect a problem with */
  printf("14!=%d \n", factorial(14)); /* integer overflow      */
  printf("15!=%d \n", factorial(15)); /* uncaught in C         */ 
  printf("16!=%d \n", factorial(16));
  printf("17!=%d \n", factorial(17)); /* obvious               */
  printf("18!=%d \n", factorial(18));

  return 0;
}

/* output of execution is:
  0!=1
  1!=1
  2!=2
  3!=6
  4!=24
  5!=120
  6!=720
  7!=5040
  8!=40320
  9!=362880
 10!=3628800
 11!=39916800
 12!=479001600
 13!=1932053504    wrong! 13! = 12! * 13, must end in two zeros
 14!=1278945280    wrong! and no indication!
 15!=2004310016    wrong!
 16!=2004189184    wrong!
 17!=-288522240    wrong and obvious if you check your results
 18!=-898433024    Only sometimes does integer overflow go negative
*/