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 <string>
#include <array>
&#160;
int main() {
&#160;&#160;&#160;&#160;std::array<bool, 1000000> is_prime_table = { false, false, true };
&#160;&#160;&#160;&#160;for( int i=3; i<=is_prime_table.size(); i+=2 ) {
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;is_prime_table[i] = [&i]() -> bool
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;{
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;for( int j=3; j*j<=i; j+=2 )
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;if ( i % j == 0 )
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;return false;
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;return true;
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;}();
&#160;&#160;&#160;&#160;}
&#160;&#160;&#160;&#160;&#160;
&#160;&#160;&#160;&#160;int num = 0;
&#160;&#160;&#160;&#160;while( std::cin >> num ) {
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;int result = num > 1 ? 1 : 0;&#160;&#160;&#160; // 2
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;for( int i=3; i<=num; i+=2 )
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;if ( is_prime_table[i] )
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;++result;
&#160;
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;std::cout << result << std::endl;
&#160;&#160;&#160;&#160;}
}

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

using namespace std;

long* primes(long high)
{
  long* p = new long[high];
  for (int i = 0; i < high; i++) {
    p[i] = 0;
  }

  long max = ceil(sqrt(high));
  for (int i = 2; i <= max; i++) {
    if (p[i - 1] == -1) continue;

    int count = 2;
    while (i * count <= high) {
      p[i * count - 1] = -1;
      count++;
    }
  }

  for (int i = 1; i < high; i++) {
    if (p[i] == -1) {
      p[i] = p[i - 1];
    } else {
      p[i] = p[i - 1] + 1;
    }
  }
  
  return p;
}

int main(int argc, char const* argv[])
{
  long* p = primes(999999);
  long n;

  while (cin >> n) {
    cout << p[n - 1] << endl;
  }
  return 0;
}

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

C++: #include<iostream>
#include<cmath>
#include<stdio.h>
using namespace std;
int main()
{int n;
 while(scanf("%d",&n))
 {int sum=1;
  int i;
  for(int m=3;m<=n;m++)
  {if(m%2==0)continue;
	  int k=int (sqrt(double(m)));
	  i=2;
   while(m%i&&i<=k)
	   i++;
   if(i>k)sum++;
  }
  printf("%d\n",sum);
 }
}

C++: #include <iostream>

using namespace std;

int main() {
	int n;
	while (cin >> n) {
		int ans = 0;
		n += 1;
		bool *p = new bool[n];
		fill(p, p + n, true);
		p[0] = p[1] = false;
		for (int i = 2; i * i < n; i++)
			if (p[i] == true)
				for (int j = i * i; j < n; j += i)
					p[j] = false;
		for (int i = 0; i < n; i++)
			if (p[i] == true)
				ans++;
		cout << ans << endl;
		delete[] p;
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
using namespace std;
int main(){
	vector<int> array;
	int n;
	bool f;
	cin >> n;
	for(int i=2;i<=n;i++){
		f=true;
		for(int j=0;j<array.size();j++)
		if(!(i%array[j])){
		f=false;
		continue;
	}
		if(f==true)array.push_back(i);
	}
	cout << array.size();
	return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int prime(int);
int main(){
    int a,sum;
    int b[999999];
    b[0]=0;
    for(int i=1;i<999999;i++){
        b[i]=prime(i+1);
    }
    while ( cin >> a){
        sum =0;
        for (int i=1;i<a;i++){
            sum +=b[i];
        }
        cout << sum << endl;
    }
}
int prime(int x){
    if(x==1)
        return 0;
    if(x==2 || x==3 || x==5 || x==7)
        return 1;
    if(x%2==0 || x%3==0)
        return 0;
    for (int i=5;i<=sqrt(x)+1;i+=6){
        if(x%i==0 || x%(i+2)==0)
            return 0;
    }
    return 1;
}

C++: #include<iostream>
#include<cstdio>
using namespace std;
int prime[1000000];
bool is_prime[100001];

int sieve(int n){
  int p=0;
  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]==true){
      prime[p++]=i;
      for(int j=2*i;j<=n;j+=i)is_prime[j]=false;
    }
  }
  return p;
}

int main(){
  int n;
  while(scanf("%d",&n)!=EOF)
    cout<<sieve(n)<<endl;
  return 0;
}

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

int prime[1000000] = {1};

int main()
{
  int n;
  prime[0] = prime[1] = 0;
  for (int i = 2; i < 1000000; i++){
    if (prime[i] == 1){
      for (int j = 2 * i; j < 1000000; j += i){
        prime[j] = 0;
      }
    }
  }
  for (int i = 0; i < 1000000; i++){
    prime[i + i] += prime[i];
  }
  while (cin >> n){
    cout << prime[n];
  }

  return 0;
}

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

int main(){
  int n;

  while(cin >> n){
    int cnt = 0;
    for(int i = 1; i <= n; i++){

      if(i % 2 == 0|| i % 3 == 0 || i % 5 == 0) continue;

      int yakusuu = 0;

      for(int j = 1; i <= i; j++){
        if(i % j == 0) yakusuu++;
      }

      if(yakusuu==2) cnt++;

    }
    cout << cnt + 3 - 1 << endl;
  }
  return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
	int a[999999]={0,0,1,2},n,k=0,count=2;
	for(int i=4;i<999999;i++){
		for(int j=2;j*j<=i;j++){
			if(i%j==0){k++;break;}
		}
		if(k==1)count++;
		a[i]=count;
		k=0;
	}
	while(cin>>n)cout<<a[n]<<endl;
	return 0;
}

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

int main(){

    while(true){
	int n;
	cin >> n;
	if(cin.eof()) break;

	int res = 0;
	bool flg = true;
	for(int i=2;i<=n;i++){
	    flg = true;
	    for(int j=2;j*j<=i;j++){
		if(i % j == 0){
                    flg = false;
                    break;
            }
	    }
	    if (flg) res++;
	}
	cout << res << endl;
    }

    return 0;
}

C++: #include<stdio.h>
#define MAX 1000010

int is_prime[MAX];

void sieve(){
  int i,j;
  for(i=0;i<=MAX;i++)is_prime[i]=1;
  is_prime[0]=is_prime[1]=0;
  for(i=2;i<=MAX;i++){
    if(is_prime[i]){
      for(j=2*i;j<=MAX;j+=i)is_prime[j]=0;
    }
  }
}

int main(){
  int a,i;
  int sum[MAX];
  sieve();
  for(i=0;i<MAX;i++)sum[i]=0;
  for(i=1;i<MAX;i++)sum[i]=sum[i-1]+is_prime[i];
  while(scanf("%d",&a)!=EOF){
    printf("%d\n",sum[a]);
  }
  return 0;
}

C++: #include<stdio.h>
#include<stdlib.h>
#include<iostream>
#include<string.h>
#include<math.h>
#include<cstring>
using namespace std;
#define NUM 1000000
int main(){
	long int n;
	int p[NUM],i,j,k,a,b;
	for(i=0;i<NUM;i++){
		p[i]=0;
	}
	for(i=2;i<NUM;i++){
		if(p[i]==0){
			p[i]=2;
			for(j=2;j<=(NUM/i);j++){
				p[i*j]=1;
			}
		}	
	}
	while(cin>>n){
		a=0;
		for(i=2;i<=n;i++){
			if(p[i]==2)a++;
		}
		cout<<a<<endl;
	}

    return 0;
}