Hi i''ve been looking for a way to write a isPrime(int x), but have only found abstract ways to solve to problem. Is there an easy way to do it? Thanks!

Hi there,
This is how i do it, its prob the slowest way, but it works, shouldn''t have any problem at all chewing through numbers up to 1 mil, i think it starts to get slow when going through numbers up to 8 digits and more.

Shabadoo

  bool isPrime( __int64 num ){	if( num == 3 )	{		return true;	}	__int64 sr = sqrt( num );	//if modding by even number there will always	//be a remainder as the primes are odd, so         //only mod with odd numbers        //only need to go up to sqrt of num to find        //out if it is prime	for( int i = 3; i <= sr; i += 2 )	{		/*test to see in num is prime */		if (num%i==0)		{			return false;  		}	}		return true;}  

(Oh my God, is this really English?...)

I don''t know what it has to do with games , but ok:

There are some sites with a lot of info to this subject, but I found one way very intresting. It is called the sieve of Erasthadocalotheasth (or something like that

// create some space:
bool *isprime = (bool *)malloc(sizeof(bool) * 1000001);

// make true
for(int counter = 0; counter < 1000000; counter++)
isprime[counter] = true;

// The numbers are prime, until you can prove the oposite.


// begin with the first one (two), then sign every number
// that is dividable by that number with a false

for(int possible = 2; possible < 1000000; possible++)
{
if(isprime[possible])
{
for(int factor = 2; factor*possible <= 1000000; factor++)
{
isprime[factor*possible] = false;
} // for
} // if
} // for

// Now you''ve an array filled with which numbers are prime
// and which are not.

I hope that that will help...

Thanks,

Those dead Greeks aren''t my best side, nice signature BTW!