Go: package main

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

func main() {
	scanner := bufio.NewScanner(os.Stdin)

	for scanner.Scan() {
		num, _ := strconv.Atoi(scanner.Text())
		flag := false
		count := 0

		for i := 2; i <= num; i++ { //1を除く入力値以下の数字
			for j := 2; j < i-1; j++ {
				if i%j == 0 && j != 1 {
					flag = true
					//fmt.Println(i, "/", j)
					break
				}
			}
			if !flag {
				count++
			} else {
				flag = false
			}
		}
		fmt.Println(count)
	}
}

C++: #include <iostream>
#include <cstring>
using namespace std;
const int maxn=1000010;
int prime[maxn];
bool isprime[maxn];

int prime_cnt(int n)
    {
        int p=0;
        memset(isprime,true,sizeof(isprime));
        isprime[0]=false;
        isprime[1]=false;
        for (int i=2;i<=n;i++)
            {
                if(isprime[i])
                    {
                        prime[p++]=i;
                        for (int j=2*i;j<=n;j+=i)
                            isprime[j]=false;
                    }
            }
        return p;
    }
int main()
    {
        int n;
        while (cin>>n)
            {
                cout<<prime_cnt(n)<<endl;
            }
        return 0;
    }

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

int main(){
  int n;
  int i,j;
  while(scanf("%d", &n) != EOF){
    int m=1;
    for(i=2;i<=n;i++){
      for(j=2;j<n;j++){
	if(i%j == 0)
	  break;
	if(j==i-1 && i%j!=0)
	  m++;
      }
    }
    printf("%d\n",m);
  }
  return 0;
}

C++: #include <iostream>                                                                                                       
#include <vector>                                                                                                         
#include <list>                                                                                                           
                                                                                                                          
#define MAX 1000000                                                                                                       
                                                                                                                          
using namespace std;                                                                                                      
                                                                                                                          
int main() {                                                                                                              
  vector<int> prime_numbers;                                                                                              
  prime_numbers.push_back(2);                                                                                             
  prime_numbers.push_back(3);                                                                                             
  prime_numbers.push_back(5);                                                                                             
  prime_numbers.push_back(7);                                                                                             
                                                                                                                          
  list<int> search_list;                                                                                                  
  for (int i = 5; i < MAX; i++) {                                                                                         
    if (i % 2 != 0 && i % 3 != 0 && i % 5 != 0 && i % 7 != 0) {                                                           
      search_list.push_back(i);                                                                                           
    }                                                                                                                     
  }                                                                                                                       
  int h = search_list.front();                                                                                            
  while (h * h < search_list.back()) {                                                                                    
    prime_numbers.push_back(h);                                                                                           
    list<int>::iterator itr = search_list.begin();                                                                        
    while (itr != search_list.end()) {                                                                                    
      if (*itr % h == 0) {                                                                                                
        itr = search_list.erase(itr);                                                                                     
      } else {                                                                                                            
        itr++;                                                                                                            
      }                                                                                                                   
    }                                                                                                                     
    h = search_list.front();                                                                                              
  }                                                                                                                       
  for (list<int>::iterator itr = search_list.begin(); itr != search_list.end(); itr++) {                                  
    prime_numbers.push_back(*itr);                                                                                        
  }                                                                                                                       
                                                                                                                          
  int n;                                                                                                                  
  while (cin >> n) {                                                                                                      
    int count = 0;                                                                                                        
    for (vector<int>::iterator itr = prime_numbers.begin(); itr != prime_numbers.end(); itr++) {                          
      if (*itr >= n) break;                                                                                               
      count++;                                                                                                            
    }                                                                                                                     
    cout << count << endl;                                                                                                
  }                                                                                                                       
                                                                                                                          
  return 0;                                                                                                               
}

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

int main(){
	int n;
	while(cin >> n){
		bool* p = new bool[n+1];
		int cou = 0;
		for(int i = 0; i < n+1; ++i){
			p[i] = true;
		}
		p[0] = false; p[1] =false;
		for(int i = 2; i*i <= n; ++i){
			for(int j = 2;i*j <= n ; ++j) {
				p[i*j] = false;
			}
		}

		for(int i = 0; i < n+1; ++i ){
			if(p[i] == true) ++cou;
		}
		cout << cou << endl;
	}
    return 0;
}