Eratosthenes筛法

Eratosthenes筛法

#include <stdio.h>
#include <string.h>
#include <math.h>

//Eratosthenes
const int maxn = 10000000 + 5;
const int maxp = 700000;
int vis[maxn];
int prime[maxp];

void sieve(int n)  //screen out the prime numbers
{
    int m = sqrt(n + 0.5);

    memset(vis, 0, sizeof(vis));
    for(int i = 2; i <= m; i++)
        if(!vis[i])
            for(int j = i * i; j <= n; j += i)
                vis[j] = 1;
}

int prime_numbers(int n)
{
    sieve(n);
    int c = 0;  //the number of prime numbers
    for(int i = 2; i <= n; i++)
        if(!vis[i])
        {
            prime[c] = i;
            c++;
        }
    return c;
}
//the end of Eratosthenes
int main()
{
    int t;

    while(scanf("%d", &t) != EOF)
    {
        printf("total prime numbers: %d\n\n", prime_numbers(t));
        for(int i = 0; i < prime_numbers(t); i++)
            printf("%d\n", prime[i]);
    }

    return 0;
}