#include <stdio.h>
double iterFact(int);
double recurFact(int);

int main(){
   int n;
   printf("n:(>=0): ");
   scanf("%d", &n);
   while (n < 0) 
   { /*error loop */
     printf("n:(>=0): ");
     scanf("%d", &n);
   }
  printf("%d factorial is %f.\n", n, iterFact(n));
  printf("%d factorial is %f.\n", n, recurFact(n));
}

double recurFact(int n)
{ /*recursive version */
  if ((n==0) || (n==1))  return 1.0;
  return n*recurFact(n-1);
}

double iterFact(int n)
{/* find the factorial, return as a double
    to keep it from overflowing */
   int i;
   double answer;
   if ((n == 0) || (n == 1)) return 1.0;
   answer = 1.0;
   for (i = n; i > 1; i--)
      answer *= i;
   return answer;
}