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<stdio.h>
int flg[1000000];
int main(void)
{

	int a,i,j,cnt;
	while(scanf("%d",&a)!=EOF){	
		for(i=2;i<=a;i++){
		flg[i]=1;
		}
		for(i=2;i*i<=a;i++){
			if(flg[i]=1){
				j=2;
				while(i*j==a){
				flg[i*j]=0;
				j++;
				}
			}
		}
		cnt=0;
		for(i=2;i<=a;i++){
			if(flg[i]=1){
			cnt++;
			}
		}
		printf("%d\n",cnt);
	}	
	return 0;
}

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

bool IsPrime(int i){
    if(i < 2) return false;
    else if(i == 2) return true;
    else if(i % 2 == 0) return false;

    double sqrtNum = sqrt(i);
    for(int j = 3; j <= sqrtNum; j += 2){
        if(i % j == 0){
            return false;
        }
    }

    return true;
}

int main(){
    int n;
    while(cin >> n){
        int k = 0;
        for(int i = 0; i <= n; ++i){
           if(IsPrime(i) == true){
               k++;
           }else ;
        }
        cout << k << endl;
    }
}

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

using namespace std;

void set_prime_number(bool* array, int num) {

  fill(array, array + num, true);
  array[0] = array[1] = false;
  for (int i = 2; i < num; i++) {
    if (!array[i]) continue;
    for (int j = i * 2; j < num; j += i) {
      array[j] = false;
    }
  }

}

int main() {
  bool prime_number[1000000];
  int num;
  int nump=0;
  int cnt=0;

	while(scanf("%d",&num)!=EOF){
	  set_prime_number(prime_number,num);
	 for (int i = 0; i < num+1; i++) {
		 if (prime_number[i]){cnt++;}
	 }
	 printf("%d ",cnt);
	 cnt=0;
	}


  return 0;
}

C++: #include<iostream>
using namespace std;
int main() {
	int a;
	int sum;
	while (cin >> a){
		sum = 0;
		for (int i = 0; i < a; i++){
			int y = 0;
			int l = i + 1;
			for (int i = 1; i != l+1; i++){
				if (l % i == 0) {
					y++;
				}
			}
			if (y == 2)sum++;
		}
		cout << sum << endl;
	}
}

C++: package util;
import java.util.Scanner;

public class Main{
    boolean isPrime[];

    void run(){
	isPrime = new boolean[1000000];
	for(int i=0;i<1000000;i++) isPrime[i] = true;

	isPrime[0] = false;
	isPrime[1] = false;

	for(int i=2;i<=1000000;i++){
	    if(!isPrime[i]) continue;
	    for(int j=i+i;j<=1000000;j+=i){
		isPrime[j] = false;
	    }
	}


	Scanner sc = new Scanner(System.in);
	while(sc.hasNext()){
	    int n=sc.nextInt();
	    int count = 0;
	    for(int i=0;i<=n;i++){
		if(isPrime[i]) count++;
	    }
	    System.out.println(count);
	}
    }

    public static void main(String[] args){
	new Main().run();
    }
}

C++: #include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <limits>
#include <map>
#include <string>
using namespace std;

bool is_prime(int n){
for(int i=2;i*i<=n;i++){
if(n % i == 0) return false;
}
return n!=1;
}

int main(){
int n;
while(~scanf("%d",&n)){
int* table =  new int[n+1];
fill((int*)table,(int*)table+n+1,1);
table[0]=0;
table[1]=0;

for(int i=2;i<=n;i++){
if(!is_prime(i)) {
for(int j=i;j<=n;j+=i){
table[j] = 0;
}
}
}
printf("%d\n",count(table,table+n+1,1));
}
}

C++: #include <iostream>
#include <stdio.h>
#include <sstream>
#include <string>
#include <vector>
#include <map>
#include <queue>
#include <algorithm>
#include <set>
#include <math.h>
#include <utility>
#include <stack>
using namespace std;
typedef pair<int,int> P;
const int INF = (int)((unsigned int)-1>>1);


int main(){
    int n;
    while(cin >> n){
        vector<int> prime;
        prime.push_back(2);

        for(int i=3;i<=n;i+=2){
            bool is_prime = true;
            for(int j=0;j<prime.size();j++){
                if (i % prime[j] == 0){
                    is_prime = false;
                    break;
                }
            }
            if(is_prime){
                prime.push_back(i);
            }
        }

        cout << prime.size() << endl;
    }

    return 0;
}

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

int main(){
 int n,i,j,x,counter;
 while (cin >> n) {
  counter = 0;
  
  for (i = 1; i <= n; i++) {
    x = 0;
    
    for (j = 1; j <= i; j++) {
      if (i % j == 0) x++;
    }
    
    if (x == 2) counter++;
  }
  
  cout << counter << endl;
  
