/*
    Factorial.java
    CIS 111
    10/29/2007
    David G. Klick

    Calculates the factorial of integers from 0 through 20.
*/

public class Factorial extends CIS111App {
    public static void main(String[] args) {
        int n = 0;
        do {
            n = getInt("Enter a number (0-20) you want the factorial of (-1 to quit): ");
            if (n >= 0 && n <= 20) {
                println(n + "! = " + fact(n));
            } else if (n != -1) {
                println("Error: Invalid input");
            }
        } while (n != -1);
    }
   
    private static long fact(long n) {
        if (n < 2) {
            return 1;
        } else {
            return n * fact(n-1);
        }
    }
}