[GrimReaperCdn]

Written in C++ and this check's if a number is prime. It's really simple, and it returns '0' if it's prime or '1' if it's not prime.

Wondering if anyone can make this more efficient?

PHP Code:

int IS_prime(double num)
{
    int isprime = 0;
    for(int i = 2; i <= sqrt(num); i += 2)
    {
        if(i % 2 == 0)
            i++;

        if((int(num)% i) == 0)
        {
            isprime = 1;
            break;
        }
    }

    return isprime;
} 


[Sylwester]

depends...
If you know the upper limit and you want to use it very often then you can use this:

PHP Code:

const max=10000;
bool isPrime[max];

// ...

for (int i=0; i<max; i++)
    isPrime[i] = true;
int i,j;
i = 2;
while (i<max)
{
    if (isPrime[i])
    {
        j = i*i;
        while (j<max)
        {
            isPrime[j] = false;
            j*=i;
        }
    }
    i++;
}

// ...

bool Is_Prime(int num)
{
    return isPrime[num];
} 


[Twilight Suzuka]

http://en.wikipedia.org/wiki/Primality_test

[GrimReaperCdn]

Now lets find the pattern to prime numbers. lol.

[TheNewt]

The pattern is... You can't divide the number of anything but itself and 1 to get another whole number.
:gasp:


5/1.... nope 5/2.. nope 5/3.... nope 5/4... nope... 5 IS PRIME!
I r teh smartiest.

[Pro Patria Finland]

/me bows down to TheNewt and ties his shoelaces around his feet

[[ --<-@ ] Black Rose]

EDIT: Crap.

[TheNewt]

You can still optimize it further with a simple check.

[GrimReaperCdn]

Wow, 2 min and 40 seconds? Mine did 1 million in 14 seconds.

[TheNewt]

modulo is a powerful mathematical tool ;)