What's wrong with the logic on this program?

Question

I have an issue with this fragment of code. It's part of a program that calculates what you would call nPr on some calculators. It compiles with no error but each time I run it and set n = 0 I get a seemingly endless list of error messages. Why can't it just print out "Error!" and terminate the program like I programmed it to?

{
    System.out.print("n = ");
    int n = j.nextInt();
    System.out.print("r = ");
    int r = j.nextInt();
    long n_fact = factorial(n);
    if ( ( (n - r) <= 0 ) || (n <= 0) || (r < 0) )
    {
        System.out.println("Error!");
        System.exit(0);
    }
    else
    {
        long nr_fact = factorial(n - r);
        long nPr = n_fact / nr_fact;
        NumberFormat f = NumberFormat.getNumberInstance();
        System.out.println("nPr = " + f.format(nPr));
    }
}

Answer 1

You're calling factorial(n) before checking whether n > 0 . Move the line

long n_fact = factorial(n);

to inside the else block.

Answer 2

Move this line in else block:

    long n_fact = factorial(n);

as it is getting triggered even when n=0 or n < 0 , as you are checking n > 0 after the above call, which you don't want. I doubt if this(n <=0) is properly handled in your factorial() method.

Alternatively put the check and handling on n in your factorial(n) method ie return 1, if n=0 and error out(throw excepion) if n < 0;