Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

C++: #include <iostream>                                                                                                     
#include <vector>                                                                                                       
                                                                                                                        
using namespace std;                                                                                                    
                                                                                                                        
int main() {                                                                                                            
  int n;                                                                                                                
  int count;                                                                                                            
  while (cin >> n) {                                                                                                    
    if (n == 1) { cout << 0 << endl; return 0; }                                                                        
    if (n == 2) { cout << 1 << endl; return 0; }                                                                        
                                                                                                                        
    vector<int> prime_numbers; // 素数リスト                                                                            
    prime_numbers.push_back(2);                                                                                         
    count = 1;                                                                                                          
    for (int i = 3; i <= n; i++) {                                                                                      
      bool prime = true;                                                                                                
      for (vector<int>::iterator itr = prime_numbers.begin(); itr != prime_numbers.end(); itr++) {                      
        if (i % *itr == 0) {                                                                                            
          prime = false;                                                                                                
          break;                                                                                                        
        }                                                                                                               
      }                                                                                                                 
      if (prime) {                                                                                                      
        prime_numbers.push_back(i);                                                                                     
        count += 1;                                                                                                     
      }                                                                                                                 
    }                                                                                                                   
    cout << count << endl;                                                                                              
  }                                                                                                                     
  return 0;                                                                                                             
}

C++: $inputArray = array();
while (!feof(STDIN)) {
  $input = trim(fgets(STDIN));
  if (empty($input)) {
    break;
  }
  array_push($inputArray, $input);
}

foreach ($inputArray as $input) {
  $count = 0;
  for ($i = 2; $i <= $input; $i++) {
    if (isSosu($i)) {
      $count++;
    }
  }
  echo $count . "\n";
}

function isSosu($num) {
  $count = 0;
  for ($i = 2; $i < $num; $i++) {
    if ($num % $i == 0) {
      $count++;
    }
  }
  if ($count == 0) {
    return true;
  } else {
    return false;
  }
}

C++: #include<bits/stdc++.h>
using namespace std;

struct SieveOfEstosthenes {
    int n;
    vector<bool> is_prime;
    vector<int> prime;
    SieveOfEstosthenes(int n) : n(n), is_prime(n, true) {
        is_prime[0] = is_prime[1] = false;
        for (int i = 2; i <= n; ++i) {
            if (is_prime[i]) {
                prime.push_back(i);
                for (int j = 2 * i; j <= n; j += i) is_prime[j] = false;
            }
        }
    }
};

/*
    verified https://onlinejudge.u-aizu.ac.jp/problems/0009
*/
int main() {
    int n;
    while (cin >> n) {
        SieveOfEstosthenes era(n);
        cout << era.prime.size() << endl;
    }
}

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

using namespace std;

int main() {
  const int N_MAX = 999999;
  const double MAX = sqrt(N_MAX);

  vector<int> preprime_list, prime_list;

  for (int i = 2; i <= N_MAX; i++) {
    preprime_list.push_back(i);
  }

  while (preprime_list[0] < MAX) {
    for (int i = 1; i < preprime_list.size(); i++) {
      if (preprime_list[i]%preprime_list[0] == 0)
        preprime_list.erase(preprime_list.begin()+i);
    }
    prime_list.push_back(preprime_list[0]);
    preprime_list.erase(preprime_list.begin()+0);
  }
  prime_list.insert(prime_list.end(), preprime_list.begin(), preprime_list.end());

  int data,count;
  while (cin >> data) {
    for (int i = 0; i < prime_list.size() && prime_list[i] <= data; i++, count = i);
    cout << count << endl;
  }
}

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

using namespace std;
int main(){
  
 int input;
 while(cin >> input){
 int res = 0;
 int   flag = 0;
 for (int i = 2; i <= input; ++i){
     for (int j = 2; j < i; ++j){
         if (i % j == 0){
             flag = 1;
         }
     }
         if (flag == 0){
             res++;
         }else{
             flag = 0;
         }
     
 }
 cout << res << endl;
}
}

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

int Era(int N) {
	int prime_num = 0;
	int* array = new int[N + 1];
	for (int i = 2; i < N + 1; i++) {
		array[i] = 1;
	}
	for (int i = 2; i <= sqrt(N); i++) {
		if (array[i]) {
			for (int j = 0; i*(j + 2) <= N; j++) {
				array[i*(j + 2)] = 0;
			}
		}
	}
	for (int i = 2; i < N + 1; i++) {
		if (array[i])prime_num++;
	}
	return prime_num;
}

int main() {
	int num;
	while (cin >> num) {
		cout << Era(num) << endl;
	}
	return 0;
}

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

int main()
{
 bool prime[10000000];

 prime[0]=prime[1]=false;
 for(int i=2;i<10000000;i++){
	prime[i]=true;
 }
 for(int i=2;i*i<10000000;i++){
	if(prime[i]){
		for(int j=i*i;j<10000000;j+=i){
			prime[j]=false;
		}
	}
 }
//???????????§?´???°??¨
 int n=0;
 int x=0;
 
 
 while(1){
	cin>>n;
  if(n==0)break;
	for(int i=n;i>10;i--){
		if(prime[i-8]&&prime[i-6]&&prime[i-2]&&prime[i]){
			cout<<i<<endl;
			break;
		}
	}
 }


 return 0;
}