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>
using namespace std;
??
#define Max 1000000
??
int prime [10000000];
	??
int main()
{
	????????int n, out;
	??
		????????for (int i = 2; i * i < Max; i++){
			????????????????if (prime[i] == 0){
				????????????????????????for (int j = i; j < Max; j += i){
					????????????????????????????????if (!(j == i)) prime[j] = 1;
					????????????????????????}
				????????????????}
			????????}
	??
		????????while (cin >> n){
			????????????????out = 0;
			????????????????for (int i = 0; i <= n; i++){
				????????????????????????if (prime[i] == 0) out++;
				????????????????}
			????????????????cout << out - 2 << endl;
			????????}
	????????return(0);
}

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

int main(){
  int n,prime[1000000],key,lf=0;
  prime[0]=2;
  key=1;
  for(int i=3;i<=999999;i+=2)
    {
      for(int j=2;j<=sqrt(i);j++)
	{
	  if(i%j == 0)
	    {
	      lf=1;
	      break;
	    }
	}
      if(lf == 1)
	{
	  lf=0;
	}
      else 
	{
	  prime[key]=i;
	  key++;
	}
    }

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

  cout << cou << endl;
 }
}

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

int main()
{	
	bool p[MAX];
	fill(p, p + MAX, 1);
	
	p[0] = p[1] = 0;
	for (int i = 2; i < sqrt(MAX) + 1; i++) {
		if (p[i]) {
			for (int j = i * 2; j < MAX; j += i)
				p[j] = 0;
		}
	}
	
	int n;
	while (cin >> n) {
		int sum = 0;
		while(n) sum += p[n--];
		cout << sum << endl;;
	}
		
	return 0;
}

C++: #include <stdio.h>
#include <stdbool.h>

int main(void)
{
  bool isPrime[1000000];
  int i,j;
  for(i=2; i<1000000; ++i){
    isPrime[i] = true;
  }
  int primeCount[1000000];

  for(i=2; i<1000000; ++i){
    if(isPrime[i]){
      primeCount[i] = primeCount[i-1]+1;
      if(i<1000){
        for(j=i*2; j<1000000; j+=i){
          isPrime[j] = false;
        }
      }
    }else{
      primeCount[i] = primeCount[i-1];
    }
  }
        
  int n;
  while(scanf("%d", &n) != EOF){
    printf("%d\n", primeCount[n]);
  }

  return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<math.h>
#define INF 999999
using namespace std;

int main()
{
	int n, rootn,ans=0;
	bool a[INF] = {true};
	while (cin >> n)
	{
		rootn = sqrt(n);
		for (int i = 2; i <= rootn; i++)
		{
			for (int j = 0; j < INF; j++)
			{
				if (j%i == 0) a[j] = false;
			}
		}
		for (int i = 0; i < INF; i++)
		{
			if (a[i])++ans;
		}
		cout << ans << endl;
	}
			
	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include<math.h>

using namespace std;

int HowManyPrimeNumbers1(const int);
int HowManyPrimeNumbers2(const int);

int main(){
	int n;

	while(cin >> n){
		printf("%d\n",HowManyPrimeNumbers1(n));
	}

	return 0;
}

int HowManyPrimeNumbers1(const int n){
	int i,j,buf,cnt = 1;

	for(i=3;i<=n;i+=2){
		buf = (int)sqrt((double)i);

		for(j=3;j<=buf;j+=2){
			if((i % j) == 0){
				cnt--;
				break;
			}
		}

		cnt++;
	}

	return cnt;
}

int HowManyPrimeNumbers2(const int n){
	return n;
}

C++: #include <iostream>
using namespace std;
int main(){
	int n , i , j;
	while(cin >> n ){
		int counter = 0;
		for( int i=1 ; i<=n ; i++ ){
			int temple=0;
			for( int j=1 ; j<=n ; j++ ){
				if( i%j == 0 )
					temple++;
			}
			if( temple == 2 )
				counter ++;
		}
		cout << counter << endl;
	}
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <iomanip>
#include <string>
#include <set>
#include <functional>

class Mod{
	const int n;
	public:
		Mod(const int _n):n(_n){}
		bool operator()(const int _src){
			return ! (_src % n);
		}
};

