seive.cpp

#define _CRT_SECURE_NO_DEPRECATE
#pragma GCC optimize("O3")
#pragma GCC target("sse4")
#include<bits/stdc++.h>
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
//using namespace __gnu_pbds;
#define int long long int
#define SYNC std::ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL);
#define FRE freopen("input.txt","r",stdin);freopen("output.txt","w",stdout);
typedef long double ld;
typedef pair<int,int> ii;
typedef pair<int,ii> iii;
typedef vector<int> vi;
typedef vector<ii>   vii;
//typedef tree<int, null_type, less<int>, rb_tree_tag, 
//             tree_order_statistics_node_update> 
//    data_set;
#define rep(i,l,r)   for(int i=(l);i<(r);i++)
#define deb(x)     cout << (#x) << " = " << (x) << endl
#define here cout << " Hey baitch!!\n";
#define  pb push_back
#define  F  first
#define  S  second
#define all(v) (v).begin(),(v).end()
#define sz(a) (int)((a).size())
#define sq(x) ((x)*(x))
const int MOD = 1e9+7;
const int MOD1 = 998244353;
const int N = 2e5+5;
const int INF = 1000111000111000111LL;
const ld PI = 3.141592653589793116;
vector <int> prime;
bool is_composite[N];

void sieve (int n) {
	std::fill (is_composite, is_composite + n, false);
	for (int i = 2; i < n; ++i) {
		if (!is_composite[i]) prime.push_back (i);
		for (int j = 0; j < prime.size () && i * prime[j] < n; ++j) {
			is_composite[i * prime[j]] = true;
			if (i % prime[j] == 0) break;
		}
	}
}
int32_t main()
{
	SYNC
	sieve(1e6);
	cout << prime.size();;
	return 0;
}