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 <stdio.h>
#include <iostream>

using namespace std;

int main() {
	int n;

	while (cin >> n) {
		int counter = 0;
		
		if(n < 1000){
			for (int i = 2; i <= n; i++) {
				int kari = 1;
				for (int j = 2; j < i; j++) {
					kari = i % j;
					if(kari == 0) {
						break;
					}
				} if (kari != 0) {
					counter++;
				}
			}
		}else if(n < 10000){
			counter = counter + 168;
			for (int i = 1000; i <= n; i++) {
				int kari = 1;
				for (int j = 2; j < i; j++) {
					kari = i % j;
					if(kari == 0) {
						break;
					}
				} if (kari != 0) {
					counter++;
				}
			}
		} else if(n < 100000){
			counter = counter + 1229;
			for (int i = 10000; i <= n; i++) {
				int kari = 1;
				for (int j = 2; j < i; j++) {
					kari = i % j;
					if(kari == 0) {
						break;
					}
				} if (kari != 0) {
					counter++;
				}
			}
		} else if(n < 200000){
			counter = counter + 9592;
			for (int i = 100000; i <= n; i++) {
				int kari = 1;
				for (int j = 2; j < i; j++) {
					kari = i % j;
					if(kari == 0) {
						break;
					}
				} if (kari != 0) {
					counter++;
				}
			}
		} else if(n < 300000){
			counter = counter + 17984;
			for (int i = 200000; i <= n; i++) {
				int kari = 1;
				for (int j = 2; j < i; j++) {
					kari = i % j;
					if(kari == 0) {
						break;
					}
				} if (kari != 0) {
					counter++;
				}
			}
		} else {
			counter = counter + 25997;
			for (int i = 300000; i <= n; i++) {
				int kari = 1;
				for (int j = 2; j < i; j++) {
					kari = i % j;
					if(kari == 0) {
						break;
					}
				} if (kari != 0) {
					counter++;
				}
			}
		}
		cout << counter << endl;
	}

	return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
typedef vector <int> vi;
vi primes;

int get_primes(int max){
	int n = (max+1)/2;
	int root = (int)(pow(n, 0.5));
	vector <bool> numbers(n, true);
	numbers[0]=false;

	if (max>1) primes.push_back(2);
	for (int i=1; i<root; i++){
		int tmp = i*2+1;
		if (numbers[i]){
			primes.push_back(tmp);
			for (int j=i+tmp; j<n+1; j+=tmp){
				numbers[j]=false;
			}
		}
	}
	for (int i=root; i<n+1; i++){
		if(numbers[i]){
			primes.push_back(2*i+1);
		}
	}
	return 0;
}

int main(void){
	vi n;
	int nmax=0;
	while (true){
		int tmp;
		cin>>tmp;
		if (cin.eof()) break;

		n.push_back(tmp);
		if (tmp>nmax) nmax=tmp;
	}

	get_primes(nmax);
	for (int i=0; i<n.size(); i++){
		cout<<upper_bound(primes.begin(), primes.end(), n[i]) - primes.begin()<<endl;
	}
	return 0;
}

C++: #include <vector>
#include <iostream>
#include <set>
#include <cstdio>
#include <queue>
#include <cstdio>
#include <cstdlib>
#include <cassert>
#include <ctime>
#include <cmath>
#include <complex>
#include <algorithm>
#include <tuple>
#include <algorithm>
#include <limits>
#include <map>
#include <valarray>
#include <list>

using namespace std;

typedef long long ll;

template<class T>
struct Pi {
    T x, y;
    Pi() {}
    Pi(T xx, T yy) {
        x = xx; y = yy;
    }
    bool operator<(const Pi &r) const {
        if (x != r.x) return x < r.x;
        return y < r.y;
    }
    bool operator>(const Pi &r) const {
        if (x != r.x) return x > r.x;
        return y > r.y;
    }
    bool operator==(const Pi &r) const {
        if (x != r.x) return false;
        return y == r.y;
    }
};

