Sum of Primes - why does my code return 0?

Question

I'm trying to answer project Euler #10, which is to find the sum of primes between 1-2000000. Every time I run my code, it gives me 0, and I can't figure out why.

public static boolean isPrime (int n) 
{
    double a = Math.sqrt(n);
    for(int i = 1; i < a; i++){
        if(n % i == 0){
            return false;
        }
    }
    return true;
}

public static void main (String[] args)
{
    long sum = 0;
    for(int i = 2; i < 2000000; i++){
        if(isPrime(i)==true){
            sum+= i;
        }
    }
    System.out.println(sum);
}

If you used a debugger, you could find out yourself: http://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems — Raedwald, Oct 22 '14 at 06:49

score 15 · Answer 1 · answered Oct 21 '14 at 06:21

15

Change

for (int i = 1; i < a; i++) {

to

for (int i = 2; i < a; i++) {

Any number is divisible by 1, even prime numbers.

Based on your current code, no number is prime.

answered Oct 21 '14 at 06:21

Eran

387,369
54
702
768

akash · Answer 2 · 2014-10-21T06:28:00.427

4

Apart from the logic in your code any integer modulo 1 is zero makes your isPrime method to break out of the loop and return false, so you should start loop from 2 instead.

 for(int i = 1; i < a; i++){
        if(n % i == 0){//for 1 loop gets break;

edited Oct 21 '14 at 06:28

answered Oct 21 '14 at 06:22

akash

22,664
11
59
87

score 2 · Answer 3 · answered Oct 21 '14 at 06:22

Please check out the typo as mentioned already..

Also in isPrime method, i should start with 2 and not 1.

public static boolean isPrime(int n){
        double a = Math.sqrt(n);
        for(int i = 2; i < a; i++){
            if(n % i == 0){
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args){
        long sum = 0;
        for(int j = 2; j < 2000000; j++){
            if(isPrime(j)==true){
                sum+= j;
            }
        }
        System.out.println(sum);
    }

score 2 · Answer 4 · answered Oct 21 '14 at 07:33

2

Your isprime logic is wrong,

if(n % i == 0)

ALWAYS returns a true statement causing it to return false. All numbers are divisible by 1.

answered Oct 21 '14 at 07:33

Luigi

439
5
23

score 1 · Answer 5 · edited Oct 29 '14 at 18:26

1

public static boolean isPrime (int n) 
{   
    double a = Math.sqrt(n);
    int b =(int)Math.sqrt(n);
    if(Math.pow(b,2)==n)
    {
        return false;
    }

    for(int i = 2; i < a; i++){
        if(n % i == 0){
            return false;
        }
    }
    return true;
}

public static void main (String[] args)
{
    long sum = 0;
    for(int i = 2; i < 200000; i++){
        if(isPrime(i)==true){
            sum+= i;
        }
    }
    System.out.println("Total is " +sum);
}

edited Oct 29 '14 at 18:26

StormeHawke

5,987
5
45
73

answered Oct 29 '14 at 18:24

user4195423

11
1

1

I fixed your code formatting for you but you'll want to be careful about that around here. Also, it's encouraged to provide a little explanation as to how your code answers the question – StormeHawke Oct 29 '14 at 18:27

Sum of Primes - why does my code return 0?

5 Answers5