Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

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

int main(){
	int n;
	 
	bool prim[1000001];
	for(int i=0;i<1000001;i++) prim[i] = true;
	prim[1]=false;
	for(int i=3;i<1000001;i++){
		if(i%2==0) prim[i] = false;
		else if(prim[i]){
			for(int j=i*2;j<=1000001;j+=i) prim[j] = false;
		}
	}
	

	while(cin >> n){
		int ans=0;
		for(int i=1;i<=n;i++){
			if(prim[i]) ans++;
		}
		cout << ans << endl;
	}
}

C++: #include<iostream>
#include<cmath>
#include<cstring>
using namespace std;
int a[1000100];
int jud(int n){
	int i;
	for (i=2;i<=sqrt(n);i++){
		if (n%i==0)
		return 0;
	}
	return 1;
}
int main(){
	int n;
	while (cin>>n){
		int i,j,res=0;
		memset(a,0,1000000*sizeof(int));
		for (i=2;i<=n;i++)
		if (!a[i]){
			if (jud[i]){
			res++;
			if (i<1000){
			for (j=i*i;j<1000000;j+=i){
				a[j]=1;
			}
			}
			}
		}
		cout<<res<<endl;
	}
	return 0;
}

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

int main()
{
	int n = 0, x = 0;
	int countdata[1000];
	while (cin >> n) {
		if (n / 1000000 > 0) {
			int count = 0;
			for (int a = 2; a <= n; a++) {
				if (a % 2 != 0 && a % 3 != 0 && a % 5 != 0 && a % 7 != 0 || a == 2 || a == 3 || a == 5 || a == 7) {
					count++;
				}
			}
			countdata[x] = count;
			x++;
		}
	}
	for (int a = 0; a < x; a++) {
		cout << countdata[a] << endl;
	}
}

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

int main(){
 int i,j,n,count,flg;
 scanf("%d",&n);
 for(i=2,count=0;i<=n;i++){
  for(j=2,flg=1;j<=i/2;j++)
   if(i%j==0){ printf("%d\n",i); flg=0; break; }
  count+=flg;
 }
 printf("%d\n",count);
 return 0;
}

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

#define MAX_NUM (1000000)

static int getPrimeNum(int maxVal);

int main(void)
{
  int n;
  
  while (std::cin >> n) {
    std::cout << getPrimeNum(n) << std::endl;
  }
  
  return 0;
}

static int getPrimeNum(int maxVal)
{
  static int searchList[MAX_NUM] = {0};
  static int primeList[MAX_NUM] = {0};
  
  for (int i = 0; i < MAX_NUM; i++) {
    searchList[i] = primeList[i] = 0;
  }
  
  double sqMaxVal = sqrt((double)maxVal) + 1;
  
  for (int i = 2; i <= sqMaxVal; i++) {
    if (searchList[i] == 0) {
      searchList[i] = 1;
      primeList[i] = 1;
      for (int j = i + 1; j <= maxVal; j++) {
        if (j % i == 0) {
          searchList[j] = 1;
        }
      }
    }
  }
  
  for (int i = 2; i <= maxVal; i++) {
    if (searchList[i] == 0) {
      primeList[i] = 1;
    }
  }
  
  int count = 0;
  for (int i = 2; i <= maxVal; i++) {
    if (primeList[i] == 1) {
      count++;
    }
  }
  
  return count;
}

C++: #include <iostream>
using namespace std;
#define x 1000000
int main(){
	bool q[x];
	for(int i=2;i<x;i++) q[i] = true;
	for(int i=2;i<x;i++) if(q[i]) for(int j=2;j<=x/i;j++) q[i*j] = false;
	int n;
	while(cin >> n){
		int a = 0;
		for(int i=2;i<x;i++) if(q[i]) a++;
		cout << a << endl;
	}
}

C++: #include <iostream>
using namespace std;
int main(){
    int a[100000];
    int n;
    a[0]=2;
    int k=1;
while(cin>>n){
    for(int i=3;i<=n;i++){
        for(int j=0;j<=k;j++){
            if(j==k){
                a[k]=i;
                k++;
                break;
             }
            if(i%a[j]==0)break;
        }
    }
    cout<<k<<endl;
    k=1;
 }
}

C++: #include <algorithm>
#include <cstdio>
#include <cstring>

using namespace std;
int N;
int primes[1000000];

void prime() {
    fill(primes, primes + 1000000, 1);
    for (int i = 2; i <= 999999; i++) {
        if (primes[i]) {
            for (int j = i + i; j <= 999999; j += i)
                primes[j] = 0;
        }
    }
}

int main() {
    prime();
    while (~scanf("%d", &N)) {
        int ans = 0;
        for (int i = 2; i <= N; ++i)
            ans += primes[i];
        printf("%d\n", ans);
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

const int maxn = 1e7+10;

int prime[maxn];
bool is_prime[maxn];

int sieve(const int& n){
	int p = 0;
	fill(is_prime,is_prime+n+1,true);
	is_prime[0] = is_prime[1] = false;

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

	return p;
}

int main(){
	int n;
	while(cin>>n){
		cout << sieve(n) << endl;
	}
}

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

long mygcd(long a, long b)
{
    if (b == 0) return a;
    if (a < b) return mygcd(b,a);
    return mygcd(a-b,b);
}

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

    while(cin >> num)
    {
        length = 0;
        int count = 0;
        for (int n = 2; n <= num; n++)
        {
            // judge whether n is prime or not
            bool isPrime = true;
            for(int i=0; i < length-1; i++)
            {
                if (!(n % prime[i]))
                {
                    isPrime = false;
                    break;
                }
            }
            // regist n as prime number
            if (isPrime)
            {
                prime[length] = n;
                length++;
            }
        }
        cout << length << endl;
    }
    return 0;
}

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

const int N = 1000009;
int dp[N];
int isprime[N];

int main()
{
    dp[1] = 0;
    for(int i=2;i<N;i++)
    {
        if(!isprime[i])
            for(int j=2*i;j<N;j+=i)
                isprime[j] = 1;
        dp[i] = dp[i-1] + isprime[i];
    }

    int y;
    while(~scanf("%d",&y))
        printf("%d\n",dp[y]);
    return 0;
}