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>

using namespace std;

int main()
{
	int n;
	int head,fin;
	int temp;
	
	
	vector<int> isPrime;
	
	while(cin>>n)
	{
		for(int i = 0; i < n; i++){
			isPrime.push_back(1);
		}
//		cout << "Corecced!" << endl;
		isPrime[0] = isPrime[1] = 0;
		temp = 0;
		
		for(;;)
		{
			//TõXgÌæªð©Â¯é
			for(head = temp+1; head < n; head++){
				if(isPrime[head] == 1){
					break;
				}
			}
			
			//æªðÜÜÈ¢{ÍSÄ¬
			for(int i = head; i < n; i++){
				isPrime[i*head] = 0;
			}
			
			//TõXgÌÅãÖ
			for(fin = n; fin > 0; fin--){
				if(isPrime[fin] == 1){
					break;
				}
			}
		//	printf("head:%d    fin:%d\n",head,fin);
			
			if(fin < head*head){
				break;
			}
			temp = head;
		}
		
		int cont = 0;
		for(int i = 0; i < n; i++){
			if(isPrime[i] == 1){
				cont++;
			}
		}
		printf("%d\n",cont);
	}
}

C++: #include<iostream>
#include<math.h>
#include<vector>
#include<algorithm>
using namespace std;

int main()
{
   int a,pp[100000];
   pp[0]=-1,pp[1]=-1;
   for(int i=2;i<100000;i++){
       if(pp[i]==0){
           pp[i]=1;
           for(int j=i*2;j<100000;j+=i){
               pp[j]=-1;
           }
       }
   }
   int now;
   while(cin>>now){
       int ans=0;
       for(int i=1;i<now+1;i++){
           if(pp[i]==1){
               ans++;
           }
       }
       cout<<ans<<endl;
   }
    return 0;
}

C++: #include <stdio.h>
int main()
{
    int low, high, i, flag,c=0,s;
    while(scanf("%d",&high)==!EOF)
       {
           int start = 2;

            while (start <= high)
            {
                flag = 0;
                s = sqrt(start);


                    for(i = 2; i <= start/2; i++)
                    {

                        if(start % i == 0)
                        {
                            flag = 1;
                            break;
                        }
                    }

                    if (flag == 0)
                    {
                          c++;

                    }
                    start++;

            }
                printf("%d\n",c); c=0;



       }
    return 0;
}

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

#define REP(i, s, n) for (int i = s; i < n; ++i)
#define rep(i, n) REP(i, 0, n)
#define SORT(c) sort((c).begin(), (c).end())
#define IINF INT_MAX
#define LLINF LLONG_MAX
#define DEBUG false

int main()
{
    int prime[1000000] = {0};
    int k = 1;
    prime[0] = 2;
    for (long long int i = 3; i < 999999; i++)
    {
        double temp = sqrt(i);
        for (int j = 0; i % prime[j] != 0; j++)
        {
            if (temp < prime[j])
            {
                prime[k++] = i;
                break;
            }
        }
    }
    int n;
    int i;
    while (cin >> n)
    {
        i = 0;
        while (true)
        {
            if (n < prime[i])
            {
                break;
            }
            i++;
        }
        cout << i << endl;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b) - 1;i>=(a);i--)
#define REP(i,n) for(int i=0;i<(n);i++)
#define RREP(i,n) for(int i=n-1;i>=0;i--)

#define PB push_back
#define INF INT_MAX/3
#define ALL(a) (a).begin(),(a).end()
#define CLR(a) memset(a,0,sizeof(a))

typedef long long int ll;

using namespace std;

int main(){
    vector<int> primes;
    primes.PB(2);
    FOR(i,3,500000){
	bool flg = true;
	REP(j,primes.size()) if(i % primes[j] == 0 ) {
	    flg = false;
	    break;
	}
	if(flg) primes.PB(i);
    }

    int n;
    while(true){
	cin >> n;
	if(cin.eof())break;
	int res = upper_bound(ALL(primes),n) - primes.begin();
	cout << res << endl;
    }

    return 0;
}

C++: #include<iostream>
#include<vector>
using namespace std;
int main(){
  int n;
  while(cin>>n){
    vector<int> vi;
    vi.push_back(2);
    vi.push_back(3);
    for(int i=5;i<=n;i+=2){
      for(int j=3;j<=i;j+=2){
	if(i==(j/2)+1)
	  vi.push_back(i);
	if(i%j==0)
	  break;
      }
    }
    cout<<vi.size()<<endl;
  }
  return 0;
}

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

#include <iostream>
#include <vector>
#include <algorithm>
#include <utility>
#include <queue>



int main() {

  bool isPrime[1000000];
  int  num[1000000];
  int  temp;
  int  n;

  for(int i = 0; i < 1000000; ++i) {
    isPrime[i] = true;
  }
  isPrime[0] = isPrime[1] = false;
  
  for(int i = 0; i < 1000; ++i) {
    if( isPrime[i] ) {
      for(int j = i+i; j < 1000000; j += i) {
	isPrime[j] = false;
      }
    }
  }

  num[0] = num[1] = 0;
  temp = 0;
  for(int i = 2; i < 1000000; ++i) {
    if( isPrime[i] ) ++temp;
    num[i] = temp;
  }

  while( std::cin >> n ) {
    std::cout << num[n] << std::endl;
  }
  
  return 0;
}

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

                                                                                                                                                               
int count_prime_number(int);

int main(void){

    int n;
    while(scanf("%d",&n) != EOF){
        std::cout << count_prime_number(n) << std::endl; 
    }

    return 0;
}

int count_prime_number(int n){

    int f,prime_num = 1;

    if (n == 2) {
        prime_num = 1;
    }
    else{
        for (int j = 3; j <= n; j += 2) {   
            f = 0;
            for(int i = 3; i <= std::sqrt(n); i+=2){
                if (j%i == 0) {
                    f = 1;
                    break;
                }
            }

            if (f == 0) {
                prime_num++;
            }
        }
    }

    return prime_num;

}

C++: #include <bits/stdc++.h>
#define debug(x) cerr << #x << ": " << x << '\n';
using namespace std;
using ll = long long;
using P = pair<int, int>;
const int INF = (int)1e9;

// エラストテネスの篩, o(n)程度
// n以下の素数の数を返す, prime[i]: i番目の素数, is_prime[i]: iが素数であるか
int sieve(int n, int prime[] = nullptr, bool is_prime[] = nullptr){
	bool flag0 = false, flag1 = false;
	if(prime == nullptr){
		flag0 = true; prime = new int[n];
	}
	if(is_prime == nullptr){
		flag1 = true; is_prime = new bool[n + 1];
	}

	int p = 0;
	fill(is_prime, is_prime + n + 1, true);
	is_prime[0] = is_prime[1] = false;

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

	if(flag0) delete prime;
	if(flag1) delete is_prime;

	return p;
}

const int MAX_N = (int)1e6;
int prime[MAX_N];
bool is_prime[MAX_N + 1];

int main(void){
	int num = sieve(MAX_N, prime, is_prime);

	int n;
	while(scanf("%d", &n)){
		if(n == EOF) break;
		cout << upper_bound(prime, prime + num, n) - prime << '\n';
	}

	return 0;
}