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<vector>
#include<algorithm>
#include<cmath>

using namespace std;

int main() {
	vector<int> prime_num{ 2, 3 };
	int n;

	while (cin >> n){
		int result;
		if (n <= 1) {
			result = 0;
		}
		else if (n == 2){
			result = 1;
		}
		else if (n < prime_num.back()){
			result = count_if(prime_num.begin(), prime_num.end(), [n](int a){return a <= n; });
		}
		else {
			for (int i = prime_num.back() + 2; i <= n; i += 2) {
				int temp = static_cast<int>( sqrt(i));
				auto it = find_if(prime_num.begin(), prime_num.end(), [temp](int a){return a > temp; });
				if (all_of(prime_num.begin(), it, [i](int a){return (i % a) != 0; })) {
					prime_num.push_back(i);
				}
			}
			result = prime_num.size();
		}

		cout << result << endl;
	}

	return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
    int a,o=0;
    bool n[1000000];
    for(int i=2;i<=999999;i++){
        n[i]=true;
    }
    n[0]=n[1]=false;
        for( int j=2;j*j<=999999;j++){
            if(n[j]){
                for(int k=j*2;k<=999999;k+=j){
                n[k]=false;
                }
            }
        }
        while(cin>>a){
    for(int k=2;k<=a;k++) if(n[k]) o++;
    cout<<o<<endl;
    o=0;
        }
}

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

int main(void) {

  int x;
  int count_cin = 0;
  while(true){
    cin >> x ;
    count_cin++;

    if(count_cin == 30) break;


    bool array[max_n];

    for(int i = 0; i < max_n; i++) {
      array[i] = true;
    }

    array[0] = false;
    array[1] = false;
    for(int i = 2; i < max_n; i++) {
      for(int j = 2; j < max_n; j++) {
        if(i * j > max_n) break;
        array[i * j] = false;
      }
    }

    int count_sosu = 0;

    for(int i = 0; i <= x; i++) {
      if(array[i] == true) count_sosu++;
    }

    cout << count_sosu << endl;
  }
  return 0;
}

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

int main(){
  int a,cou,j;
  while(cin >> a){
    if(a>2){
      cou=0;
      for(int i=3;i<=a;i+=2){
	for(j=3;j*j<=i;j+=2){
	  if(i%j==0)break;
	}	  
	if(j*j>i)cou++;
      }
      cout << cou+1 << endl;
    }else cout << "1"<< endl;
  }
  return 0;
}

C++: #include <iostream>
#include <vector>
#include <set>
#include <algorithm>
#include <string>
#include <istream>
 
#define INPUT_FROM_FILE (0)
#if INPUT_FROM_FILE
#include <fstream>
#endif

//素数判定クラス
class PrimeTest{
private:
	std::vector<bool> is_prime_;					//nが素数かどうかをis_prime[n]で判定
	std::vector<unsigned int> prime_numbers_;		//素数の集合

public:
	PrimeTest();

	~PrimeTest();

	const std::vector<unsigned int>& get_prime_numbers() const;

	bool is_prime(unsigned int n);

};


PrimeTest::PrimeTest(){
	is_prime_.push_back(false);	//0
	is_prime_.push_back(false);	//1
	is_prime_.push_back(true);	//2
	is_prime_.push_back(true);	//3

	prime_numbers_.push_back(2);
	prime_numbers_.push_back(3);
}
PrimeTest::~PrimeTest(){}

const std::vector<unsigned int>& PrimeTest::get_prime_numbers() const{
	return prime_numbers_;
}

bool PrimeTest::is_prime(unsigned int n){
	if(n >= is_prime_.size()){
		unsigned int i_s = is_prime_.size();
		is_prime_.resize(n + 1, false);

		//i_sからnまでの各数が素数かどうか判定
		for(unsigned int i = i_s; i <= n; i++){
			unsigned int max_val = int(sqrt((double)i));
			bool b = true;
			for(unsigned int j = 0; j < prime_numbers_.size(); j++){
				if(prime_numbers_[j] > max_val){ break;}
				if(i % prime_numbers_[j] == 0){
					b = false;
					break;
				}
			}
			if(b){
				prime_numbers_.push_back(i);
				is_prime_[i] = true;
			}
		}

