Go: package main

import (
        "fmt"
)

const MAX int = 1000000
var primes [MAX]int

func main() {
        Init()
        var n, cnt int
        for {
                _, err := fmt.Scanf("%d", &n)
                if err != nil {
                        break
                }
                cnt = 0
                for i := 2; i <= n; i++ {
                        cnt += primes[i]
                }
                fmt.Printf("%d\n", cnt)
        }
}

func Init() {
        for i := 0; i < MAX; i++ {
                primes[i] = i
        }

        Soe()

        for i := 0; i < MAX; i++ {
                primes[i] %= 2
        }
        primes[1] = 0
        primes[2] = 1
}

func Soe() {
        for i := 2; i < MAX; i++ {
                if primes[i] > 0 {
                        for j := 2; j < MAX/i; j++ {
                                if i * j < MAX {
                                        primes[i*j] = 0
                                }
                        }
                }
        }
}

C++: #include <iostream>
#include <string>
#include <cctype>
#include <cmath>
#include <stdio.h>
#include <algorithm>
#include <vector>
#include <map> // pair
#include <set>
#include <string>

#define MAXSIZE 200000
#define PI 3.141592653589
using namespace std;

template<typename T>
T add(T a,T b){
  return a+b;
}

namespace A
{
  int f(void){
    return 1;
  }
}

int main(){
  int n;
  while (std::cin >> n) {
    int count=0;//素数の数
    for(int i=2;i<=n;i++){
      int flag=0;
      if(i==2){
        count++;
        continue;
      }else if(i%2==0){
        continue;
      }
      
      for(int j=3;j<=sqrt(i);j=j+2){
        if(i%j==0){
          flag=1;
          break;
        }
      }
      if(flag==0){
        count++;
      }
    }
    std::cout << count << '\n';
  }

}

C++: #include <stdio.h>
#include <string.h>
int main(void){
    int n=999999,a,b,c;
    char x[1000000];
    memset(x,0,sizeof(x));
    for(a=2,c=0;a<=n;a++){
        if(x[a]){x[a]=c;continue;}
        x[a]=++c;
        for(b=a*2;b<=n;b+=a)x[b]=-1;
    }
    while(~scanf("%d",&n)){
        printf("%d\n",x[n]);
    }
    return 0;

C++: #pragma GCC optimize(2)
#pragma GCC optimize(3)
#include<bits/stdc++.h>
#define fo(x,a,b) for(register int x=a;x<=b;x++)
#define dfo(x,a,b) for(register int x=a;x>b;x--)
using namespace std;
int n,a[1000000];
bool ans[1000000];
int main ()
{	
	fo(i,2,999999)
	{
		ans[i]=true;
		fo(j,2,sqrt(i))
		{
			if (i%j==0) {ans[i]=false;break;}
		}
		
	}
	do
	{
		scanf("%d",&n);
		if (getchar()!='\n') break;
		int te=0;
		if (a[n]==0)
		{
			fo(i,2,n)
			{
				if (ans[i]) te++;
			}
			a[n]=te;
		}
		printf("%d\n",a[n]);
	} while (true);
	
	return 0;
}

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

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
using namespace std;
const int MAX_N = int(1e6);

int main(){

    int n, i, j, maxin = 2;
    bool jud;
    vector <int> vec;
    for (i = 2; i <= MAX_N; i++){
        jud = 0;
        for (j = 0; j < vec.size(); j++){
            if (vec[j] * vec[j] > i) break;
            if (!(i % vec[j])){
                jud = 1;
                break;
            }
        }
        if (!jud)
            vec.push_back(i);
    }
    while (cin >> n){
        cout << upper_bound(vec.begin(), vec.end(), n) - vec.begin() << endl;
    }
}

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

bool check(int in){
	for (int i = 3; i<=sqrt(in); i+=2){
		if (i != 5 && i % 5 == 0)	continue;
		if (in % i == 0)
			return false;
	}
	return true;
}

int PrimeNumber(int in, int start){
	int count = 0;
	if (in > 2)	count++;
	for (int i = start; i <=in; i+=2){
		if (check(i))	count++;
	}

	return count;
}

int main(){
	int in, count500000 = PrimeNumber(500000, 3);

	while (cin >> in, !cin.eof()){
		if (in <= 500000)
			cout << PrimeNumber(in, 3) << endl;
		else
			cout << count500000 + PrimeNumber(in, 500001) << endl;
	}
}

