#include <iostream>
#include <cstdlib>	//for EXIT_SUCCESS, EXIT_FAILURE, and the exit function
using namespace std;

void factorial(int n);

int main()
{
	//Each expression in parentheses here in main is called an
	//"actual argument" of the function.

	factorial(4);   //Pass the value 4 to the function.
	factorial(5);   //Pass the value 5 to the function.
	int i {6};
	factorial(i);	//Create a copy of i and pass the copy to the function.
	factorial(i+1);	//Pass the value 7 to the function.

	return EXIT_SUCCESS; //Inside of main is the only place you can do this.
}

void factorial(int n) //n is a "formal argument" or "parameter" of the function.
{
	if (n < 0) {
		cerr << "argument " << n << " can't be negative.\n";
		exit(EXIT_FAILURE);  //Outside of main, exit instead of return.
	}

	int f {1};

	for (int i {1}; i <= n; ++i) {
		f *= i;   //means f = f * i
	}

	cout << "The factorial of " << n << " is " << f << ".\n";
}