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++: #include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <algorithm>
#include <iostream>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
#include <cmath>
using namespace std;

#define FOR(i,s,e) for (int i = int(s); i != int(e); i++)
#define REP(i,e) for (int i = 0; i != int(e); i++)
#define FORIT(i, A) for (auto i : A)
#define ISEQ(c) (c).begin(), (c).end()
#define PRINT(x) cout << (x) << endl
#define SZ(x) ((int)(x).size())


int mem[1000000];
int ans[1000000];

int main(int argc, char const *argv[])
{
	int max_num = 1000000;
	int loop_max = sqrt(1000000);
	memset(mem, 0, 1000000);
	for (int i = 2; i < loop_max; i++)
	{
		int c = i * i;
		while(c < max_num)
		{
			mem[c] = 1;
			c+= i;
		}
	}
	int n;
	int sum = 0;
	ans[0] = ans[1] = 0;
	for (int i = 2; i < max_num; i++)
	{
		sum += mem[i] ? 0 : 1;
		ans[i] = sum;
	}
	while(scanf("%d", &n) != EOF)
	{
		printf("%d\n", ans[n]);
	}
	return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <vector>
#include <cmath>
using namespace std;
typedef vector <int> vi;
vi primes;

int get_primes(int max){
	int n = (max+1)/2;
	int root = (int)(pow(n, 0.5));
	vector <bool> numbers(n, true);
	numbers[0]=false;

	if (max>1) primes.push_back(2);
	for (int i=1; i<root; i++){
		int tmp = i*2+1;
		if (numbers[i]){
			primes.push_back(tmp);
			for (int j=i+tmp; j<n+1; j+=tmp){
				numbers[j]=false;
			}
		}
	}
	for (int i=root; i<n+1; i++){
		if(numbers[i]){
			primes.push_back(2*i+1);
		}
	}
	return 0;
}

int main(void){
	vi n;
	int nmax=0;
	while (true){
		int tmp;
		cin>>tmp;
		if (cin.eof()) break;

		n.push_back(tmp);
		if (tmp>nmax) nmax=tmp;
	}

	get_primes(nmax);
	for (int i=0; i<n.size(); i++){
		cout<<upper_bound(primes.begin(), primes.end(), n[i]) - primes.begin()<<endl;
	}
	return 0;
}

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

int main(){
    int n=11, t;
    while(cin >> n){
      t = 0;
      for(int i = 2;i <= n;++i){    
        for(int j = 2;j < i;++j){
          if(i % j == 0) break;
          if(j == i - 1) ++t;
        }
      }
      if(n >= 2) ++t;
      cout << t << endl;
    }
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
bool flg[1000000];
int main(void){
	int n;
	int i,j;
	int cnt=0;
	while(1){
		cin>>n;
		if(cin.eof())	break;
		fill(flg,flg+n+1,0);
		for(i=2;i*i<=n;i++){
			//cout<<"I="<<i<<endl;
			for(j=2;j<=n;j++){
				//cout<<"J="<<j<<endl;
				flg[i*j]=1;
			}
		}
		cnt=0;
		for(i=2;i<=n;i++)	if(flg[i]==0)	cnt++;
		cout<<cnt<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <map>
#include <vector>
#include <set>
#include <iomanip>
#include <fstream>
#include <cstdint>
#include <cmath>
#include <algorithm>
#include <utility>
#include <numeric>
using namespace std;

#define REP(i,f,n) for(int i=(f); i < (n); ++i)
#define PER(i,f,n) for(int i=(n-1); i >= f; --i)

//----------------------------------
//----------------------------------

constexpr int64_t CALCNUM = 2000000;

bool primes[CALCNUM];

signed main()
{
	for(int i = 0; i < CALCNUM; i++)
		primes[i] = true;
    ios::sync_with_stdio(false);
	vector<int> prime;

	for(int i = 2; i <CALCNUM ; i++)
	{
		if(primes[i])
		{
			prime.push_back(i);
			for(int j = 2 * i; j < CALCNUM; j += i)
				primes[j] = false;
		}
	}
	int n;
	while(cin >> n)
	{
		for(int i = 0; i < prime.size(); ++i)
			if(prime[i] > n){
				cout << i <<'\n'; break;

C++: #include    <iostream>
 
using namespace std;
 
bool F[1000003] = { false };
int DP[80000] = { 0 };
void prepa_Prime();
int count_Prime(int);
 
int main() {
    int n;

    prepa_Prime();
    while (cin >> n) {
        cout << count_Prime(n) << endl;;
    }
    return 0;
}
 
void prepa_Prime() {
    int i, c = 2;

