埃氏筛

写个埃氏筛模板

#include<bits/stdc++.h>
using namespace std;
bool pri[1000005];
void shai(int n){
	pri[0] = pri[1] = false;
    for(int i = 2;i * i <= n;i++){
    	if(pri[i]){
        	for(int j = i * i;j <= n;j += i) pri[j] = false;
        }
    }
}
int main(){
    memset(pri,true,sizeof pri);
    int n; cin >> n;
    shai(n);
    int ans = 0;
    for(int i = 1;i <= n;i++){
    	if(pri[i] == true) ans++;
    }
    cout << ans;
    return 0;
}