Euler problem #10

[J. Kevin Robbins]

Another spoiler. This code works for values of 10 and 100, but according to the Euler site, I'm getting the wrong sum for the value of 2000000.

I've spent hours looking at this and can't figure out why it's inconsistent. Can anyone spot the problem?

public class Euler10 {

    private final static int LIMIT = 2000000;

    public static void main(String[] args) {

        int sum = 0;
        for (int i = 0; i <= LIMIT; i++) {
            if (isPrime(i)) {
                sum += i;
            }
        }
        System.out.println("total of all primes is " + sum);
    }

    private static boolean isPrime(int num) {
        if (num <= 1){
            return false;
        }
        int sqr = (int) Math.sqrt(num);
        for (int i = 2; i <= sqr; i++) {
            if (num % i == 0) {
                return false;
            }
        }
        //System.out.println(num + " is prime");
        return true;
    }
}

[Matthew Brown]

Overflow?

[J. Kevin Robbins]

Nope, the answer I get is 1,179,908,154. An int will hold up to 2,147,483,647.

I'm thinking maybe the square root cast to an int is causing a rounding error which might cause it to falsely identify a prime or two. My next step is to remove the use of the square root and just test all possible divisors.

[Matthew Brown]

Jk Robbins wrote:Nope, the answer I get is 1,179,908,154. An int will hold up to 2,147,483,647.

Yes, but what if it's already gone all the way around? You're storing the answer in an int - of course it's going to look like it's within range! Just switch to a long, see what happens.

[J. Kevin Robbins]

Doh! Got it!

It didn't even occur to me that it might have wrapped around. I thought it would throw an error if I tried to exceed the max value.

Live and learn. That's why we do these exercises, right?

Thanks!!