 }
 return 0;
}

C++: #include <iostream>
#include <cmath>
#include <vector>
#include <time.h>     // for clock()
using namespace std;

/*
1???2 ?????? n ?????§?????´??°???????????????
2??????????????£???????????°????????§???????°?????????????????????? p ??¨?????????????????????????????????????????°?????? p ??¨?????????
	p ??\?????? p ????????°?????¨????¶????????????????
3???2??????????????°?????????????????????p ??? ???n ????¶??????????????????????????????????????????£?????????????´???°???
*/
// ??¨????????????????????????
int SieveOfEratosthenes(int num){
	// num???????´???°?????°
	int count = 0;
	
	// Route Num?????´??°??¨??????????????????
	int routeN_int = (int)sqrt(num);
	//cout << routeN_int << endl;
	
	// ???n?????§?????´??°?????????????´???°????????°????????????
	vector<bool> prime_flag_set(num + 1, true);
	
	for(int i = 0; i < prime_flag_set.size(); i++){
		//cout << "Flag" << i << " " << prime_flag_set[i] << endl;
	}
	//cout << endl;
	// 1???2 ?????? n ?????§?????´??°????????????p????????°????????? = false
	for(int p = 2; p < prime_flag_set.size(); p++){
		if( prime_flag_set[p] == false){
			continue;
		}
		int j = 2;
		// 3???2??????????????°?????????????????????p ??? ???n ????¶????????????????
		while(p < routeN_int){
			if(p*j > num+1){
				// num??\???
				break;
			}
			prime_flag_set[p * j] = false;
			//p????????°????±???????
			j++;
		}
	}
	// 2??\???
	for(int i = 2; i < prime_flag_set.size(); i++){
		if(prime_flag_set[i] == true){
			count++;
		};
		//cout << "Flag" << i << " " << prime_flag_set[i] << endl;
	}
	
	return count;
}

int main(){
	
	int num = 0;
	while(cin >> num, num != 0){
		clock_t start = clock();    // ??????????????????
		cout << SieveOfEratosthenes(num) << endl;
		clock_t end = clock();     // ????????????
    	std::cout << "duration = " << (double)(end - start) / CLOCKS_PER_SEC << "sec.\n";
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
#define MAX 1000000

int main(){
	bool p[MAX+1];
	int n, ans;

	for(int i=0 ; i<=MAX ; i++){
		p[i] = (i<=1)? false : true ;
	}

	for(int i=2 ; i<=sqrt(MAX)+1 ; i++){
		if(p[i]){
			for(int j=i*2 ; j<=MAX ; j+=i){
				p[j] = false;
			}
		}
	}

	while( cin >> n ){		
		ans = 0;
		for(int i=0 ; i<=n ; i++){
			if(p[i]){
				ans++;
			}
		}
		cout << ans << endl;
	}
}

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

#define M 1000001

int main(void)
{
    int x, i, j;
    int p[M];
    
    for (i = 0; i < M; i++) {
        p[i] = 1;
    }
    p[0] = p[1] = 0;
    for (i = 2; i * i < M; i++) {
        if (p[i] == 1) {
            for (j = i * i; j * j < M; j += i) {
                p[j] = 0; 
            }
        }
    } 
    for (i = 0; i * i < M-1; i++) {
        p[i+1] += p[i];
    }
    while (scanf("%d", &x) != EOF) {
        printf("%d\n", p[x]);
    } 
    return (0);
}

C++: #include <iostream>
using namespace System;
using namespace std;
const int MAX_N =1000000;
int main(){
	bool sosu[MAX_N]={};//false:sosu
	for(int i=2;i<MAX_N;i++){
		if(!sosu[i]){
			for(int j=2;j*i<MAX_N;j++){
				sosu[j*i]=true;
			}
		}
	}
	for(int n;cin>>n;){
		int ans=0;
		for(int i=2;i<=n;i++){
			if(!sosu[i])ans++;
		}
		cout<<ans<<endl;
	}
    return 0;
}