/* Implements a factorial function fact recursively.  If the user inputs
   a negative number by mistake, it returns the error value 0.  This 
   avoids the possibility of infinite recursion.  It will still give the
   wrong value if n>12 because the value will go beyond the maximum range
   of long unsigned int.  */

#include<iostream.h>

long unsigned int fact(int);

void main()
{
  int num;

  cout << "Enter a nonnegative integer: ";
  cin >> num;

  cout << num << "! is " << fact(num) << ".\n";
}

long unsigned int fact(int n)
{
  if (n < 0)
    return 0;
  if (n == 0) 
    return 1; 
  return n*fact(n-1); 
}