divide by zero error

Question

here is the code (java):

class prime
{

    public static boolean prime (int a, int b) 
    { 
        if (a == 0) 
        {
            return false; 
        }
        else if ((a%(b-1) == 0) && (b>2))
        {
            return false; 
        }
        else if (b>1) 
        {
            return (prime (a, b-1)) ;
        }
        else
        {
            return true; 
        }

    }

    public static void main (String[] arg) 
    {
        System.out.println (prime (7, 7)) ; 
    }
}

This is the error message i get when i try to run it (it compiles fine):

Exception in thread "main" java.lang.ArithmeticException: / by zero
    at prime.prime(prime.java:10)
    at prime.prime(prime.java:16)
    at prime.prime(prime.java:16)
    at prime.prime(prime.java:16)
    at prime.prime(prime.java:16)
    at prime.prime(prime.java:16)
    at prime.prime(prime.java:16)
    at prime.main(prime.java:27)

So this means i devided by zero some how right? or does it mean something else? I don't see how i'm dividing by zero. What went wrong?

Answer 1

I assume any code of the form x % 0 will throw this error. Your code does not guard against this possibility.

Answer 2

Because of your recursive call:

return (prime (a, b-1)) ;

You will at some point be calling prime with a value for b of 1. Which means on your second condition you will be testing a%0. Since the modulo operator (%) is essentially a divide, that is bringing your divide by zero issue.

The solution is probably to catch this case to enforce b > 2 in your condition before doing the %.

Answer 3

Try turning this around

if ((a%(b-1) == 0) && (b>2))

to

   if ((b>2) && a%(b-1)==0)

What's happening is that the a%(b-1) operation is being executed before the b>2 test.

After the switch, you are taking advantage of short-circuit evaluation. Once the b>2 test returns false, then there's no need to calculate the modulus (hence avoiding the division)