Prime Number Algorithm

Question

Can anyone tell me how to implement Sieve of Eratosthenes algorithm in C? I need to generate prime numbers but my algorithm is slow.

My code:

#inclu         


        

Answer 1

Toomtarm Kung' answer is great, thank's a lot.

There's however still one caveat I stumbled upon: (i*i) may overflow for i>46340 on 32 bit and i>3037000499 on 64-bits, producing incorrect results, i.e. 2147483647 will not be recognised as a prime when using 32-bit integers.

To avoid the integer overflow, one can replace (i * i)<=number by i <= number / i

Now, to avoid a double division, the code may be written as follows:

int isPrime(int number){
    if(number < 2) return 0;
    if(number == 2) return 1;
    if(number % 2 == 0) return 0;
    int j;
    for(int i=3; ((j=number/i) >= i); i+=2){
        if(number - (j * i) == 0 ) return 0;
    }
    return 1;
}