Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextInt() int {
	i, e := strconv.Atoi(scanner.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func check(n int) int {
	eratos := make([]bool, n+1)
	cnt := 0
	for i := 2; i < len(eratos); i++ {
		if !eratos[i] {
			cnt++
			for j := i*i; j < len(eratos); j += i {
				eratos[j] = true
			}
		}
	}
	return cnt
}

func main() {
	for scanner.Scan() {
		n := nextInt()
		fmt.Println(check(n))
	}
}

C++: #include <iostream>
using namespace std;
int main()
{
    int n;
    while(cin >> n)
    {
        int cnt = 0;
        for(int i = 2; i <= n; i++)
        {
            bool is_prime = true;
            for(int j = 2; j <= i / 2; j++)
            {
                if(i % j == 0)
                {
                    is_prime = false;
                    break;
                }
            }
            if(is_prime) cnt++;
        }
        cout << cnt << endl;
    }
}

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

int main() {
	int n;
	while (std::cin >> n) {
		int a = 0;
		if (n == 0) {
			break;
		}
		for (int i = 2; i <= n; ++i) {
			bool f = true;
			for (int j = 2; j < i&&f; ++j) {
				if (i%j == 0) {
					f = false;
				}
			}
			if (i == 2) {
				++a;
			}
			else if (f) {
				++a;
			}
		}
		std::cout << a << std::endl;
	}
	return 0;
}

C++: #include <stdio.h>
const int MAX_V = 1000000; // 100???????????????????f???????¢\????°???¬?????
int prime[MAX_V+1] ,cnt[MAX_V+1] = {0};
int main(){
		int k,i,n; // 1????????f???????C0????????f????????????????????¢
	for(i = 2; i <= MAX_V; i++) {
		prime[i] = 1; // 2???????£????f????????????°??????¨
	}
	for(i = 2; i*i <= MAX_V; i++) {
		cnt[i] = cnt[i-1];
		//printf("")
		if(prime[i]) {
			for(k = 2 * i; k <= MAX_V; k += i) {
				prime[k] = 0; // ???f??????2???{???????£????{?????????f????????????????????¢
			}
			cnt[i]++;
		}
	}
	while(scanf("%d",&n) != EOF){
		printf("%d\n",cnt[n]);
	}
	return 0;
}

C++: #include <iostream>
#include <sstream>
#include <fstream>

#include <ctime>
#include <cstdlib>
#include <cstring>
#include <cctype>

#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>

#include <cmath>
#include <algorithm>
#include <functional>
#include <numeric>

using namespace std;

template<typename T> ostream& operator<<(ostream& os, const vector<T>& v){
    os << "{ ";
    for(typename vector<T>::const_iterator it=v.begin(); it!=v.end(); ++it)
        os << '\"' << *it << '\"' << (it+1==v.end() ? "" : ", ");
    os << " }";
    return os;
}

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<bool> vb;
typedef vector<vb> vvb;
#define vv(type,w,h,init) vector<vector<type>>(h,vector<type>(w,init))
typedef vector<string> vs;
typedef long long ll;
typedef unsigned uint;
typedef unsigned long ul;
typedef unsigned long long ull;

#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define loop(i,a,b) for(int i=(a); i<(int)(b); i++)
#define rep(i,b) loop(i,0,b)
#define pb push_back
#define mp make_pair

string const resstr[] = {"Impossible","Possible","No","Yes"};

bool p[1000100];

void erato(){
	fill(p, p+1000100, true);
	p[0] = p[1] = false;
	
	for(int i=0; i < 1000100; i++){
		if(!p[i]) continue;
		for(int j=i+i; j < 1000100; j+=i){
			p[j] = false;
		}
	}
}

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

C++: #include<stdio.h>
#include<math.h>
int isprime(int x){
	int i;
	i = 3;
	while( i <= sqrt(x) ){
		if(x%i==0){
			return 0;
		}
		i = i + 2;
	}
	return 1;
}
int main(){
	int n;
	while(scanf("%d",&n)!=EOF){
		int c=1;
		for(int i=3;i<=n;i+=2){
			if(isprime(i)){
				c++;
			}
		}
		printf("%d\n",c);
	}
	return 0;
}

