Go: package main

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

func main() {
    var limit = 1000000
    var root = 1000
    var isPrime [1000000]bool
    for i := 0; i < limit; i++ {
        isPrime[i] = true
    }
    isPrime[0] = false
    isPrime[1] = false
    for i := 2; i <= root; i++ {
        if isPrime[i] {
            for j := i * i;j < limit; j+=i {
                isPrime[j] = false
            }
        }
    }
    var primeCnt [1000000]int
    var cnt = 0
    for i := 0; i < limit; i++ {
        if isPrime[i] {
            cnt++
        }
        primeCnt[i] = cnt
    }

    var sc = bufio.NewScanner(os.Stdin)
    for sc.Scan() {
        n, _ := strconv.Atoi(sc.Text())
        fmt.Println(primeCnt[n])
    }
}

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 prime[max_n];

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

int main(){
    rep(i,max_n) prime[i]=true;
    for(int i=2;i<max_n;i++){
        if(!prime[i]) continue;
        for(int j=2;i*j<max_n;j++) prime[i*j] = false;
    }

    while(cin>>N){
        solve();
    }
}

C++: #include<stdio.h>
int isprime(int n){
	int i;
	for(i=2;i*i <= n;i++){
		if(n%i==0)return 0;
	}
	return 1;
}
int main(){
	int sum[1000];
	int i, n;
	sum[0]=0;
	while(scanf("%d",&n) != EOF){
		for(i=2;i<=n;i++){
			if(isprime(i)){
				sum[i+1]=sum[i]+1;
			}else{
				sum[i+1]=sum[i];
			}
		}
		printf("%d\n",sum[n+1]);
	}
	return 0;
}

C++: 
#include<iostream>
#include<string>
using namespace std;
int main(){
    int n;
    bool a[1000000]={false};
    for(int i=2;i<1000;i++){
        bool t=false;
        for(int j=2;j*j<=i;j++){
            if(i%j==0)
                t=true;
        }
        if(i==2)
            t=false;
        if(t==false){
            for(int j=i*2;j<1000000;j+=i)
                a[j]=true;
            a[i]=false;
        }
    }
    while(cin>>n){
        int count=0;
        for(int i=0;i<=n;i++){
            if(a[i]==false)
                count++;
        }
        cout<<count-2<<endl;
    }
    return 0;
}

C++: #include <stdio.h>

const int MAX_V = 1000000;
int prime[MAX_V+1];

void init(){
    int i, k, v;
    for(i=2; i<=MAX_V; i++){
        prime[i] = 1;
    }
    for(i=2; i*i<=MAX_V; i++){
        if(prime[i]){
            for(k=2*i; k<=MAX_V; k+=i){
                prime[k]=0;
            }
        }
    }   
}

int isprime(int n){
    return prime[n];
}

int main(){
    init();
    int num;
    while(scanf("%d", &num) != EOF){
        int count=0;
        for(int i=2; i<=num; i++){
            if(isprime(i)==1)
                count++;
        }
    printf("%d\n", count);
    }
    return 0;
}

C++: #include<stdio.h>
int main(){
	int a,b,c,n;
	c=0;
	scanf("%d",&n);
		for(a=2;a<=n;a++){
			for(b=2;b<=n;b++){
				if(a%b==0&&a!=b){
					break;
				}else if(a%b==0&&a==b){
					c++;
				}else{
				}
			}
		}
		printf("%d\n",c);
	return 0;
}

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


int main(){
	int number,cnt=0;
	cin>>number;
	int* arr = new int[number+1];
	for (int i = 1; i <= number; i+=2)
	{
		arr[i-1]=0;
		arr[i]=1;
	}
	for (int i = 3; i < sqrt(number); i+=2)
		if (arr[i])
			for (int j = i+i; j <=number; j+=i)
				arr[j]=0;
	arr[2]=1;arr[1]=0;
	for (int i = 0; i < number; i++)
		if (arr[i])
			++cnt;
	cout<<cnt;
}

C++: #include <iostream>
#include <cmath>

using namespace std;


int main(){
	const int MAX_NUM=1000000;
	int table[MAX_NUM]={};
	for(int i=2;i<1001;i++){
		if(table[i]==1)continue;
		for(int j=i*2;j<MAX_NUM;j+=i){
			table[j]=1;
		}
	}

	int input;
	while(cin>>input){
		int count=0;
		for(int i=2;i<=input;i++){
			//cout<<"table:"<<i<<"="<<table[i]<<"\n";		
			if(table[i]==0)count++;
		}
		cout<<count<<"\n";
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
int main()
{
	bool p[1000000] = { false };
	int n;
	for (int i = 2; i <1000000; i++){
			for (int i2 = 2; i*i2 <= n; i2++){
				p[i*i2] = true;
			}
		}
		p[2] = false;
		p[3] = false;
	while (cin >> n){
		
		
		int c = 0;
		for (int i = 2; i <= n; i++){
			if (!p[i])c++;
		}
		cout << c << endl;
	}
}

C++: #include<cstdio>
using namespace std;
const int N = 1000000;

int main(){

  int IsPrime[N];
  IsPrime[0] = 1;
  IsPrime[1] = 1;
  for(int i=2; i*i<=N;i++){
    if(IsPrime[i]==0){
      for(int j=2*i; j<=N; j+=i)
        IsPrime[j] = 1;
      }
  }
  int max;
  while(~scanf("%d",&max)){
    int count=0;
    for(int i=1; i<max+1; i++)
      if(IsPrime[i]==0) count++;
    printf("%d\n",count);
  }
}

C++: // http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0009
// for ref

#include <iostream>
#include <cstring>
#define Rep(i, b, e)\
    for (int i = (int)b; i < (int)e; i++)

using namespace std;

const int MAXN = 999999;
bool isPrime[MAXN];

int countPrime(int n) {
    memset(isPrime, true, sizeof isPrime);
    Rep(i, 0, 1000)
        isPrime[i] = true;

    isPrime[0] = false;
    isPrime[1] = false;
    // isPrime[2] = true;
    // isPrime[3] = true;

    Rep(i, 2, n) {
        Rep (k, 2, 100) {
            isPrime[i*k] = false;
        }
    }

    int primeCount = 0;
    int j = 0;
    // while (primeCount < n) {     // GS q
    while (j <= n) {
        if (isPrime[j]) {
            // cout << primeCount << " th prime\t";
            // cout << j << endl;
            primeCount++;
        }
        j++;
    }

    return primeCount;
}

int num;

int main()
{
    while (cin >> num) {
        cout << countPrime(num) << endl;
    }
 //   cout << "Hello world!" << endl;
    return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
	int n, cnt;	

	while(cin >> n){
		bool prime[n+1] = {false};
		prime[1] = true;
		
		// ??¨??????????????????????????????
		for(int j = 2; j*j <= n; j++){
			if(prime[j] == false){
				for(int k = j; k*j <= n; k++){
					prime[j*k] = true;
				}
			}
		}
		cnt = 0;
		for(int i = 1; i <= n; i++){
			if(prime[i] == false) cnt++;
		}
		
		cout << cnt << endl;
	}
}