	}
	return is_prime_[n];
}

int count_prime(PrimeTest& prime_test, int n){
	int count = 0;

	for(int i = 2; i <= n; i++){
		if(prime_test.is_prime(i)){
			count++;
		}
	}
	return count;
}

 int main(int argc, char **argv){
	int n = 0;
	std::string buf;
	int count = 0;
	PrimeTest prime_test;
 
#if INPUT_FROM_FILE
	std::ifstream ifs("test.txt");
#endif
 
 
#if INPUT_FROM_FILE
	while(std::getline(ifs, buf)){
#else
	while(std::getline(std::cin, buf)){
#endif
		n = std::atoi(buf.c_str());
		count = count_prime(prime_test, n);

		std::cout << count << std::endl;
		
	}
    return 0;
}

C++: #include<bits/stdc++.h>
#define sd(x) scanf("%d",&x)
#define ss(x) scanf("%s",x)
#define sc(x) scanf("%c",&x)
#define sf(x) scanf("%f",&x)
#define slf(x) scanf("%lf",&x)
#define slld(x) scanf("%lld",&x)
#define me(x,b) memset(x,b,sizeof(x))
#define pd(d) printf("%d\n",d);
#define plld(d) printf("%lld\n",d);
#define eps 1.0E-8
// #define Reast1nPeace

typedef long long ll;

using namespace std;

const int INF = 0x3f3f3f3f;

bool p[1000010];

void P(){
	memset(p,1,sizeof(p));
	p[0] = p[1] = 0;
	for(int i = 2 ; i*i<=1000000 ; i++){
		if(p[i]){
			for(int j = i+i ; j<=1000000 ; j+=i){
				p[j] = 0;
			}
		}
	}
}

int main(){
#ifdef Reast1nPeace
	freopen("in.txt", "r", stdin);
	freopen("out.txt", "w", stdout);
#endif
	ios::sync_with_stdio(false);
	P();
	
	int n;	
	while(cin>>n){
		int ans = 0;
		for(int i = 1 ; i<=n ; i++){
			if(p[i]){
				ans++;
			}
		}
		cout<<ans<<endl;	
	}
	return 0;
}

C++: #include<iostream>
#include<vector>
#define MAX 1000000
using namespace std;
int main(){
    vector<int> v(MAX);
    for(int i = 0;i<MAX;i++)
      v[i]=true;

      v[0]=v[1]=false;
    for(int i = 2;i<MAX;i++){
      if(v[i]){
        for(long long int j=(long long int)i*(long long int)i;j<MAX;j+=i){
          cout <<i<<" "<<j<<endl;
         v[j]=false;
       }
      }
    }
    for(int i = 2;i<MAX;i++)v[i]+=v[i-1];
    while(true){
      int a;
      cin >> a;
      if(cin.eof())break;;
      cout << v[a] << endl;
    }
    return 0;
}

C++: #include <iostream>

using namespace std;

int main(){
	int n,count = 0;
	int result = 0;
	cin >> n;
	for (int i = 1; i <= n; i++)
	{
		count = 0;

		for (int j = 1; j <= i; j++)
		{
			if (i%j == 0)
			{
				count++;
			}
		}
		if (count == 2)
		{
			result++;
		}
	}
	cout << result << endl;
}

C++: #include <iostream>
#include <vector>
using namespace std;
int main( void )
{
	int n;
	cin >> n;
	
	vector <bool> p(n, true);
	p[2] = true;
	for ( int i = 3; i * i < n; i += 2 )
	{
		if ( p[i] )
		{
			for ( int j = i+i; j < n; j += i )
			{
				p[j] = false;
			}
		}
	}

	int kotae = 1;
	for ( int i = 3; i < n; i += 2 )
	{
		if ( p[i] )
			kotae++;
	}

	cout << kotae << endl;

	return 0;
}

C++: #include <iostream>
#include <sstream>
#include <vector>
#include <algorithm>
#include <string>
#include <functional>
#include <numeric>
#include <ctype.h>
#include <cstdlib>
#include <string.h>
#include <cmath>
#include <cstdio>





using namespace std;

bool isPrime(int num);

int main()
{
	int input, count = 0;

	while (cin >> input)
	{
		count = 0;

		for (int i = 2; i <= input; i++)
		{
			if (isPrime(i))
			
				count++;
	
		}
		cout << count << endl;
	}

	
}

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

	double sqrtNum = sqrt(num);

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

	return true;
}