素数筛选法

#include <cstdlib>
#include <iostream>
#include <string.h>
#include <math.h>

using namespace std;
const int MAX = 1000000+5;

long vis[MAX],primes[MAX];

void sieve(long n)
{
     memset(vis,0,sizeof(vis));
     
     int m = (int)sqrt(n + 0.5);//最大到 n的 开方 
     for(long i = 2; i < m; i++)if(!vis[i])
        for(long j = i*i; j < n; j += i) 
            svis[j] = 1;
               
}

int gen_primes(long n)
{
    sieve(n);
    long c = 0;
    for(long i = 2; i < n; i++)
        if(!vis[i])primes[c++] = i;
    return c; 
}

int main(int argc, char *argv[])
{
    memset(primes,0,sizeof(primes));
    gen_primes(100000);
    for(long i = 0; primes[i]; i++)printf("%ld ===>%ld\n",i,primes[i]); 
    system("PAUSE");
    return EXIT_SUCCESS;
}

用高效的方法筛选小于n的素数