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;

#define int long long
typedef vector<int>vint;
typedef pair<int,int>pint;
typedef vector<pint>vpint;
#define rep(i,n) for(int i=0;i<(n);i++)
#define reps(i,f,n) for(int i=(f);i<(n);i++)
#define all(v) (v).begin(),(v).end()
#define each(it,v) for(__typeof((v).begin()) it=(v).begin();it!=(v).end();it++)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
template<typename A,typename B>inline void chmin(A &a,B b){if(a>b)a=b;}
template<typename A,typename B>inline void chmax(A &a,B b){if(a<b)a=b;}

int f[1000000];

signed main(){
    fill_n(f,1000000,1);
    f[0]=f[1]=0;
    for(int i=2;i<1000000;i++){
        if(!f[i])continue;
        for(int j=i+i;j<1000000;j+=i)f[j]=0;
    }
    for(int i=1;i<1000000;i++)f[i]+=f[i-1];

    int n;
    while(cin>>n)cout<<f[n]<<endl;
    return 0;
}

C++: #include<cstdio>
#include<cstring>
using namespace std;
bool isPrime(int n)
{
    for(int i = 2; i * i <= n; i ++)
    {
        if(n % i == 0)
        {
            return false;
        }
    }
    return true;
}
int main()
{
    int n;
    while(scanf("%d", &n) != EOF)
    {
        int res = 0;
        for(int i = 2; i <= n; i ++)
        {
            if(isPrime(i))
            {
                res ++;
            }
        }
        printf("%d\n",res);
    }
    return 0;
}

C++: import java.io.*;

import java.util.*;
import java.math.*;



class Main{

	private static ArrayList<Integer> sum_check(int a){

		int count = 0;
		ArrayList<Integer> s = new ArrayList<Integer>();
		
		for(int i=0;i<=a;i++){
			s.add(0);
		}
		s.set(2,1);
		for(int i=3;i<=a;i=i+2) s.set(i,1);
		for(int i=3;i<=Math.sqrt(a);i=i+2){
			if(s.get(i) == 0) continue;
			for(int j=i+i;j<=a;j=j+i)s.set(j,0);
		}		
		return s;

	}

	

		public static void main(String args[]) throws IOException{

			Scanner scan = new Scanner(System.in);

			int n,count;
			ArrayList<Integer> a = new ArrayList<Integer>();

			ArrayList<Integer> res = new ArrayList<Integer>();


			while(scan.hasNext()){
				count = 1;

				String str1 = scan.next();

				n = Integer.valueOf(str1).intValue();

				if(n != 1){
				a = sum_check(n);
				for(int i=3;i<=n;i=i+2){
					if(a.get(i) == 1)count++;
				}
				res.add(count);
				}
				else res.add(0);

			}
			for(int i=0;i<res.size();i++){
				System.out.println(res.get(i));
			}

		}

}

C++: #include<iostream>
#include<stdio.h>
#define M 1000000
using namespace std;
int a[M]={0};
int main()
{
    int n,i,j,k=0,l;

    a[0]=1;
    a[1]=1;
    for(i=2;i<=M;i++){
        for(j=2;i*j<M && a[i]!=1;j++){
            a[i*j]=1;
        }
    }
    while(cin>>n)
    {
        k=0;


    for(i=1;i<=n;i++){
        if(a[i]==0)k++;
    }
    cout<<k<<endl;
    //k=0;
    }



    return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <string.h>

int prime(int n) {
  int k;
  for(k = 2; k * k <= n; k++) {
    if(n % k == 0) return 0;
  }
  return 1;
}

int cntPrime(int n) {
  int cnt = 0, i, k;
  for(i = 2; i <= n; i++) {
    cnt += prime(i);
  }
  return cnt;
}

int main() {
  int a;
  int arg[1024] = { 0 };
  int cnt = 0;
  while(scanf("%d", &a) != EOF) {
    printf("%d\n", cntPrime(a));
  }
  int i;
  for(i = 0; i < cnt; i++) {
  }
}

C++: #include <iostream>
int main(int argc, const char * argv[])
{

    // insert code here...
    
    while(true)
    {
        int n;
        std::cin >> n;
        if(std::cin.eof())break;
        
        int count = 0;
        std:
        for(int i=2;i<n;i++)
        {
            bool flag = true;
            for(int j=2;j<i/j+1;j++)
            {
                if(i%j==0)
                {
                    flag=false;
                    break;
                }
            }
            if(flag)
            {
                count++;
            }
        }
        
        std::cout << count <<"\n";
        
    }
    
    return 0;
}

C++: #include <iostream>
#include <math.h>

int prime[1000000];


bool isPrime(int n) {
  int x = (int)sqrt(n);
  for (int i = 3; i <= x; i += 2) {
    if (n % i == 0) {
      return false;
    }
  }
  return true;
}


void get_prime(int max_num) {
  int primecount = 1;
  prime[2] = primecount;
  for (int i = 3; i <= max_num; i += 2) {
    if (isPrime(i))
      prime[i] = ++primecount;
    else
      prime[i] = primecount;
  }
}


int main () {
  int ns[31];
  int i = 0, max_num;
  while (std::cin >> ns[i]) {
    max_num = std::max(max_num, ns[i]);
    if (ns[i] % 2 == 0)
      ns[i] --;
    i ++;
  }
  get_prime(max_num);
  for (int j = 0; j < i; j ++) {
    std::cout << prime[ns[j]] << std::endl;
  }
  return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#define N 500000
using namespace std;

int main(){

  int n;
  int a,b,i;
  int sosu[N];
  bool judge;

  sosu[0] = 2;
  for(a=3,i=1;a<1000000;a++){
    judge = true;
    for(b=0;sosu[b]*sosu[b] <= a;b++){
      if(a%sosu[b] == 0){
	judge = false;
	break;
      }
    }
    if(judge){
      sosu[i] = a;
      i++;
    }
  }
  while(cin >> n){
    if(0<n && n<1000000){
      for(i=0;sosu[i]<=n;i++);
      cout << i << endl;
    }
  }

  return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <list>
#include <map>
#include <algorithm>
#include <stack>
#include <string>

using namespace std;


int main(){
	int n = 0;
	int count;
	bool flag = false;
	while(cin >> n){
		count = 0;
		for(int i = 1; i <= n; i++){
			if(i < 2);
			else if(i == 2){
				count++;
			}
			else if(i % 2 != 0){
				for(int j = 3; j * j <= i; j += 2){
					if(i % j == 0){
						flag = true;
						break;
					}
				}
				if(flag != true){
					count++;
				}
				flag = false;
			}
		}

		cout << count << endl;

	}

	return 0;
}

C++: #include <stdio.h>

const int MAX_V = 10000;
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"bits/stdc++.h"
using namespace std;

#define ll long long
#define INF INT_MAX/3
#define pi 2*acos(0.0)
#define eps 1e-14

const int MAX_N = 1e+3;
const int MAX_M = 1e+2;
int n;
int a[MAX_N];
bool is_prime[1000000];
int prime[1000000];

int sieve(int n) {
	int p = 0;
	for (int i = 0; i <= n; i++) is_prime[i] = true;
	is_prime[0] = is_prime[1] = false;
	for (int i = 2; i <= n; i++) {
		if (is_prime[i]) {
			prime[p++] = i;
			for (int j = 2 * i; j <= n; j += i) is_prime[j] = false;
		}
	}
	return p;
}

int main() {
	while (cin >> n)
		cout << sieve(n) << endl;
	return 0;
}