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<iostream>
using namespace std;
int main(){
	long long int N,count[1000000];
		count[1] = 1;
		if(N<0)	break;
	for(int x=2;x<=1000000;x++){
		int flag = 0;
		for(int y=2;y<x;y++){
			if((x%y)==0)	flag = 1;
		}
		if(flag == 0)	count[i] = count[i-1]+1;
	}
	while(cin >> N){
		cout << count[N] << endl;
	}
	return 0;
}

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

int a[1000000];

int main()
{
  int n;
  int count;

  for (int i = 2; i < 1000000; i++){
    a[i] = 1;
  }

  for (int i = 0; i * i < 1000000; i++){
    if  (a[i] == 1){
      for (int j = 2; j * i <= 100000; j++){
        a[j * i] = 0;
      }
    }
  }

  while (cin >> n){
    count = 0;
    for (int i = 0; i < 1000000; i++){
      if (a[i] == 1){
        count++;
      }
    }
  }

  cout << count << endl;

  return (1);
}

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

bool is_prime(int n){
  if(n<=1)
    return false;

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

  return true;
}

int main(){
  bool is_prime[1000000];

  is_prime[0] = is_prime[1] = false;
  for(int i=2; i<1000000; i++){
    is_prime[i] = true;
  }

  for(int i=2; i*i<1000000; i++){
    if(is_prime[i]){
      for(int j=i*i; j<1000000; j+=i){
        is_prime[i] = false;
      }
    }
  }

  int n_prime[100000];

  int cnt = 0;
  for(int i=0; i<1000000; i++){
    if(is_prime[i]){
      cnt++;
    }
    n_prime[i] = cnt;
  }

  while(true){
    int n;
    cin >> n;

    if(cin.eof())
      break;

    cout << n_prime[n] << endl;
  }

  return 0;
}

C++: 
#include<iostream>
#define N 1000000


using namespace std;
void PrimeNum(int n,bool Primeif[]){
  for(int i = 2;i*i < n;i++){
    if(Primeif[i] == true){
      for(int k = 2;k * i <= n;k++){
	Primeif[k*i] = false;
}
    }
  }
}
int main(){
  int n,num;
  bool Primeif[N+1];
  Primeif[1] = false;
  for(int i = 2;i < N;i++){
    Primeif[i] = true;
  }
  PrimeNum(N+1,Primeif);
  
  while(cin >> n){
    num = 0;
    for(int i = 0;i < n;i++){
      if(Primeif[i] == true){
	num = num+1;
      }
    }
     cout << num << endl;
     
   } 
    
}

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

#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define in(x) cin >> x
#define out(str) cout << str << endl

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

int n,ans;

int main(){
    int N=1000000;
    int prime[N];
    rep(i,N) prime[i] = 1;
    prime[0] = prime[1] = 0;
    rep(i,N){
        if(prime[i]){
            for(int j = i + i; j < N; j+=i){
                prime[j] = 0;
            }
        }
    }
    while(cin>>n){
        ans=0;
        rep(i,n+1)ans+=prime[i];
        out(ans);
    }
}

C++: #include<iostream>
using namespace std;
int main(){
	bool num[1000000] = {0};
	for(int i=2; i<1000; i++){
		if(!num[i]){
			for(int j=i*2; j<1000000; j+=i){num[j] = true;}
		}
	}
	int n;
	while(cin >>n){
		int f = 0;
		for(int i=2; i<=n; i++){
			if(!num[i]){f++;}
		}
		cout <<f<<endl;
	}
}

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

struct IsPrime {
 public:
  explicit IsPrime(int max);
  bool operator()(int n);

 private:
  std::vector<bool> is_prime;
};

