Speed up program?

[Zigster]

I created a program to give me the sum of all prime numbers below 2 million. It works, but it takes a really long time to process. I have tried to fiddle around with it, trying to figure out where I can take out ifs and such, but hasn't made any difference. How can I make it go faster?

public class Problem10 {
	public static void main (String[] args){
		
		int x, y;
		long a = 0;
		
		for (x= 2; x <= 2000000; x++)
		{
			if (x==2000000)
			{
			System.out.println("The sum of all the primes below two million:" + a);
			System.exit(0);
			}
			
			y=2;
			outerloop:
				do 
				{
					
					if (PrimeN(x, y))
					{
					break outerloop;
					}
					y++;
					}
					while (y<x);
			
				if (x==y)
				{
				a +=x;
				
				}
					
				
		}
	}	
	
	
	public static boolean PrimeN(int a, int b){

		if (a%b==0){
			return true;
		}

		else
		{
			return false;
		}
		
	}

}

Cheers!

[DarrylBurke]

For one, you could cut down on the iterations by using while(y * y < x). think about why that's valid first.

There are probably other optimizations possible.

db

[Norm]

Remove the PrimeN method. It only does one thing that could be done in-line.

[Zigster]

Yes, because if y * y = x, x is not a prime number. Thanks, I will try it and see if it speeds up. Ok thanks I will remove the PrimeN method too.

[JosAH]

Use a Sieve for finding those prime numbers; a couple of million bits isn't that much and the creation of a Sieve is fast.

kind regards,

Jos

[Zigster]

Use a Sieve for finding those prime numbers; a couple of million bits isn't that much and the creation of a Sieve is fast.

kind regards,

Jos

Thanks Jos, that looks awesome!