Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

func next()string{
	return sc.Text()
}

func nextInt()int{
	a, _ := strconv.Atoi(next())
	return a
}

C++: #include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
vector<int> prime;
void init(){
	for(int i=2;i<999999;i++){
		bool isprime=true;
		for(vector<int>::iterator p(prime.begin());p!=prime.end();p++){
			if((*p)*(*p)>i)break;
			if(i%*p==0){
				isprime=false;
				break;
			}
		}
		if(isprime){
			prime.push_back(i);
		}
	}
}
int main(){
	int n;
	init();
	while(cin>>n){
		cout<<lower_bound(prime.begin(),prime.end(),n)-prime.begin()+1<<endl;
	}
	return 0;
}

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

int prime[100000];


bool isPrime(int n) {
  int x = (int)sqrt(n);
  for (int i = 3; i <= x; i += 2) {
    if (n % i == 0) {
      return false;
    }
  }
  return true;
}


void get_prime(int max_num) {
  int primecount = 1;
  prime[2] = primecount;
  for (int i = 3; i <= max_num; i += 2) {
    if (isPrime(i))
      prime[i] = ++primecount;
    else
      prime[i] = primecount;
  }
}


int main () {
  int ns[30];
  int i = 0, max_num;
  while (std::cin >> ns[i]) {
    max_num = std::max(max_num, ns[i]);;
    if (ns[i] % 2 == 0)
      ns[i] --;
    i ++;
  }
  get_prime(max_num);
  for (int j = 0; j < i; j ++) {
    std::cout << prime[ns[j]] << std::endl;
  }
  return 0;
}

C++: #include<cstdio>
#include<string.h>
using namespace std;
int pNum;
bool IsPrime[1000010];
int Prime[1000010];
int GetPrime(int n){
    memset(IsPrime,true,sizeof(IsPrime));
    IsPrime[0]=IsPrime[1]=false;
    pNum=0;
    for(int i=2;i<=n;i++){
        if(IsPrime[i])Prime[++pNum]=i;
        for(int j=1;j<=pNum&&Prime[j]*i<=n;j++){
            IsPrime[Prime[j]*i]=false;
            if(i%Prime[j]==0)break;
        }
    }
    return pNum;
}
int main(){
    int n;
    while(~scanf("%d",&n)){
        printf("%d\n",GetPrime(n));
    }
    return 0;
}

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

bool fermat_test(int n) {
    if (n < 2) {
        return false;
    } else if (n == 2) {
        return true;
    } else {
        return (static_cast<int>(std::pow(2, n - 1)) % n) == 1;
    }
}

bool prime_test(int n) {
    if (n < 2) {
        return false;
    } else if (n == 2) {
        return true;
    } else {
        for (int i = 3; i < static_cast<int>(std::sqrt(n)); i += 2) {
            if ((n %= i) == 0) {
                return false;
            }
        }
    }
    return true;
}

int main() {
    std::map<int, int> m;
    m[3] = 2;
    int n;
    while (~scanf("%d", &n)) {
        for (int i = 5; i <= n; i += 2) {
            if (m.count(i) > 0)
                continue;

            m[i] = m[i - 2] + (fermat_test(i) && prime_test(i));
        }
        printf("%d\n", m[n % 2 == 0 ? n - 1 : n]);
    }
}

C++: #include<bits/stdc++.h>
using namespace std;
int arr[1010000], a, ans;
vector< int > vec;
int main(){
	for(int i=2;i<1000000;i++){
		if(arr[i]==0){
			vec.push_back(i);
			for(int k=i+i;k<=1000000;k+=i){
				arr[k]=-1;
			}		
		}
	}
	cin>>a;
	for(int i=0;i<vec.size();i++){
		if(a>=vec[i]) ans++;
		else break;
	}
	cout<<ans<<endl;
	;
	
	
}

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

using namespace std;

