Question

I’m trying to sort out some homework regarding “repetition” with the while statement. The homework is asking me to input a number and tell if said number is prime. So far, I’ve come with this:

class Prime {
    boolean esPrime(int n) {
        boolean prime = true;
        int divisor = 2;
        while (prime && divisor != n) {
            if (n % divisor == 0) {
                prime = false;
            } else {
                divisor++;
            }
        }

        return prime;
    }
}

Then I stated this “boolean test” in the main method to check if that piece of code worked:

boolean testEsPrime = esPrime(2) == false;

public static void main(String[] args) {
    Prime p = new Prime();
    System.out.println("testEsPrime = " + p.testEsPrime);
}

And whenever I run it I get false and I can’t seem to be able to spot the error. Any clue why this happens?

Answer 1

It’s not the most efficient code in the world, but it works for me (see below). I’m not sure what the problem is here! Incidentally, you only need to test 2 and odd factors up to the square root of the thing you’re testing, and a more efficient approach in general (for determining what numbers are primes in bulk) is to use a Sieve of Erastothenes (http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes).

The reason you get false for the test you do is that esPrime(2) == false is false – that is, esPrime(2) == true.

class Prime {
    boolean esPrime(int n) {
        boolean prime = true;
        int divisor = 2;
        while (prime && divisor != n) {
            if (n % divisor == 0) {
                prime = false;
            }
            else {
                divisor++;
            }
        }
        return prime;
    }

    public static void main(String[] args) {
        Prime p = new Prime();
        for(int i=2; i<10; ++i) {
            System.out.println("esPrime(" + i + ") = " + p.esPrime(i));
        }
    }
}

Output:

esPrime(2) = true
esPrime(3) = true
esPrime(4) = false
esPrime(5) = true
esPrime(6) = false
esPrime(7) = true
esPrime(8) = false
esPrime(9) = false