Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

C++: //
//  main.cpp
//  CppTest
//
//  Created by Ryu on 2017/02/08.
//  Copyright ?? 2017??´ Ryu. All rights reserved.
//

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cmath>
using namespace std;

int main(int argc, const char * argv[]) {
    // insert code here...
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    
    long in[30];
    long out[30];
    int n = 0;
    long max = 0;
    while(cin >> in[n]){
        if(max < in[n]){
            max = in[n];
        }
        n++;
    }
    //printf("%d", n);
    
    long p[100000];
    p[0] = 2;
    int m = 1;
    for (int i = 3; i <= max; i += 2) {
        bool prime = true;
        for(int j = 0; j < m; j++){
            if (i % p[j] == 0) {
                prime = false;
                break;
            }
            if (i < pow(p[j], 2)) {
                break;
            }
        }
        if (prime) {
            p[m] = i;
            //printf("%ld\n", p[m]);
            m++;
        }
    }
    
    
    for (int i = 0; i < n; i++) {
        int s = 0;
        while (in[i] >= p[s] && p[s] != 0) {
            //printf("in: %ld, p: %ld\n", in[i], p[s]);
            s++;
        }
        out[i] = s;
        printf("%ld\n", out[i]);
    }
    
    return 0;
}

C++: #include <iostream>
#include <vector>

using namespace std;

class Prime {
// n以下の素数を列挙する
public:
  const int n;
  vector<bool> is_prime;
  vector<int> primes;
  Prime(int size) : n(size), is_prime(n+1, true) {
    is_prime[0] = false;
    is_prime[1] = false;
    for (int i = 2; i <= n; ++i) {
      if (!is_prime[i]) continue;
      primes.push_back(i);
      int tmp = 2*i;
      while (tmp <= n) {
        is_prime[tmp] = false;
        tmp += i;
      }
    }
  }

  bool check(int x) { return is_prime[x]; }
};

int main() {
  Prime prime(1000001);
  int n;
  while (cin >> n) {
      int ans = 0;
      for (int p : prime.primes) {
          if (p > n) break;
          ++ans;
      }
      cout << ans << endl;
  }
}

C++: #include<stdio.h>
int main(void){
   int s[10000], l, m, n, i, sum;
   sum = 0;
   for ( i = 1; i <= 1000000; i++ ) {
      s[i] = 0;
   }
   for ( k = 2; k <= 1000000; k++ ){
      if (s[k] == 0) {
         for ( l = 2; l <= 100000 / k; l++ ) {
            s[k * l] = 1;
         }
      }
   }
   while(scanf("%d", &n) != EOF){
      for(i = 1; i <= n; i++){
         if(s[i] == 0){
            sum++;
         }
      }
   }
   printf("%d", sum);
   return 0;
}

C++: #include <iostream>
#include <vector>

using namespace std;

int main(void) {
	int n;
	
	vector<int> prmcnt_v;

	while(cin >> n) {
		int pcnt = 0;
		for (int i = 2; i <= n; i++) {
			for (int j = 2; j <= i; j++) {
				if (i % j == 0){
					if (i == j) {
						pcnt++;
					}
					else {
						break;
					}
				}
			}
		}
		prmcnt_v.push_back(pcnt);
	}


	for (int k = 0; k < prmcnt_v.size(); k++) {
		cout << prmcnt_v[k] << endl;
	}

	return 0;
}

C++: #include<iostream>
#include<cstdio>
using namespace std;
const int maxn = 1e6+7;
int prime[maxn];
bool is_prime[maxn];
int n,p=-1;

void Eular()
{
    for(int i=1;i<=maxn-7;i++)
        is_prime[i]=true;
    for(int i=2;i<=maxn-7;i++)
    {
        if(is_prime[i])
            prime[++p]=i;
        for(int j=0;j<=p && i*prime[j] <= maxn-7;j++)
            is_prime[i*prime[j]]=false;
    }
}

int main()
{
    Eular();
    while(~scanf("%d",&n))
    {
        int i=0,ans=0;
        while(prime[i]<=n) i++,ans++;
        printf("%d\n",ans);
    }
    return 0;
}

C++: #include<iostream>
using namespace std;
int a[1000000];
int sum[1000000];
int n;
int main(){
 for (int i=2;i<1000000;i++){
  if (!a[i])
   for (int j=i*2;j<1000000;j+=i)
    a[j]=1;
 }
 for (int i=2;i<1000000;i++) sum[i]=sum[i-1]+(a[i]==0?1:0);
 while (cin>>n)
  cout<<sum[n]<<endl;
 return 0;
}

C++: #include <bits/stdc++.h>

#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define llong long long
#define pb(a) push_back(a)

#define INF 100000000
#define MOD 1000000007
#define writeln(n) cout<<n<<endl
using namespace std;
typedef pair<int, int> P;
typedef pair<llong, llong> LP;
typedef pair<int, P> PP;
typedef pair<llong, LP> LPP;
//vector<tuple<long long, long long, long long>>vec;

template<class T>
inline bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
inline bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}

int dy[] = {0, 0, 1, -1, 0};
int dx[] = {1, -1, 0, 0, 0};

const int inf=100000000;

bool isprime(int p) {
    if (p == 1) return false;
    for (int i = 2; i < p; i++) {
        if (p%i == 0) return false;
    }
    return true;
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n; cin>>n;
    int cnt=0;
    for(int i=1; i<=n; i++)
    {
        if(isprime(i)) ++cnt;
    }
    while(cin>>n){cout<<cnt<<endl;}
}

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

int main(void) {
    int* array;
    int n, count;
    
    while(cin >> n) {
        array = new int[n + 1];
        count = 0;

        for(int i = 0; i <= n; ++i) {
            array[i] = 0;
        }
        for(int i = 2; i <= n; ++i) {
//iの倍数に全て印をつける。
            if(array[i] == 0) {
                for(int j = i; j <= n; j += i) {
                    array[j] = 1;
                }
                count++;
            }
        }
        cout << count << endl;
        delete[] array;
    }
}