// factor.c -- uses loops and recursion to calculate factorials
#include <stdio.h>

long fact(int n);
long rfact(int n);

int main(void)
{
	int num;
	
	printf("This program calculates factorials.\n");
	printf("Enter a value in the range 0-12 (q to quit): ");
	while (scanf("%d", &num) == 1)
	{
		if (num < 0)
			printf("No negative numbers, please.\n");
		else if (num > 12)
			printf("Keep input under 13.\n");
		else
		{
			printf("loop: %d factorial = %ld\n",
			       num, fact(num));
			printf("recursion: %d factorial = %ld\n",
			       num, rfact(num));
		}
		printf("Enter a value in the range 0-12 (q to quit): ");
	}
	printf("Bye.\n");
}

// loop-based function
long fact(int n)
{
	long ans;
	
	for (ans = 1; n > 1; n--)
		ans *= n;
	return ans;
}

// recursive version
long rfact(int n)
{
	long ans;
	
	if (n > 0)
		ans = n * rfact(n-1);
	else
		ans = 1;
	return ans;
}