const std::vector<int> Eratosthenes(std::vector<int>& src){
	std::vector<int> result;
	int cnt(0);
	while(!src.empty()){
		int n(src[0]);
		result.push_back(n);
		Mod mod(n);
		std::vector<int>::iterator it= std::remove_if( src.begin(), src.end(), mod );
		src.erase(it,src.end());
		if(cnt<10 || cnt%10==0){
			src.swap(src);
		}
	}
	return result;
}
static int const MAX(1000000);

int main(int,char**){
	std::vector<int> vec;
	int max_2(MAX-MAX/2-MAX/3-MAX/5+MAX/6+MAX/10+MAX/15-MAX/30+3);
	vec.reserve(max_2);
	for(int i(3);i<=MAX;i+2){
		if(i!=3 && i%3==0) continue;
		if(i!=5 && i%5==0) continue;
		vec.push_back(i);
	}
	std::vector<int> primes;
	primes.reserve(10000);
	primes =Eratosthenes(vec);
	int n;
	while(std::cin >>n){
		std::vector<int>::const_iterator const it= std::upper_bound(primes.begin(),primes.end(),n);
		int off=it -primes.begin();
		if(n>=2){ ++off; }
		std::cout << off <<std::endl;
	}
	return 0;
}

C++: #include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
vector<int> prime;
void init(){
	for(int i=2;i<999999;i++){
		bool isprime=true;
		for(vector<int>::iterator p(prime.begin());p!=prime.end();p++){
			if(i%*p==0){
				isprime=false;
				break;
			}
		}
		if(isprime){
			prime.push_back(i);
		}
	}
}
int main(){
	int n;
	init();
	cerr<<"init"<<endl;
	while(cin>>n){
		cout<<lower_bound(prime.begin(),prime.end(),n)-prime.begin()+1<<endl;
	}
	return 0;
}

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

#define maxPN 1000000
#define PNs  1000100
int countPN = 0, countPNs = 0;
int PrimeNum[maxPN], temp;
bool PN[PNs];

bool IsPrimeNumber(int x);

void PrimeN(){
	int i, j;
	PrimeNum[0] = 2;
	PN[2] = true;PrimeNum[countPN++] = 2;
	PN[3] = true;PrimeNum[countPN++] = 3;
//	PN[5] = true;PrimeNum[countPN++] = 5;
	
	for(i = 6; i  < PNs;i+=6){
		if(IsPrimeNumber(i-1)){
			PrimeNum[countPN++] = i-1;
			PN[i-1] = true;//cout<<i-1<<endl;
		}
		if(IsPrimeNumber(i+1)){
			PrimeNum[countPN++] = i+1;
			PN[i+1] = true;//cout<<i+1<<endl;
		}
		countPNs = i+1;//cout<<i-1<<" "<<PN[i-1]<<endl;
	}
}

bool IsPrimeNumber(int x){
	//if(x == 2 || x == 3) return true;
	//if(x <= 4 || x%2 == 0 || x%3 == 0)   return false;
	//if(x < countPNs) return PN[x];
	for(int i = 0; PrimeNum[i] * PrimeNum[i] <= x; i++)
		if(x%PrimeNum[i] == 0)return false;
	return true;
}


int main(){
//	clock_t start, end;
	int n = true, counta;
//	start = clock();
	PrimeN();
	cout<<PrimeNum[countPN-1]<<" "<<countPNs<<endl;
//	cout<<clock()-start<<"ms"<<endl;
	
	while(cin>>n){
		counta = 1;
		
		
		//cout<<(IsPrimeNumber(n)?"yes":"no")<<endl;
	//	start = clock();
		//cout<<IsPrimeNumber(n)<<endl;
		for(int i = 1; i <= n; i+=2){
			if(PN[i])counta++;
		//	cout<<i<<" "<<PN[i]<<endl;
		}
		if(n == 1) counta = 0;
		cout<<counta<<endl;
	//	cout<<clock()-start<<"ms"<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <math.h>
#include <iomanip>
#include <vector>
#include <map>

int main() {
	int a;
	while (std::cin >> a) {
		int nSum = 0;
		for (int i = 2;i <= a;i++) {
			int flag = 0;
		for (int j = 2;j < i;j++) {
			if (i%j == 0) {
				flag = 1;
				break;
			}
		}
		if (flag == 0) {
			nSum += 1;
		}
	}
		std::cout << nSum << std::endl;
	}
	return 0;
}