#include <stdio.h>

int main() 
{
    int num, product, i;
	
    /* Prompt user for a number and read it in */
    printf("Enter number: ");
    scanf("%d", &num);
	
    /* One way to do it - using a while loop, work backwards */
    product = 1;
    i = num;
    while(i >= 1) {
        product *= i;	
        i = i - 1;	/* Can also be written as "i--" or "i -= 1" */
    }
    printf("Factorial of %d = %d\n", num, product);
	
    /* Another way to do it - using a for loop */
    product = 1;
    for(i=1; i <= num; i++) {
	product *= i;
    }
    printf("Factorial of %d = %d\n", num, product);
	
    return 0;
}

/* Try determining factorial of 12 and then the factorial of 13.
** The latter should (of course) be 13 times bigger than the 
** former, but you'll notice this is not the case. 
**
** 12! = 479001600
** 13! = 6227020800 (answer reported by this program: 1932053504)
**
** Try 17 as input - you'll notice you get a negative number out!
**
** 32 bit two's complement can represent numbers from
** -2^31 to (2^31)-1
** i.e.-2147483648 to +2147483647
*/