Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
/*{{{*/  //template
#define rep(i,n) for(int i=0;i<n;i++)
constexpr int INF = numeric_limits<int>::max()/2;
constexpr long long LINF = numeric_limits<long long>::max()/3;
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define sz(x) (int)(x).size()
#define debug(x) cerr<<#x<<":"<<x<<endl
#define debug2(x,y) cerr<<#x<<","<<#y":"<<x<<","<<y<<endl
//struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double{ double d; explicit Double(double x) : d(x){} };
ostream& operator<<(ostream& os,const Double x){ os << fixed << setprecision(20) << x.d; return os; }
template<typename T> ostream& operator<<(ostream& os,const vector<T>& vec){ os << "["; for(const auto& v : vec){ os << v << ","; } os << "]"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os,const pair<T,U>& p){ os << "(" << p.first << ","<< p.second <<")"; return os; }
template<typename T> ostream& operator<<(ostream& os,const set<T>& st){ os<<"{"; for(T v:st) os<<v<<","; os <<"}"; return os; }
template<typename T,typename U> inline void chmax(T &x,U y){ if(y>x) x = y; }
template<typename T,typename U> inline void chmin(T &x,U y){ if(y<x) x = y; }
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a,ll b){ if(b==0) return a; else return gcd(b,a%b); }
//constexpr double eps = 1e-14; 
constexpr double eps = 1e-10; 
constexpr ll mod = 1e9+7;
const int dx[]={1,0,-1,0} ,dy[] = {0,1,0,-1};
/*}}}*/

ll N;

constexpr ll max_n = 1000006;
bool is_prime[max_n];

void solve(){
    int cnt=0;
    for(int i=2;i<=N;i++) if(is_prime[i]) cnt++;
    cout << cnt << endl;
}

int main(){
    rep(i,max_n) is_prime[i]=true;

    for(int i=2;i<max_n;i++){
        if(!is_prime[i]) continue;
        for(int j=2;i*j<max_n;j++) is_prime[i*j]=0;
    }
    while(cin>>N){
        solve();
    }
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <string>
#include <map>
#include <bitset>
#include <vector>
#include <queue>

using namespace std;

typedef long long ll;
#define FOR(i,a,b)   for(ll i = (a); i < (b); i++ )
#define REP(i, n)    FOR(i,0,n)
typedef pair< ll, ll >   cp2;
typedef pair< ll, cp2 >   cp3;
#define fi first
#define se second
#define sec  se.fi
#define thr  se.se
const ll mod = 1000000007;
//              123456789

 
///////////////////////////////////////////////
//?????§??¬?´???°??¨????°???¬?????°
//O(1)?????????
///////////////////////////////////////////////

////////////////////////////////////////////////
////////////////////////////////////////////////

#define NUM_MAX   10000000
bool flag[NUM_MAX];
vector< int >prm;

int N;
int ans = 0;
int K;

void eratosthenes(){
	if( N < 2 ) return;
	fill( flag, flag+NUM_MAX, 0 );
	int idx = 2;
	while( idx <= K ){
		while( flag[idx] ) idx++;
		if( idx > N ) break;
		prm.push_back( idx );
		for( int i = idx; i <= N; i += idx ) flag[i] = 1;
		ans++;
	}
	while( idx < N ){
		idx++;
		if( !flag[idx] ){
			prm.push_back( idx );
			ans++;
		}
	}
}


int main(){
	
	while( cin>>N, N != EOF ){
		K = sqrt( N );
		K++;
		
		eratosthenes();
		
		cout<<ans<<endl;
		/*
		REP( i, prm.size() ){
			cout<<prm[i]<<endl;
		}*/
		prm.clear();
		ans = 0;
	}	
	return 0;
	
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main(){

  int n;
  int a,b,i;
  bool count[1000000];
  count[2] = true;
  for(a=3;a<1000000;a++){
    i = 0;
    if(a%2!=0){
      for(b=3,i=1;b<=a/3;b++){
	if(b%2!=0){
	  if(a%b == 0){
	    i = 0;
	    break;
	  }
	}
      }
    }
    if(i)
      count[a] = true;
    else
      count[a] = false;
  }

  while(cin >> n){
    for(a=2,b=0;a<n;a++)
      if(count[a] == true)
	b++;
    cout << b << endl;
  }
  return 0;
}

C++: #include<bits//stdc++.h>
using namespace std;
int main(){
	int a[1000000];
	a[0]=2;
	int z=1;
	for(int i=2;i<100000;){
		for(int j=0;j<z;j++){
			if(i%a[j]==0)goto stop;
		}
		a[z]=i;
		z++;
		stop:;
		if(i==2) i++;
		else i+=2;
	}
	int b;
	while(cin>>b){
		int c=0;
		for(int i=0;i<z;i++){
			if(a[i]>b)break;
			else c++;
		}
		cout<<c;
	}
}

C++: #include <iostream>
#include <cmath>
using namespace std;

int main() {
	int n, ans; bool not_prime[1000000 + 1];
	not_prime[0] = true;
	not_prime[1] = true;
	for (int i = 2; i <= sqrt(1000000); i++) {
		if (!not_prime[i]) {
			for (int j = i * 2; j <= 1000000; j += i) {
				not_prime[j] = true;
			}
		}
	}
	for (int i = 0; i < 1000000 + 1; i++) {
		if (!not_prime[i]) { not_prime[i] = false; }
	}
	while (cin >> n) {
		ans = 0;
		for (int i = 2; i <= n; i++) {
			if (not_prime[i] == false) { ans++; }
		}
		cout << ans << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#include <stdio.h>
using namespace std;
int isprime(int x){
    if(x==2||x==3) return true;
    if(x<5||x%2==0||x%3==0) return false;
    else{
        int i=5;
        while(i<=sqrt(x)){
            if(x%i==0) return false;
            i = i+2;
        }
        return true;
    }
}
int main() {
    int n;
    while(cin >> n){
        int ans=0;
        int i=2;
        while(n-i){
            if (isprime(i)) {
                ans++;
            }
            i++;
        }
        
    cout << ans << endl;
    }
}

C++: #include<iostream>

using namespace std;

int main() {
    int n = 999999;
    char* p;
    int t = 2;

   

    p = new char[n];
    for(int i = 0; i < n; i++) {
        p[i] = 1;
    }
    p[0] = p[1] = 0;

    for(int i = 2; i < n; i++) {
        if(p[i] == 0)
            continue;
        for(int point = i * 2; point < n; point += i) {
            p[point] = 0;        
        }
    }
    
    while(cin >> n) {
        int c = 0;
        for(int i = 1; i <= n; i++) {
            if(p[i]) {
                c++;
            }
        }
        cout  << c << endl;
    }



    return 0;
}

C++: #include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<iostream>
#include<string>
using namespace std;

bool isPrime(int num) {
	if (num == 1)return false;
	if (num == 2)return true;


	if (num % 2 == 0)return false;
	for (int i = 3; i *i <= num; i += 2) {
		if (num % i == 0)return false;
	}
	return true;
}


int main() {

	int num;
	int count;
	while (cin >> num) {
		count = 0;
		for (int i = 1; i <= num; i++) {
			if (isPrime(i))count++;
		}
		printf("%d\n", count);

	}

	//system("pause");
	return 0;
}