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 <algorithm>
#include <string>
#include <vector>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <ctime>
#define INF 999999999
#define mod 1000000007

#define rep(i,n) for(int i=0;i<int(n);i++)
#define rrep(i,n) for(int i=(n);i>=0;i--)
#define REP(i,a,b) for(int i=(a);i<int(b);i++)
#define all(x) (x).begin(),x.end()
#define pb push_back
#define MOD(x) (x%(mod))
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef pair<int,int> pi;

bool debug=false;
/*---------------------------------------------------*/

bool prime[1000000];
int ans[1000000];

void is_prime(){
  rep(i,1000000)prime[i]=true;
  prime[0]=prime[1]=false;
  REP(i,2,1000000){
    if(prime[i]==true){
      for(int j=i+i;j<1000000;j+=i)prime[j]=false;
    }
  }
  if(debug)rep(i,1000)if(prime[i])cout<<i<<endl;
}

void calc(){
  ans[0]=0;
  REP(i,1,1000000){
    ans[i]=ans[i-1]+prime[i];
  }
}

int main(){
  int n;
  is_prime();
  calc();
  while(cin>>n){
    cout<<ans[n]<<endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;

#define FOR(i,k,n) for(int i = (k); i < (n); i++)
#define REP(i,n) FOR(i,0,n)
#define INF 114514810
#define ELEM(array) (sizeof (array)/sizeof *(array))
#define MAX_N 1000005
#define SAFE_FREE(ptr) if( ptr != NULL ){ free(ptr); ptr = NULL; }
typedef unsigned int UINT;
#define ll long long
typedef pair<int, int> P;

bool prime[MAX_N];

void solve()
{
	memset(prime, true, sizeof(prime));
	prime[0] = prime[1] = false;
	FOR(i, 2,(int)sqrt(MAX_N))
	{
		if (prime[i] == true)
		{
			for (int j = 2; j*i <= 1000000; j++){
				if (prime[i*j] == true)prime[i*j] = false;
			}
		}
	}
}

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

		int ans = 0;

		solve();
		REP(i, n + 1){
			if (prime[i] == true){
				ans++;
			}
		}
		cout << ans << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<math.h>
using namespace std;
int isPrime(int n) {
	if (n == 2) {
		return 1;
	}
	if ((n <2) || (n % 2 == 0)) {
		return 0;
	}
	int i = 3;
	while (i <= sqrt(n)) {
		if (n % i == 0) {
			return 0;
			
		}
		i += 2;
	}
	return 1;
}
int main()
{
	int x[30];
	int i = 0;
	while (cin >> x[i]) {
		int k = 0;
		for (int j = 0; j < x[i]; j++) {
			
			k += isPrime(j+1);
		}
		cout << k << endl;

		i++;
	}

	return 0;
}

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

C++: 
import java.util.Scanner;
public class Main{
	public static void main(String[] args){
		while(true){
			Scanner sc=new Scanner(System.in);
			int INF=1000;
			int cnt=0;
			int n=sc.nextInt();
			int[] prime=new int[n+1];
			for(int i=0;i<=n;i++){
				prime[i]=INF;
			}
			for(int i=2;i<=n;i++){
				if(prime[i]==INF&&n%i==0){
					prime[i]=0;
					cnt++;
					for(int j=2;j*i<=n;j++){
						prime[j*i]=1;
					}
				}
			}
sc.close();
			System.out.println(cnt);
		}
	}
}

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

int a[1000000];

int main()
{
  int n;
  int count;

  for (int i = 2; i < 1000000; i++){
     a[i] = 1;
   }

   for (int i = 0; i * i < 1000000; i++){
     if  (a[i] == 1){
       for (int j = 2; j * i <= 100000; j++){
         a[j * i] = 0;
       }
     }
   }

   while (cin >> n){
     count = 0;
     for (int i = 0; i < 1000000; i++){
       if (a[i] == 1){
         count++;
       }
     }
   }

   cout << n << endl;

   return (1);
}

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

int arr[1000000];

void ar()
{
	int cnt;
	arr[2] = 1;
	int j;
	int i = 3;
	for (; i < 1000000; ++i) {
		cnt = 1;
		for (j = 2; j < i; ++j) {
			if (i%j == 0) {
				arr[i] = arr[i - 1];
				--cnt;
				break;
			}
		}
		arr[i] = arr[i - 1] + cnt;
	}
}

int main()
{
	int x;
	ar();
	while (cin >> x) {
		cout << arr[x] << endl;
	}
	return 0;
}

C++: #include<stdio.h>
const int size=1000000;
int main(){
  int i,k,n;
  int data[size];
  int sum[size];
  for(i=2;i<size;i++){
    data[i]=1;
  }
  for(i=2;i<size;i++){
    if(data[i]){
      for(k=2*i;k<=size;k+=i){
        data[k]=0;
      }
    }
  }
  for(i=2;i<size;i++){
    sum[i+1]=sum[i]+data[i];
  }
  while(scanf("%d",&n)!=EOF){
    printf("%d\n",sum[n+1]);
  }
}

C++: import math
import sys

def prime_calc(n):
    rootN = math.floor(math.sqrt(n))
    prime = [2]
    data = [i + 1 for i in range(2,n,2)]
    while True:
        p = data[0]
        if rootN < p:
            return len(prime + data)
        prime.append(p)
        data = [e for e in data if e % p != 0]

def main():
    l = []

    for line in sys.stdin:
        l.append(line)        

    for line in l:
        print(prime_calc(line))

if __name__ == "__main__":
    main()

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

int count_prime_number(int);

int main(void){

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

int count_prime_number(int n){ 

    int f,prime_num = 1;

    if (n == 2) {
        prime_num = 1;
    }   
    else{
        for (int j = 3; j <= n; j += 2) {
            f = 0;
            for(int i = 3; i <= std::sqrt(n); i+=2){
                if (j%i == 0) {
                    f = 1;
                    break;
                }
            }   

            if (f == 0) {
                prime_num++;
            }   
        }
    }   

    return prime_num;

}

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

#define MAX 1000000

int main(){
	int i,j,k,n;
	bool arr[MAX];
		i=MAX; while(i--) arr[i]=true;

	while(scanf("%d", &n)!=EOF){

	for(j=2; j<=n; j++){
		if(arr[j]==true){
		for(k=2; j*k<=n; k++){
			arr[j*k]=false;
		}
		}
	}

	for(j=2, k=0; j<=n; j++) if(arr[j]==true) k++;

	printf("%d\n", k);
	}
 return 0;
}