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 <math.h>
using namespace std;

int main(){
  int a;
  int i, j, k;
  int past=1, a_sqrt;
  int count;
  bool chk[1000000];

  for(i=1; i<1000000; i++) chk[i]=true;
  while(cin>>a){
    count=0;
    if(past>=a){
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
    else {
      a_sqrt=(int)sqrt((double)a);
      for(i=2; i<=a_sqrt; i++){
        k=past/i;
        if(k<2) k=2;
        for(j=k; j<=a/i; j++){
          chk[j*i]=false;
        }
      }
      past=a;
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
  }

  return 0;
}

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

int main(){
  bool prime[1000000] = {};
  int sum[1000000]; sum[0] = sum[1] = 0; sum[2] = 1;
  for(int i = 3 ; i < 1000000 ; i += 2 ){
    sum[i] = sum[i + 1] = sum[i - 1];
    if(!prime[i]){
      for(int j = i + i + i ; j < 1000000 ; j += i + i ){
        prime[j] = true;
      }
      sum[i]++;
      sum[i+1]++;
    }
  }

  int n;
  while(cin >> n){
    cout << sum[n] << endl;
  }
}

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

using namespace std;

int setPrimeNumberCount(int n) {
    int count = 0;
    vector<int> primeList;
    bool findList[n+1];
    
    if(n >= 2) {
        primeList.push_back(2);
        count++;
        
        for(int i = 2; i <= n; i++){
            findList[i] = false;
        }
        for(int i = 2; i <= sqrt(n); i++){
            if(findList[i]) {
                continue;
            }
            for(int j = i * 2; j <= n; j += i) {
                if(!findList[j]) {
                    findList[j] = true;
                }
            }
        }
    }
    for(int i = 3; i <= n; i += 2){
        if(!findList[i]) {
            primeList.push_back(i);
            count++;
        }
    }
    return count;
}

int main(void) {
    int n;
    int count = 0;
    
    while(cin >> n) {
        count = setPrimeNumberCount(n);
        cout << count << endl;
        count = 0;
    }
    return 0;
}

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

void count_Prime(int n);
int IsPrime(int num);


int main()
{
	int Data = 0;
	while (cin >> Data)
	{
		count_Prime(Data);
	}

	return 0;
}

void count_Prime(int n)
{
	int counter = 0,  i = 0;
	for (i = 2; i <= n; ++i)
	{
		counter += IsPrime(i);		
	}

	cout << counter << endl;

}

int IsPrime(int num)
{
	if (num < 2) return 0;
	else if (num == 2) return 1;
	else if (num % 2 == 0) return 0; // ??¶??°????????????????????????

	double sqrtNum = sqrt(num);
	for (int i = 3; i <= sqrtNum; i += 2)
	{
		if (num % i == 0)
		{
			// ?´???°??§?????????
			return 0;
		}
	}

	// ?´???°??§??????
	return 1;
}

C++: #include <bits/stdc++.h>
using namespace std;
const long long INF = 1e12;

template<typename T>
bool is_prime(T _Num){
    if(_Num < 2)return false;
    if(_Num % 2 == 0){
        if(_Num == 2)return true;
        else return false;
    }
    bool prime = true;
    for(int i = 3; i <= sqrt(_Num); i += 2){
        if(_Num % i == 0){
            prime = false;
            break;
        }
    }
    return prime;
}

int main(){
    long long cnt = 0;
    vector<long long> prime(1010000, 0);
    for(int i = 1; i < 1010000; i++){
        if(is_prime(i))cnt++;
        prime[i] = cnt;
    }
    
    while(true){
        int n = 0;
        cin >> n;
        if(n == 0)break;
        cout << prime[n] << endl;
    }
    return 0;
}

C++: #include<bits/stdc++.h>
#define rep(i,n) for(int i=0; i<n; i++)
#define rrep(i,n) for(int i=n-1; i>=0; i--)