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

const int MAX = 1000000;
int cnt[MAX];
bool isCmp[MAX] = {true};


int main() {
    ios::sync_with_stdio(false);
    int n;
    cnt[1] = 0;
    for(int i = 2; i < MAX; i++){
      if(! isCmp[i]){
        for(int j = 2; i*j < MAX; j++){
          isCmp[i*j] = true;
        }
      }
      cnt[i] = cnt[i-1] + !isCmp[i];
    }

    while(cin >> n){
      cout << cnt[n] << endl;
    }
    return 0;
}

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

bool x[1000000];

void pri(int num){
	for(int i = 0; i <= num; i++) cout <<"x["<<i<<"] = "<<x[i]<<endl;
}

void setbool(int num){
	x[0] = false; x[1] = false;
	for(int i = 2; i <= 1000000; i++) x[i] = true;
	for(int i = (num + 1); i <= 1000000; i++) x[i] = false;
}

void loop(int num){
	for(int i = 2; i <= num; i++){
		if(x[i] == false) continue;
		else{
			int j = i;
			while(j < num){
				j++;
				if(x[j] == true && j % i == 0) x[j] = false;
			}
		}
	}
}

int ans(){
	int cou = 0;
	for(int i = 0; i <= 1000000; i++) if(x[i] == true) cou++;
	return cou;
}

int main(){
	int num, cou = 0, i;
	
	while(cin >>num){
		setbool(num);
		loop(num);
		cout <<ans()<<endl;
	}
	
	return 0;
}

C++: #include <iostream>
using namespace std;
int main(){
    int a[10000];
    int n;
    a[0]=2;
    int k=1;
while(cin>>n){
    for(int i=3;i<=n;i++){
        for(int j=0;j<=k;j++){
            if(j==k){
                a[k]=i;
                k++;
                break;
             }
            if(i%a[j]==0)break;
        }
    }
    cout<<k<<endl;
    k=1;
 }
}

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

using namespace std;

const int MAX_N = 1000000;

int prime[MAX_N];
bool is_prime[MAX_N];

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()
{
	int n;
	while(scanf("%d",&n) > 0){
		printf("%d\n",sieve(n));
	}
}

C++: /*
Á·Ï°Ìâ£ºPrime Number_AOJ 0009 
*/

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

const int MAX_N=1000007;

//°£ÊÏÉ¸·¨(Sieve of Eratosthenes)
int prime[MAX_N]; //µÚi¸öËØÊý£¬´ËÎª´æ·ÅËØÊýµÄÊý×é 
bool is_prime[MAX_N]; //is_prime[]ÎªtrueÊ±ÎªËØÊý

//·µ»ØnÒÔÄÚËØÊýµÄ¸öÊý
int sieveErato(int n)
{
	int p=0;
	for(int i=0; i<=n; i++)
		is_prime[i]=true;
	is_prime[0]=false;
	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()
{
	int n;
	while(scanf("%d", &n)!=EOF)
	{
		printf("%d\n", sieveErato(n));
	}
	return 0;
}

C++: #include "bits/stdc++.h"
#include <cassert>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define all(a)  (a).begin(),(a).end()
#define vi vector<int>
#define pb push_back
#define INF 999999999
//#define INF (1LL<<59)

int isPrime[1000000];


void era(){
    for(int i=2;i<1000000;i++){
        if( isPrime[i]==1 ){
            int k = 2;
            while(k*i<1000000){
                isPrime[k*i]=0;
                k++;
            }
        }
    }
}


int main(){
    for(int i=0;i<1000000;i++){
        isPrime[i]=1;
    }
    isPrime[0]=isPrime[1]=0;
    
    era();
    
    int n;
    while(cin>>n){
        int count=0;
        for(int i=0;i<=n;i++){
            if( isPrime[i] )count++;
        }
        cout<<count<<endl;
    }
    
    
    
}