The factorial of a positive integer n, denoted by n!, is the product of all positive integers less than or equal to n. For example, 6! = 1 x 2 x 3 x 4 x 5 x 6 = 720.
 

#include<iostream>

using namespace std;

int main(){
	unsigned long int product = 1;
	int initial = 1;
	int n;

	cout << "enter positive integer: ";
	cin >> n;

	if (n >= 0){
		while(initial <= n){
			product = product * initial;
			initial ++;
		}
	}
	else{
		cout << "error: please enter positive number";
		return 0;
	}

	cout << n << "! = " << product;
 	return 0;
}

output: