Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
#include <numeric>
#include <functional>
#include <set>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cctype>
#include <climits>
#include <fstream>
#include <bitset>
#include <time.h>
#include <assert.h>

#define LL long long
#define VI vector<int>
#define VB vector<bool>
#define VL vector<long long>
#define FOR(i,a,b) for(int i= (a); i<((int)b); ++i)
#define RFOR(i,a) for(int i=(a); i >= 0; --i)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define DUMP(x)  cerr << #x << " = " << (x) << endl;
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v,x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());
#define EPS 1e-14

template <class T> int find_index(T x, const std::vector<T> &v) {
    auto it = std::find(v.begin(), v.end(), x);
    if (it != v.end()) { return (int)std::distance(v.begin(), it); }
    return -1;
}

template <class T> void print(T x) {
    std::cout << x << std::endl;
}

using namespace std;

// nまでの素数のリストを作成(nを含む) O(N log log N)
vector<int> make_prime_list(int n) {
    vector<bool> is_prime(n + 1, true);
    is_prime[0] = false;
    is_prime[1] = false;

    vector<int> prime_list;
    for (int i = 2; i < n + 1; ++i) {
        if (!is_prime[i]) { continue; }
        prime_list.emplace_back(i);

        for (int j = i + i; j < n + 1; j += i) {
            is_prime[j] = false;
        }
    }
    return prime_list;  // 素数のリスト
    //return is_prime;  // すべての数値について素数かどうか
}

int main(int argc, char *argv[]) {
    cin.tie(0);
    ios::sync_with_stdio(false);

    vector<int> prime_list = make_prime_list(1000100);

    int n;
    while(cin >> n) {
        int ans = 0;
        for(int i = 0;;++i)  {
            if(prime_list[i] <= n){
                ans++;
            }
            else {
                break;
            }
        }

        cout << ans << endl;
    }

    return 0;
}

C++: /*
 * main.cpp
 *
 *  Created on: 2011/04/08
 *      Author: satoki
 *      5:09 ~ 6:03
 *      Wrong Answer : 2
 *      nÈºÌfð¢ÅÀ
 *      Runtime Error : 4
 *      PÂ½ßÉfðZbg
 */
#include <iostream>
#include <string.h>
using namespace std;

int iN;
int iDp[78499];

void init() {
  int i, j, cnt = 0;
  for (i = 2; i < 1000004; i++) {
    for (j = 0; j < cnt; j++) {
      if (iDp[j] * iDp[j] > i) {
        j = cnt;
        break;
      } else if (i % iDp[j] == 0) break;
    }
    if (j == cnt) {
      iDp[cnt] = i;
      cnt++;
    }
  }
}

bool input() {
  if (scanf("%d", &iN) == EOF) return false;
  return true;
}

int manage() {
  int i;
  for (i = 0; iDp[i] <= iN; i++);
  return i;
}

int main() {
  init();
  while (1) {
    if (!input()) break;
    printf("%d\n", manage());
  }
  return 0;
}

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

std::vector<int> PRIMES;

void init_primes(int max)
{
  if (max < 2) return;
  std::set<int> candidate;
  PRIMES.push_back(2);
  for (int i = 3; i < max; i+=2) {
    candidate.insert(i);
  }
  for (int i = 3; i < sqrt(max); i+=2) {
    std::set<int>::iterator it = candidate.find(i);
    if (it != candidate.end()) {
      PRIMES.push_back(i);
      int del_num = i;
      while (del_num < max) {
        it = candidate.find(del_num);
        if (it != candidate.end()) candidate.erase(it);
        del_num += i;
      }
    }
  }
  for (std::set<int>::iterator it = candidate.begin();
       it != candidate.end(); ++it) {
    PRIMES.push_back(*it);
  }
}

int main()
{
  const int max = 1000000;
  init_primes(max);
  int i;
  while (std::cin >> i) {
    std::vector<int>::iterator it = PRIMES.begin();
    int n = 0;
    while (*it <= i) {
      n++;
      it++;
    }
    std::cout << n << std::endl;
  }
}

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

int main(){
	int n;
	static bool c[1000000]={true};
	static int t[1000000];t[0] = 0;
	for(int i = 2;i < 1000000;i++){
		if(!c[i]){
		for(int j = 2;i*j < 1000000;j++){
			c[i*j] = true;
		}
		}
		t[i]=t[i-1]+!c[i];
	}
	for(int n;cin>>n;n++){
		cout << t[n] << endl;
	}
	return 0;
}

C++: #include <iostream>

int main()
{
        int p_table[1000000];
        for(int i = 0; i < 1000000; ++i)
                p_table[i] = i;
        p_table[1] = 0;
        for(int i = 2; i < 1000; ++i)
                if(p_table[i] != 0)
                        for(int j = 2; j * i < 1000000; ++j)
                                p_table[i*j] = 0;

        int n, count;
        while(std::cin >> n)
        {
                count = 0;
                for(int i = 0; i <= n; ++i)
                        if(p_table[i] != 0)
                                ++count;
                std::cout << count << std::endl;
        }
        return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <stdlib.h>
#include <iomanip>
#include <string>
#include <cmath>
#define PI 3.14159265359
using namespace std;

bool prime_judge(int x)
{
	for (int i = 2; i < x; ++i) {
		if (x%i == 0)return false;
	}
	return true;
}

int main()
{
	int x, ans;
	while (cin >> x) {
		ans = 0;
		for (int i = 2; i <= x;++i) {
			if (prime_judge(i))++ans;
		}
		cout << ans << endl;
	}
	return 0;
}

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

void Sieve(int n)
{
	bool prime[n+1];
	int c=0;
	memset(prime, true, sizeof(prime));
	int x=sqrt(n*1.0)+2;

	for (int p=2; p<=x; p++)
	{
		if (prime[p] == true)
		{
			for (int i=p*p; i<=n; i+=p)
				{
				    prime[i] = false;
				}

		}
	}
	for (int p=2; p<=n; p++)
	if (prime[p])
		c++;
		cout<<c<<endl;
}

int main()
{
	int n;
	while(cin>>n){
        Sieve(n);
	}

	return 0;
}

C++: #include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define INF 1<<30
#define LINF 1LL<<60

/*
<url:>
問題文============================================================

=================================================================

解説=============================================================

================================================================
*/
vector<int> prime;

void Prime() {
    prime.emplace_back(2);
    for (int i = 3; i <= 999999; i += 2) {
        bool f = false;
        for (int j = 3; j <= sqrt(i); j += 2) {
            if (i%j == 0) {
                f = true;
                break;
            }
        }
        if (!f) prime.emplace_back(i);
    }
}

int main() {
    cin.tie(0); ios::sync_with_stdio(false);
    Prime();
    int n;
    while (cin >> n) {
        cout << upper_bound(prime.begin(), prime.end(), n) - prime.begin() << endl;
    }
}