筛素数

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

#define MAXN 2000000+10

int vis[MAXN];

int main() {
    int beg, end; scanf("%d%d", &beg, &end);
    int m = sqrt(end+0.5);
    memset(vis, 0, sizeof(vis)); vis[1] = 1;
    int i, j;
    for(i = 2; i <= m; i++) if(!vis[i])
        for(j = i*i; j <= end; j += i) vis[j] = 1;
    int cnt = 0;
    for(i = beg; i <= end; i++) if(!vis[i]) {
        cnt++;
        printf("%d ", i);
        if(cnt == 5) { cnt = 0; printf("\n"); }
    }
    return 0;
}