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pi;
typedef long double lb;

const int INF=1<<27;
const int MOD=(int)1e9+7;

int main(void)
{
	int n[30];
	bool po[1000001];
	rep(i,1000001)po[i]=true;
	int prime[1000001]={0};
	po[0]=false;
	po[1]=false;
	int cnt;
	while(cin>>n[cnt]) cnt++;

	int m=0;
	rep(i,cnt) m=max(n[i],m);

	for(int j=2;j<=m;j++){
		if(po[j]){
			prime[j]=prime[j-1]+1;
			for(int k=1;j*k<=m;k++) {
				po[j*k]=false;
			}
		}else{
			prime[j]=prime[j-1];
		}
	}

	rep(i,cnt) cout << prime[n[i]] << endl;

        return 0;
}

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


int main(){
	int number,cnt=0;
	cin>>number;
	int* arr = new int[number+1];
	for (int i = 1; i <= number; i+=2)
	{
		arr[i-1]=0;
		arr[i]=1;
	}
	for (int i = 3; i < sqrt(number); i+=2)
		if (arr[i])
			for (int j = i+i; j <=number; j+=i)
				arr[j]=0;
	arr[2]=1;arr[1]=0;
	for (int i = 0; i <= number; i++)
		if (arr[i])
			++cnt;
	cout<<cnt;
}

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

const int MAX_N = 1000000;

vector<int> init(void){
    vector<int> prime;
    bool is_prime[MAX_N];
    memset(is_prime, true, MAX_N);
    
    is_prime[0] = is_prime[1] = false;
    for(int i=2; i<MAX_N; i++){
        if(is_prime[i]){
            prime.push_back(i);
            for(int j=2*i; j<MAX_N; j+=i) is_prime[j] = false;
        }
    }
    return prime;
}

int main(int argc, const char * argv[])
{
    int n;
    vector<int> prime = init();
    while(cin>>n){
        cout << upper_bound(prime.begin(), prime.end(), n)-prime.begin() << endl;
    }
    return 0;
}

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

int main(int, char**)
{
	std::vector<int> nums;
	nums.reserve(30);
	std::vector<bool> pegion;
	pegion.resize(1000000, false);
	pegion[2] = true;

	int buff;
	int max = 0;
	while (std::cin >> buff)
	{
		nums.emplace_back(buff);
		if (max < buff)
		{
			max = buff;
		}
	}

	for (int target = 3; max >= target; ++target)
	{
		if (0 == target % 2)
		{
			continue;
		}
		int count = 0;
		bool flag = true;
		for (count = 3; target >= count * count; ++count)
		{
			if (0 == target % count)
			{
				flag = false;
				break;
			}
		}
		pegion[target] = flag;
	}

	for (const int num : nums)
	{
		int count = 0;
		for (int index = 0; num >= index; ++index)
		{
			if (pegion[index])
			{
				count += 1;
			}
		}
		std::cout << count << std::endl;
	}
}

C++: #include<iostream>
#include<vector>
#include<cmath>
using namespace std;
int main(){
  int n,k;
  vector<int> vi;
  vi.push_back(2);
  for(int i=3;i<=999999;i+=2){
    k=0;
    for(int j=3;j<=sqrt(i);j+=2){
      if(i%j==0){
	k=1;
	break;
      }
    }
    if(k==0)vi.push_back(i);
  }	
  while(cin>>n){
    int l;
    for(l=0;vi[l]<=n;l++);
    cout<<l<<endl;
  }
  return 0;
}

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

int main(){
    int n;
    vector<int> S;

    while(cin>>n){
        S.push_back(2);
        int ans=0;
        for(int i=3; i<=n; i+=2){
            int judge=0;
            for(int j=0; j<S.size(); j++){
                if(i%S[j]==0)judge=1;
            }
            if(judge==0){
                ans++;
                S.push_back(i);

            }
        }
        cout<<ans+1<<endl;
        S.clear();
    }
    return 0;
}