    F[0] = F[1] = true;
    DP[0] = 1;

    DP[1] = 2;
    for (i = 4; i <= 1000003; i += 2) { F[i] = true; }
    
    for (i = 3; i <= 1000003; i += 2) {
        if (F[i] == false) {
            DP[c++] = i;
            for (int j = i * 2; j <= 1000003; j += i) {
                F[j] = true;
            }
        }
    }
}

int count_Prime(int n) {
    int i = 0;
    for (i; DP[i + 1] <= n; i++) {}
    return i;
}

C++: #include <iostream>
using namespace std;
int main() {
  int n;
  while(cin >> n) {
	int cnt = 0;
	for(int i = 0; i <= 9; ++i) {
	  if(i > n) break;
	  for(int j = 0; j <= 9; ++j) {
		if(i+j > n) break;
		for(int k = 0; k <= 9; ++k) {
		  if(i+j+k > n) break;
		  for(int l = 0; l <= 9; ++l) {
			if(i+j+k+l > n) break;
			else if((i+j+k+l) == n) cnt++;
		  }
		}
	  }
	}
	cout << cnt << endl;
  }
  return 0;
}

C++: #include<stdio.h>
const int MAX_V = 10000;
int prime[MAX_V+1],sum[MAX_V+1];
int main(void){
    int i,k,n,ans;
    for(i = 0; i < MAX_V + 1; i ++){
        prime[i] = 1;
        sum[i] = 0;
    }
    prime[0] = 0;
    prime[1] = 0;
    for(i = 2; i * i < MAX_V; i ++){
        if(prime[i]){
            for(k = 2 * i; k < MAX_V; k += i){
                prime[k] = 0;
            }
        }
    }
    for(i = 2; i < MAX_V; i ++){
        sum[i+1] = sum[i] + prime[i];
    }
    while(scanf("%d",&n) != EOF){
        ans = sum[n+1];
        printf("%d\n",ans);
    }
    return 0;
}

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

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

	cin >> n;
	int *arr = new int[n];

	for(int i = 0; i < n + 1; i++) arr[i] = 1;

	for(int i = 2; i < sqrt(n); i++){
		if(arr[i]) {
			for(int j = 0; i * (j + 2) <= n; j++) {
				arr[i * (j + 2)] = 0;
			}
		}
	}
	for(int i = 2; i <= n; i++){
		if(arr[i]) count++;
	}

	cout << count << "\n";

	delete[] arr;
}

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

using namespace std;

int isprime(int x){
	int i;
	for(i = 3; i <= sqrt(x); i += 2){
		if(x % i == 0) return 0;
	}
	return 1;
}

int main(){
	int n, i, count;
	while(cin >> n){
		count = 0;
		for(i = 2; i <= n; i++){
			if(i == 2 || i == 3) count++;
			else if(isprime(i) == 1 && i % 2 != 0) count++;
		}
		cout << count << endl;
	}
	return 0;
}

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

int main() {
	int n,yakusuu,sosuu;
	while (cin >> n) {
		sosuu = 0;
		for (int i = 1; i <= n; i++) {
			yakusuu = 0;
			for (int j = 1; j <= i; j++){
				if (i%j == 0)yakusuu++;
			}
			if (yakusuu == 2)sosuu++;
		}
		cout << sosuu << endl;
	}
	return 0;
}

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

using namespace std;

int main(){
	int n,i,j,buf,cnt;
	bool flag;

	while(cin >> n){
		cnt = 1;
		for(i=3;i<=n;i+=2){
			flag = false;
			buf = (int)sqrt((double)i);

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

			if(flag == false){
				cnt++;
			}
		}

		cout << cnt << endl;
	}

	return 0;
}

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

int main(){
    int n, cnt;
    bool prime[1000000];
    prime[0] = prime[1] = false;
    
    for(int i = 2;i <= 1000000;i++){
        prime[i] = true;
    }

    while(cin >> n){
        cnt = 0;
        
        for(int i = 2;i <= sqrt(n);i++){
            if(prime[i]){
                for(int j = 2*i;j<= n;j++){
                    if(j%i == 0){
                        prime[j] = false;
                    }
                }
            }
        }
        
        for(int i = 2;i <= n;i++){
            if(prime[i]){
                cnt++;
            }
        }
        
        cout << cnt << endl;
    }

    return 0;
}