template<class T>
struct Ti {
    T x, y, z;
    Ti() {}
    Ti(T xx, T yy, T zz) {
        x = xx; y = yy; z = zz;
    }
    bool operator<(const Ti &r) const {
        if (x != r.x) return x < r.x;
        if (y != r.y) return y < r.y;
        return z < r.z;
    }
    bool operator>(const Ti &r) const {
        if (x != r.x) return x > r.x;
        if (y != r.y) return y > r.y;
        return z > r.z;
    }
    bool operator==(const Ti &r) const {
        if (x != r.x) return false;
        if (y != r.y) return false;
        return z == r.z;
    }
};

int main() {
    int n;
    cin >> n;
    int res = 0;
    for (int i = 2; i <= n; i++) {
        bool f = true;
        for (int j = 2; j*j <= i; j++) {
            if (!(i%j)) {
                f = false;
                break;
            }
        }
        if (f) {
            //printf("%d\n", i);
            res++;
        }
    }
    printf("%d\n", res);
    return 0;
}

C++: #include <iostream>
#include <vector>
//#include <math.h>
//#include <string>
#include <cmath>
#include <iomanip>

int main();
int count_prime_number(int n);

int main(){
	int input;
	std::vector<int> answer;
	
	
	//for (int i = 0; i < 3; i++){
		//std::cin >> input;
	while (std::cin >> input){
		answer.push_back(count_prime_number(input));
	}

	for (int i = 0; i < answer.size(); i++){
		std::cout << answer[i] << std::endl;
	}

	while (1);
	return 0;
}

/**
* nまでの素数の個数を出力する
* args:数値n
* return:素数の個数
* 参考:http://tsg.ne.jp/TT/tsg/c/mansaku/P01.html
*/
int count_prime_number(int n){
	int rtn = 0;

	//2だけ個別チェック
	if (n >= 2){
		rtn++;
	}

	for (int i = 3; i <= n; i += 2)
	{
		bool is_prime_num = true;
		for (int j = 3; j <= sqrt(i); j += 2)		//ルートi以上のiの約数はルートi以下のiの約数と対になっているので範囲はルートiまででいい
		{
			if (i % j == 0)
			{
				is_prime_num = false;
				break;
			}
		}

		if (is_prime_num){
			rtn++;

			//デバッグ
			//std::cout << i << std::endl;
		}
	}

	return rtn;
}

C++: #include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include <algorithm>
#include <cmath>
#include <queue>
#include <set>
#include <map>
#include <vector>
#include <iomanip>
#include <sstream>

using namespace std;

#define ll long long
#define eps 1e-7
#define inf 0x1fffffff
#define maxn 1000010



bool is[maxn];
vector<int>p;



int main(int argc, char *argv[])
{
	ios::sync_with_stdio(false);
	int n;
	memset(is,true,sizeof(is));
	for(int i=2;i<maxn;i+=2)
		is[i] = false;
	is[0] = is[1] = false;
	p.push_back(2);
	for(int i=3;i<maxn;i+=2)
	{
		if(is[i])
		{
			p.push_back(i);
			for(int j=i+i;j<maxn;j+=i)
			{
				is[j] = false;
			}
		}
	}
	while(cin>>n)
	{
		cout<<upper_bound(p.begin(),p.end(),n)-p.begin()<<endl;
	}
	return 0;
}

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

int main() {
  constexpr int max = 1000000;
  vector<bool> isPrime(max, true);
  isPrime[0] = isPrime[1] = 0;
  for (int i = 2; i * i < max; i++) {
    if (isPrime[i]) {
      for (int j = 2; i * j < max; j++) {
        isPrime[i * j] = false;
      }
    }
  }
  int n;
  while (cin >> n) {
    int ans = 0;
    for (int i = 0; i <= n; i++) {
      if (isPrime[i]) ans++;
    }
    cout << ans << endl;
  }
  return 0;
}

C++: #include <iostream>

using namespace std;

int main(){
  int n,visited,cnt=0;
  
  while(1){
    int count =0;
    cnt++;
    if(cnt==30)break;
    cin >> n;
    if(n==0)count =0;
    for(int j=2;j<=n;j++){
      visited=0;
      for(int i = 2;i<=n;i++){
	
	if(j%i==0&&j!=i)visited=1;
	else if(j%i==0&&j==i&&visited==0)count++;
	
      }
    }
    cout << count << endl;
  }
  return 0;
}