project euler #10: optimization with primes

Michael P. baseball.mjp at gmail.com
Wed Apr 1 04:58:10 PDT 2009


Spacen Jasset Wrote:

> Michael P. wrote:
> > Hey, I've started to do some of the problems on Project Euler to practice my programming skills. I'm doing #10 right now, and I think I've got a working solution. It's just that it's way too slow.
> > Here's the link:
> > http://projecteuler.net/index.php?section=problems&id=10
> > The code works fine with sum of primes below 10. It output 17.
> > But for 2 million, it takes quite a while, and an FAQ on the page said most problems are made with being run within a minute. Mine was gonna quite a bit longer.
> > Here is my code:
> > 
> > //Import
> > import std.stdio;
> > import std.math;
> > 
> > //Main
> > int main()
> > {
> > 	int sum = 2;
> > 	long bigNum = 10; //Or 2 million
> > 	for ( int i = 3; i < bigNum; i += 2 )
> > 	{
> > 		if( isPrime( i ) )
> > 		{
> > 			sum += i;
> > 		}
> > 	}
> > 	writefln( sum );
> > 	return 0;
> > }
> > 
> > //Functions
> > bool isPrime( long n )
> > {
> > 	for( int i = 2; i < n; i++ )
> > 	{
> > 		if ( n % i == 0 )
> > 		{
> > 			return false;
> > 		}
> > 	}
> > 	return true;
> > }
> > 
> > I'm pretty sure the isPrime() function can be done better.
> 
> One thing that may help a little is that you need only loop from 2 to 
> sqrt(n) in the isPrime function, otherwise you end up testing for 
> factors > sqrt(n) twice.  A factor over sqrt(n) must have a 
> corresponding factor less than sqrt(n) for a given n.

That helped. Ran in about 10-15sec I think after doing that. 
Thanks for the help everyone. 
-Michael P.


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