C++: #include <iostream>

using namespace std;

int isprime(int x){
	if(x < 2){
		return false;
	}else{
		for(int i=2;i<x;i++){
			if(x%i==0){
				return false;
			}
		}
		return true;
	}
}
int main(){
	int n;
	cin >> n;
	int a = 0;
	for(int i=2;i<=n;i++){
		if(isprime(i)){
				a++;
		}
	}
				cout << a << endl;
				return 0;
}

C++: #include <iostream>

using namespace std;
int main()
{
    long long n;
    long long prime = 0;

    cin >> n;
    
    for(int x=1; x<=n; x++)
    {
            int counter = 0;
            for(int y=1; y<=x; y++)
            {
                    if(x%y==0)
                    counter++;
            }
            
            if(counter==2)
            {
                    prime++;
            }
    }
    
    cout << prime << endl;
    
return 0;
}

C++: #include <iostream>
#include <algorithm>
using namespace std;
int main(void) {
	long long int n;
	while (cin >> n) {
		int m = 0;
		long long int prime[2000000];
		bool is_prime[2000000];
		for (int i = 0; i <= n; i++) {
			is_prime[i] = true;
		}
		is_prime[0] = is_prime[1] = false;
		for (int i = 2; i <= n; i++) {
			if (is_prime[i]) {
				m++;
				for (int j = i * 2; j <= n; j += i) {
					is_prime[j] = false;
				}
			}
		}
		cout << m << endl;
	}
	return 0;
}

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

int main(void)
{
    long num, length, prime[1000];

    while(cin >> num)
    {
        length = 0;
        int count = 0;
        for (int n = 2; n <= num; n++)
        {
            bool isPrime = true;
            for(int i=0; i < length-1; i++)
            {
                if (!(n % prime[i]))
                {
                    isPrime = false;
                    break;
                }
            }
            if (isPrime)
            {
                prime[length] = n;
                length++;
            }
        }
        cout << length << endl;
    }
    return 0;
}

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

using namespace std;

int main(){
        int prime[40];
        bool is_prime[40];
        int i=2,j=2,n,count=0;
        while(scanf("%d",&n) != EOF){
                int p = 0;
                for(i = 0;i <= n; i++){
                        is_prime[i] = true;
                }
                is_prime[0]=is_prime[1]=false;
                for(i = 2; i<=n;i++){
                        if(is_prime[i]){
                                prime[p++] = i;
                                for(j = 2*i; j <= n; j+=i) is_prime[j] = false;
                        }
                }
                cout << p << endl;
        }
        return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
#define MAX 100

bool prime[MAX]={};

void run(){
  for(int i=2;i<MAX;i++){
    prime[i]=true;
  } 

  for(int i=2;i<MAX;i++){
    if(!prime[i])continue;
    for(int j=2;j*i<MAX;j++){//i*1 is prime number
      prime[i*j]=false;
    }
  }
  return;
}

int main(){

  run();
  
  int n;
  while(cin>>n){
    int sum=0;
    for(int i=2; i<=n; i++){
      if(prime[i])sum++;
    }
    cout<<sum<<endl;
  }
  return 0;

}

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

#define LIM 1000000
using namespace std;

void prime();

vector<int> prime_map;

int main(){
  int num, i=0,j=0;
  vector<int> count;
  prime();
  cout<<"plese"<<endl;
  while(cin){
    cin>>num;
    i=0;
    while(prime_map[i]<=num){
      i=i+1;
    }
    count.push_back(i);
    j++;
  }

  i=0;
  while(i<j){
    cout<< count[i]<<endl;
    i++;
  }
}

void prime(){
  bool judge;
  for(int i = 2; i <= LIM;i++){
    judge=1;
    for(int j= 2; j < i/2; j++){
      if(i%j == 0)judge = 0 break;
    }
    if(judge == 1)prime_map.push_back(i);
  }
}