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 <list>
#include <algorithm>

using namespace std;

int main() {
  int n;
  
  while(!cin.eof()) {
    cin >> n;
    list<int> search_list(n-1);
    list<int> prime_number;
    while(n!=1) {
      search_list.push_front(n);
      n--;
    }
    while(!search_list.empty()) {
      int m = search_list.front();
      prime_number.push_back(m);
      search_list.pop_front();
      list<int>::iterator it = search_list.begin();
      while(it!=search_list.end()) {
	if((*it % m) == 0){
	  it = search_list.erase(it);
	  continue;
	}
	it++;
      }				   
    }
    cout << prime_number.size()-1 << endl;
    prime_number.erase();
    search_list.erase();
  }
}

C++: #include <iostream>
#include <stdio.h>
#include <vector>
#include <algorithm>

using namespace std;


bool isPrime( int n ) {
	for( int i = 2; i * i <= n; i++ ) {
		if( n % i == 0 ) return false;
	}
	return true;
}


int main(void){


	int n;
	while( scanf("%d", &n) != EOF ) {

		int cnt = 0;
		for( int i = 2; i <= n; i++ )
			if( isPrime(i) )
				cnt ++;
		cout << cnt << endl;

	}

    return 0;
}

C++: #include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<iostream>
#include<string>
using namespace std;

bool isPrime(int num) {
	if (num == 1)return false;
	if (num == 2)return true;

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


int main() {

	int num;
	int count;
	while (cin >> num) {
		count = 0;
		for (int i = 1; i <= num; i++) {
			if (isPrime(i))count++;
		}
		printf("%d\n", count);

	}

	//system("pause");
	return 0;
}

C++: #include<iostream>

using namespace std;

int isPrime(int n) {
  for(int i=2;i*i<=n;i++){
    if(n%i==0) return 0;
  }
  return 1;
}

int main() {
  int a;
  int b[1000000];
  for(int i=2;i<=1000000;i++){
    if(isPrime(i)) b[i]=b[i-1]+1;
    else b[i]=b[i-1];
  }
  while(cin>>a){
    cout << b[a] << endl;
  }
  return 0;
}

C++: #include <iostream>

using namespace std;

bool isPrime(int n)
{
 for(int i=2; i * i <= n; i++) {
  if(n % i == 0) return false;
 }
 return true;
}

int main()
{
 int n, cnt;
 
 while(cin >> n)
 {
  cnt = 0;
  for(int i=2; i <= n; i++) {
   if(isPrime(i)) cnt++;
  }
  cout << cnt << endl;
 }
 return 0;
}

C++: #include<stdio.h>
int main ()
{
    int n,i,m,sum;
    while(1)
    {
        sum=0;
        scanf("%d",&n);
        for(m=2;m<=n;m++)
        {
            for(i=2;i<=m;i++)
            {
                if(m%i==0 && i<m)
                {
                    break;
                }
                if(i==m)
                {
                    sum=sum+1;
                }
            }
        }
        printf("%d\n",sum);
    }
    return 0;
}

C++: #include<bits/stdc++.h>
#define rep(i, x) for(int i = 0; i < (int)(x); i++)
#define reps(i, x) for(int i = 1; i <= (int)(x); i++)
#define UNIQUE(v) v..erase( unique(v.begin(), v.end()), v.end() );
#define Size(x) ((int)(x).size())
using namespace std;

const int N = 1000000;
int for_erat[N];
vector<int> prime;
void erat(){
    for(int i = 0; i < N; i++){
        for_erat[i] = 1;
    }
    for(int i = 2; i < sqrt(N); i++){
        if(for_erat[i]){
            for(int j = 0; i * (j + 2) < N; j++){
                for_erat[i *(j + 2)] = 0;
            }
        }
    }

    for(int i = 2; i < N; i++){
        if(for_erat[i]){
            prime.push_back(i);
        }
    }
}

int main(){
	erat();
	int n;
	while(cin >> n){
		int cnt = 0;
		rep(i, Size(prime)){
			if(prime[i] <= n) cnt++;
			else break;
		}
		cout << cnt << 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((*p)*(*p)>i)break;
			if(i%*p==0){
				isprime=false;
				break;
			}
		}
		if(isprime){
			prime.push_back(i);
		}
	}
}
int main(){
	int n;
	init();
	while(cin>>n){
		cout<<lower_bound(prime.begin(),prime.end(),n)-prime.begin()+1<<endl;
	}
	return 0;
}

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

int prime[100000];


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<cstdio>
#include<string.h>
using namespace std;
int pNum;
bool IsPrime[1000010];
int Prime[1000010];
int GetPrime(int n){
    memset(IsPrime,true,sizeof(IsPrime));
    IsPrime[0]=IsPrime[1]=false;
    pNum=0;
    for(int i=2;i<=n;i++){
        if(IsPrime[i])Prime[++pNum]=i;
        for(int j=1;j<=pNum&&Prime[j]*i<=n;j++){
            IsPrime[Prime[j]*i]=false;
            if(i%Prime[j]==0)break;
        }
    }
    return pNum;
}
int main(){
    int n;
    while(~scanf("%d",&n)){
        printf("%d\n",GetPrime(n));
    }
    return 0;
}

C++: #include <cmath>
#include <cstdio>
#include <map>

bool fermat_test(int n) {
    if (n < 2) {
        return false;
    } else if (n == 2) {
        return true;
    } else {
        return (static_cast<int>(std::pow(2, n - 1)) % n) == 1;
    }
}

bool prime_test(int n) {
    if (n < 2) {
        return false;
    } else if (n == 2) {
        return true;
    } else {
        for (int i = 3; i < static_cast<int>(std::sqrt(n)); i += 2) {
            if ((n %= i) == 0) {
                return false;
            }
        }
    }
    return true;
}

int main() {
    std::map<int, int> m;
    m[3] = 2;
    int n;
    while (~scanf("%d", &n)) {
        for (int i = 5; i <= n; i += 2) {
            if (m.count(i) > 0)
                continue;

            m[i] = m[i - 2] + (fermat_test(i) && prime_test(i));
        }
        printf("%d\n", m[n % 2 == 0 ? n - 1 : n]);
    }
}

C++: #include<bits/stdc++.h>
using namespace std;
int arr[1010000], a, ans;
vector< int > vec;
int main(){
	for(int i=2;i<1000000;i++){
		if(arr[i]==0){
			vec.push_back(i);
			for(int k=i+i;k<=1000000;k+=i){
				arr[k]=-1;
			}		
		}
	}
	cin>>a;
	for(int i=0;i<vec.size();i++){
		if(a>=vec[i]) ans++;
		else break;
	}
	cout<<ans<<endl;
	;
	
	
}