Project Euler problem 10

[rajarshi roy]

I was trying to solve problem 10 of Project Euler .I wrote a code ,but it does not work for higher values.

  public class P10 {

	/**
	 * @param args
	 */
	public long findAns(long n){
		long ret=0;
		for (long i = 0;i<n; i++) {
			if(ifPrime(i)){
				ret+=i;
			}
		}
		return ret;
	}
	public boolean ifPrime(long num){  
        boolean b=false;  
        long i;  
        for (i=2; i < num ;i++ ){  
          long n = num%i;  
         if (n==0){  
            b=false;  
            break;  
          }  
        }  
       if(i == num){  
          b=true;  
        }  
       return b;  
    }  
	public static void main(String[] args) {
		// TODO Auto-generated method stub
System.out.println(new P10().findAns(2000000));
	}
}
  

Can anyone please help me out???

[Wouter Oet]

You'll probably overflow the long. Try it with BigInteger.

[Ulf Dittmer]

What do you mean by "higher values" - higher than 2000000? I think even an "int" should work fine for all involved fields for the original problem.

If you start with "boolean b = true", then you don't need the "if (i == num)" block. One should avoid accessing loop variables outside of the "for" block.

[Mike Simmons]

rajarshi roy wrote:I wrote a code ,but it does not work for higher values.

In what way does it not work? Do you get the wrong answer? Does it throw an exception? Does it take an unacceptably long time to finish?

Looking at your code, I suspect it's the latter. It's going to take a long time to check all those values to see if they're prime. To find is a value n is prime, do you really have to loop through every number less than n? There are a couple of huge improvements you can make on this method that will make it much faster.

Wouter Oet wrote:You'll probably overflow the long. Try it with BigInteger.

I'm pretty sure this problem can be done without BigInteger. All the primes are less than 2 million, and there are much less than 2 million of them, so their sum should be much less than 2 million times 2 million, which is 4 trillion. That should fit easily inside a long.

[Hunter McMillen]

You should look up something called the Sieve of Eratosthenes. It is a really quick way to test if a number is prime.

-Hunter

[rajarshi roy]

the code works fine for smaller values,say below 100000.But it takes exceptionally long time to finish for values like 1000000.I will try to change the code.Thanks for your advices.