Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

func next()string{
	return sc.Text()
}

func nextInt()int{
	a, _ := strconv.Atoi(next())
	return a
}

C++: #include <bits/stdc++.h>
#define debug(x) cerr << #x << ": " << x << '\n';
using namespace std;
using ll = long long;
using P = pair<int, int>;
const int INF = (int)1e9;

// エラストテネスの篩, o(n)程度
// n以下の素数の数を返す, prime[i]: i番目の素数, is_prime[i]: iが素数であるか
int sieve(int n, int prime[] = nullptr, bool is_prime[] = nullptr){
	bool flag0 = false, flag1 = false;
	if(prime == nullptr){
		flag0 = true; prime = new int[n];
	}
	if(is_prime == nullptr){
		flag1 = true; is_prime = new bool[n + 1];
	}

	int p = 0;
	fill(is_prime, is_prime + n + 1, 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;
		}
	}

	if(flag0) delete prime;
	if(flag1) delete is_prime;

	return p;
}

const int MAX_N = (int)1e6;
int prime[MAX_N];
bool is_prime[MAX_N + 1];

int main(void){
	int num = sieve(MAX_N, prime, is_prime);

	int n;
	while(scanf("%d", &n) != EOF){
		cout << upper_bound(prime, prime + num, n) - prime << '\n';
	}

	return 0;
}

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

int main(){
    int n, cnt;
    bool prime[1000000];
    prime[0] = prime[1] = false;
    
    for(int i = 2;i < 100000;i++){
        prime[i] = true;
    }

    while(cin >> n){
        cnt = 0;
        
        for(int i = 2;i <= sqrt(n);i++){
            if(prime[i]){
                for(int j = 2*i;j<= n;j++){
                    if(j%i == 0){
                        prime[j] = false;
                    }
                }
            }
        }
        
        for(int i = 2;i <= n;i++){
            if(prime[i]){
                cnt++;
            }
        }
        
        cout << cnt << endl;
    }

    return 0;
}

C++: #include <iostream>
using namespace std;
int main(){
  int NUM = 1000000;
  bool prime[NUM];
  prime[0] = false;prime[1] = false;
  for(int i=2;i<NUM;i++)prime[i] = true;
  for(int i=2;i*i<NUM;i++)
    for(int k=2;i*k<NUM;k++)
      prime[i*k]=false;
  int n,c=0,i=0;
  for(cin>>n;i<n;i++)if(prime[i])c++;
  cout<<c<<endl;
  return 0; 
}

C++: #include<bits/stdc++.h>
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
#define pb push_back
int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};

#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
bool hurui[1000000];
int main(){
    int n;
    while(scanf("%d",&n) != EOF){
        for(int i = 0;i <= n;i++)
            hurui[i] = true;

        for(int i = 2;i <= (int)sqrt(n);i++) {
            if(!hurui[i])
                continue;
            int m = i * 2;
            while(m <= n) {
                hurui[m] = false;
                m += i;
            }
        }
        int cou = 0;
        for(int i = 2;i <= n;i++) {
            if(hurui[i]) {
                cou++;
            }
        }
        printf("%d\n",cou);
    }
    return 0;
}

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

const int MAX = 1000000;
bool prime[MAX] = { true };
int pcount[MAX];

int main(){
	int n;
	prime[0] = prime[1] = false;
	for(int i = 2;i < MAX;++i) prime[i] = true;
	for(int i = 2;i <= sqrt(MAX);++i){
		if(prime[i]){
			for(int j = i + i;j < MAX;j += i){
				prime[j] = false;
			}
		}
	}
	pcount[0] = 0;
	for(int i = 1;i < MAX;++i){
		pcount[i] = pcount[i - 1];
		if(prime[i]) pcount[i]++;
	}
	while(cin >> n){
		cout << pcount[n] << endl;
	}
	return 0;
}

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

int is_prime_f(int n) {
    int i;
    if(n < 2)
        return -1;
    else if(n == 2)
        return 1;

    if(n % 2 == 0)
        return -1;

    for(i = 3; i <= n / i; i += 2)
        if(n % i == 0)
            return -1;
    return 1;
}

unordered_map<int, int> map;

bool is_prime_map(int n) {
    int is_prime = map[n];
    if (is_prime == 0) {
        is_prime = is_prime_f(n);
        map[n] = is_prime;
    }
    return is_prime == 1;
}

int prime_num_count(int max) {
    int count = 0;
    for (int i = 1; i <= max; ++i) {
        if (is_prime_map(i)) {
            count++;
        }
    }
    return count;
}

int main() {
    string str;
    while(getline(cin, str)) {
        cout << prime_num_count(stoi(str.c_str())) << endl;
    }
    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[30];
  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 <stdio.h>
#include <string.h>
#include <math.h>
#include <algorithm>
#include <stdlib.h>

using namespace std;


int main(){

	int i, j, k;
	int n;

	while(cin >> n){

		int count = 0;

		if(n>2){
			count ++;
			for(i=3 ; i<=n ; i+=2){
				for(j=3 ; j<=(int)sqrt(i) ; j+=2){
					k = i % j;
					if(k == 0) break;
				}
				if(j > (int)sqrt(i)) count ++;
			}
		}
		cout << count << endl;

	}


	return 0;
}

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

int main() {
	vector<bool>hantei(1000000, true);
	vector<int>sosuu(1000000);
	int s = 0;
	for (int a = 2; a < 1000000; a++) {
		if (hantei[a]) {
			s++;
			for (int b = a * 2; b < 1000000; b += a)hantei[b] = false;
		}
		sosuu[a] = s;
	}
	int c;
	while (cin >> c, c) {
		cout << sosuu[c] << endl;
	}
}

C++: #include<iostream>

int main(){

	bool data[100001];

	for (int i = 0; i < 100001; i++)
		data[i] = true;

	int in;

	int res=0;

	for(int i=2;i<=100000;i++){
		for (int j = 2; j < i; j++) {
			if (i%j == 0){
				data[i] = false;
				goto fl;
		}
	}
	fl:;
	}

	while (true) {

		std::cin >> in;

		for (int i = 2; i <= in; i++) {
			if (data[i] == true)
				res++;
		}

		std::cout << res << std::endl;

	}

		return 0;
}

C++: #include <bits/stdc++.h>

const static double	de_PI	= 3.14159265358979323846;
const static int	de_MOD	= 1000000007;
const static int	de_MAX	= 999999999;
const static int	de_MIN = -999999999;

inline int Sieve_of_Eratothenes(const int n) {

	std::vector<bool> flg(n + 1, true);
	for (int i = 2; i < n; i++) {
		if (flg[i]) {
			for (int j = i + i; j <= n; j += i) {
				flg[j] = false;
			}
		}
	}

	return(std::count(flg.begin(), flg.end(), true) - 2);

}

int main(void) {

	//std::ifstream in("123.txt");	std::cin.rdbuf(in.rdbuf());

	int a = 0;
	std::vector<int> A;
	while (std::cin >> a) {
		A.push_back(a);
	}

	std::for_each(A.begin(), A.end(),
		[](int i) {std::cout << Sieve_of_Eratothenes(i) << std::endl; });
}

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