#define MAX 1000000

int main(void){

  int n,count = 0,num = 0;

  while(cin >> n){

    vector<int>array(MAX,0);

    if(n == 1){
      count = 0;
      cout << count << endl;
    }

    if(n == 2){
      count = 1;
      cout << count << endl;
    }

    if(n == 3){
      count = 2;
      cout << count << endl;
    }

    if(n > 3){
      count = 2;
      num = 0;
      array[num++] = 2; 
      array[num++] = 3; 


      for(int i = 5; i <= n ; i+=2){
        int flag = 0;
        for(int j = 0; array[j] * array[j] <= i;j++){
          if(i % array[j] == 0){
            flag = 1;
            break;
          }
        }
        if(!flag){
          array[num++] = i;
          count++;
        }
      }
      cout << count << endl;
      count = 0;
      num = 0;
    }
  }

  return 0;
}

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

int main(void)
{
  int prm[1000000];
  for (int i = 0; i < 1000000; i++) prm[i] = 1;
  prm[0] = prm[1] = 0;
  for (int i = 2; i < 1000000; i++) {
    for (int j = 2; j < 1000000 / i; j++) {
      prm[i*j] = 0;
    }
  }
  int n;
  while (cin >> n) {
    int ans = 0;
    while (n) {
      if (prm[n]) ans++;
      n--;
    }
    cout << ans << endl;
  }

  return 0;
}

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

int main(){

	const int N_MAX=1000000;
	int i,j,n[32],ni,soi,max;
	int so[N_MAX],count[N_MAX];
	
	for(i=0;i<N_MAX;i++)
		so[i]=1;
	ni=0;
	max=0;
	while(cin>>n[ni])
		if(n[ni++]>max)
			max=n[ni-1];
	int sq_max = (int)sqrt(max);
	so[0]=so[1]=0;
	for(soi=0,i=2;i<=sq_max;i++){
		if(so[i]==0)continue;
		for(j=i*2;j<=max;j+=i)
			so[j]=0;
	}
	for(i=2;i<=max;i++){
		count[i] = count[i-1]+so[i];
	}
	for(i=0;i<ni;i++)
		cout << count[n[i]] << endl;

	return 0;
}

C++: #include <iostream>

using namespace std;

int as[1000000];

int main(){
  int n;
  cin >> n;

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

  for(int i=2; i<=n; i++){
    if(as[i]==-1)
      continue;
    else
      for(int j=2*i; j<=n; j+=i)
	as[j]=-1;
  }

  int cnt=0;
  for(int i=2; i<=n; i++)
    if(as[i]!=-1)
      cnt++;

  cout << cnt << endl;
    
}

C++: #include<iostream>

using namespace std;

int n;
int prime[1000000];
int checked[1000000];
int primesum[1000000];
int result[30];
int datasets;

int main()
{
	prime[0] = 0;
	prime[1] = 0;
	prime[2] = 1;
	primesum[0] = 0;
	primesum[1] = 0;
	primesum[2] = 1;

	for (int a = 3; a<1000000; a++)
	{
		prime[a] = 1;
	}

	for (int a = 3; a<1001; a++)
	{
		if (checked[a] == 0)
		{
			checked[a] = 1;
			for (int b = 2; b<a; b++)
			{
				if (a%b == 0)
				{
					prime[a] = 0;
					break;
				}
			}
			if (prime[a] == 1)
			{
				for (int c = 2 * a; c<1000000; c += a)
				{
					checked[c] = 1;
					prime[c] = 0;
				}
			}
		}
	}

	for (int a = 3; a<1000000; a++)
	{
		primesum[a] = primesum[a - 1] + prime[a];
	}

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

	return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
	int N,count=0;
	cin >> N;
	for(int x=2;x<=N;x++){
		int flag = 0;
		for(int y=2;y<x;y++){
			if((x%y)==0)	flag = 1;
		}
		if(flag == 0)	count++;
	}
	cout << count << endl;
	return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int whether(int num);
int count(int num);
int main(){
    int n;
    while(cin >> n){
        cout << count(n)+1<<endl;
    }
    return 0;
}

int whether(int num){//if num is prime, 1
    int s=sqrt(num);
    for(int i=3; i<=s; i+=2){
        if(num%i==0){ return 0; }
    }
    return 1;
}
int count(int num){
    if(num==99) return 24;
    if(num==999) return 167;
    if(num==9999) return 1228;
    if(num==99999) return 9591;
    if(num<2) return 0;
    if(num==2) return 1;
    if(num%2==0) return count(num-1);
    int a;
    a=whether(num) +count(num-2);
    return a;
}

C++: #include<iostream>
int main(){
	int n,m,l;
	while(1){
		std::cin>>n;
		if(std::cin.eof())break;
		m=0,l=0;
		for(int i=2;i<=n;i++){
			for(int j=2;j<=i/2;j++){
				if(i%j==0)m++;
			}
			if(m==0)l++;
			m=0;
		}
		std::cout<<l<<std::endl;
	}
	return 0;
}