IsPrime::IsPrime(int max) : is_prime(max + 1, true) {
  is_prime[0] = false;
  is_prime[1] = false;
  for (auto i = 2; i <= std::sqrt(max); i++) {
    if (!is_prime[i]) {
      continue;
    }

    for (auto mult = 2 * i; mult <= max; mult += i) {
      is_prime[mult] = false;
    }
  }
}

bool IsPrime::operator()(int n) { return is_prime[n]; }

int CountPrime(IsPrime& is_prime, int upto) {
  int cnt = 0;
  for (int i = 2; i <= upto; i++) {
    if (is_prime(i)) {
      cnt++;
    }
  }
  return cnt;
}

int main(void) {
  std::vector<int> inputs;
  int n;
  while (std::cin >> n) {
    inputs.push_back(n);
  }

  int max = *std::max_element(inputs.begin(), inputs.end());
  IsPrime is_prime{max};

  for (auto n : inputs) {
    std::cout << CountPrime(is_prime, n) << '\n';
  }
  return 0;
}

C++: #include <iostream>
#include <vector>
#include <math.h>
using namespace std;

bool testinPrimeSet(int num, vector<int> & pset) {
	for (vector<int>::iterator it = pset.begin(); it < pset.end() && sqrt(num) >= (*it); it++) {
		if ((num % (*it) == 0))
			return false;
	}
	return true;
}

int CountPrime(int num, vector<int> & pset) {
	int be = pset.at(pset.size() - 1);
	while (++be <= num) {
		if(testinPrimeSet(be, pset))
			pset.push_back(be);
	}
	return 0;
}

int getdst(int num, vector<int> & pset) {
	int i = 0;
	for (vector<int>::iterator it = pset.begin(); it < pset.end(); it++)
		if ((*it) <= num)
			i++;
		else
			break;
	return i;
}

int main() {
	int n = 0;
	int a[5] = { 2,3,5,7,11 };
	vector<int> dst;
	vector<int> primeset(a, a + 5);
	while (cin >> n) {
		CountPrime(n, primeset);
		dst.push_back(n);
	}
	for (vector<int>::iterator it = dst.begin(); it < dst.end(); it++)
		cout << getdst(*it, primeset) << endl;
	return 0;
}

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

int sieve(int n) {
    if (n <= 1) return 0;
    vector<bool> is_prime(n+1, true);
    is_prime[0] = is_prime[1] = false;

    int res = 0;
    for (int i = 2; i <= n; i++) {
        if (is_prime[i]) {
            res++;
            for (int j = i+i; j <= n; j += i) {
                is_prime[j] = false;
            }
        }
    }
    return res;
}

int main() {
    while (1) {
        int n;
        int ret = scanf("%d", &n);
        if (ret < 0) break;
        printf("%d\n", sieve(n));
    }
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int a[1000000];
void sive()
{
    int i,j;
    a[0]=a[1]=1;
    for(i=4;i<=1000000;i+=2) a[i]=1;
    for(i=3;i<=1000;i+=2){
        if(a[i]==0)
        {
            for(j=i*i;j<=1000000;j+=2*i) a[j]=1;
        }
    }
}
int main()
{
    sive();
    int n,i,j=0;
    cin>>n;
    for(i=1;i<=n;i++)
    {
        if(a[i]==0) j++;
    }
    cout<<j<<endl;

    return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
#include <stack>
#include <fstream>
#include <vector>
#include <map>
#include <set>
#include <cmath>
#include <algorithm>
#include <functional>
#define inf 0x3f3f3f3f
#define inff -30000000
using namespace std;
typedef long long ll;
const int MAXN=1e5+10;
const int MAX=1e6+10;
int prime[MAX];
bool notprime[MAX];
int num=0;
void init(){
    memset(notprime,0,sizeof(notprime));
    for(int i=2;i*i<=MAX;i++){
        if(!notprime[i]){
            prime[num++]=i;
            for(int j=i*i;j<=MAX;j+=i)
                notprime[j]=1;
        }
    }
}
int main(){
    #ifdef SIYU
    freopen("in.txt","r",stdin);
    #endif // SIYU
    int n;
    init();
    while(cin>>n){
        //int *p=upper_bound(prime,prime+num,n);
        //int ans=p-prime;
        //cout<<ans<<endl;
        int ans=0;
        for(int i=0;i<num;i++){
            if(prime[i]<=n)
                ans++;
        }
        cout<<ans<<endl;
    }
    return 0;
}