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 <sstream>
#include <fstream>

#include <ctime>
#include <cstdlib>
#include <cstring>
#include <cctype>

#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>

#include <cmath>
#include <algorithm>
#include <functional>
#include <numeric>

using namespace std;

template<typename T> ostream& operator<<(ostream& os, const vector<T>& v){
    os << "{ ";
    for(typename vector<T>::const_iterator it=v.begin(); it!=v.end(); ++it)
        os << '\"' << *it << '\"' << (it+1==v.end() ? "" : ", ");
    os << " }";
    return os;
}

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<bool> vb;
typedef vector<vb> vvb;
#define vv(type,w,h,init) vector<vector<type>>(h,vector<type>(w,init))
typedef vector<string> vs;
typedef long long ll;
typedef unsigned uint;
typedef unsigned long ul;
typedef unsigned long long ull;

#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define loop(i,a,b) for(int i=(a); i<(int)(b); i++)
#define rep(i,b) loop(i,0,b)
#define pb push_back
#define mp make_pair

string const resstr[] = {"Impossible","Possible","No","Yes"};

bool p[1000100];

void erato(){
	fill(p, p+1000100, true);
	p[0] = p[1] = false;
	
	for(int i=0; i < 1000100; i++){
		if(!p[i]) continue;
		for(int j=i+i; j < 1000100; j+=i){
			p[j] = false;
		}
	}
}

int main(){
	int n;
	erato();
	while(cin >> n){
		int count = 0;
		for(int i=0; i <= n; i++){
			if(p[i]) count++;
		}
		cout << count << endl;
	}
}

C++: #include<stdio.h>
#include<math.h>
int isprime(int x){
	int i;
	i = 3;
	while( i <= sqrt(x) ){
		if(x%i==0){
			return 0;
		}
		i = i + 2;
	}
	return 1;
}
int main(){
	int n;
	while(scanf("%d",&n)!=EOF){
		int c=1;
		for(int i=3;i<=n;i+=2){
			if(isprime(i)){
				c++;
			}
		}
		printf("%d\n",c);
	}
	return 0;
}

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

const int MAX = 1000000;
int cnt[MAX];
bool isCmp[MAX] = {true};


int main() {
    ios::sync_with_stdio(false);
    int n;
    cnt[1] = 0;
    for(int i = 2; i < MAX; i++){
      if(! isCmp[i]){
        for(int j = 2; i*j < MAX; j++){
          isCmp[i*j] = true;
        }
      }
      cnt[i] = cnt[i-1] + !isCmp[i];
    }

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

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

bool x[1000000];

void pri(int num){
	for(int i = 0; i <= num; i++) cout <<"x["<<i<<"] = "<<x[i]<<endl;
}

void setbool(int num){
	x[0] = false; x[1] = false;
	for(int i = 2; i <= 1000000; i++) x[i] = true;
	for(int i = (num + 1); i <= 1000000; i++) x[i] = false;
}

void loop(int num){
	for(int i = 2; i <= num; i++){
		if(x[i] == false) continue;
		else{
			int j = i;
			while(j < num){
				j++;
				if(x[j] == true && j % i == 0) x[j] = false;
			}
		}
	}
}

int ans(){
	int cou = 0;
	for(int i = 0; i <= 1000000; i++) if(x[i] == true) cou++;
	return cou;
}

int main(){
	int num, cou = 0, i;
	
	while(cin >>num){
		setbool(num);
		loop(num);
		cout <<ans()<<endl;
	}
	
	return 0;
}

C++: #include <iostream>
using namespace std;
int main(){
    int a[10000];
    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<iostream>
#include<cstdio>

using namespace std;

const int MAX_N = 1000000;

int prime[MAX_N];
bool is_prime[MAX_N];

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]){
			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) > 0){
		printf("%d\n",sieve(n));
	}
}

C++: /*
Á·Ï°Ìâ£ºPrime Number_AOJ 0009 
*/

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

const int MAX_N=1000007;

//°£ÊÏÉ¸·¨(Sieve of Eratosthenes)
int prime[MAX_N]; //µÚi¸öËØÊý£¬´ËÎª´æ·ÅËØÊýµÄÊý×é 
bool is_prime[MAX_N]; //is_prime[]ÎªtrueÊ±ÎªËØÊý

//·µ»ØnÒÔÄÚËØÊýµÄ¸öÊý
int sieveErato(int n)
{
	int p=0;
	for(int i=0; i<=n; i++)
		is_prime[i]=true;
	is_prime[0]=false;
	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(scanf("%d", &n)!=EOF)
	{
		printf("%d\n", sieveErato(n));
	}
	return 0;
}

C++: #include "bits/stdc++.h"
#include <cassert>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define all(a)  (a).begin(),(a).end()
#define vi vector<int>
#define pb push_back
#define INF 999999999
//#define INF (1LL<<59)

int isPrime[1000000];


void era(){
    for(int i=2;i<1000000;i++){
        if( isPrime[i]==1 ){
            int k = 2;
            while(k*i<1000000){
                isPrime[k*i]=0;
                k++;
            }
        }
    }
}


int main(){
    for(int i=0;i<1000000;i++){
        isPrime[i]=1;
    }
    isPrime[0]=isPrime[1]=0;
    
    era();
    
    int n;
    while(cin>>n){
        int count=0;
        for(int i=0;i<=n;i++){
            if( isPrime[i] )count++;
        }
        cout<<count<<endl;
    }
    
    
    
}

C++: #include <iostream>
#include <math.h>
using namespace std;
int main() {
	unsigned long long int i;
	int n, p, cnt,fg;
	while (cin >> n) {
		i = 3;
		cnt = 1;
		fg = 0;	
		
		if ((int)log10(n) == 3) {
			cnt = 168;
			i = 1001;
		}
		else if ((int)log10(n) == 4) {
			cnt = 1229;
			i = 10001;
		}
		else if ((int)log10(n) == 5) {
			cnt = 9592;
			i = 100001;
		}
		if (110000<n) {
			cnt = 10453;
			i = 110001;
		}
		if (120000 < n) {
			cnt = 11301;
			i = 120001;
		}
		if (130000 < n) {
			cnt = 12159;
			i = 130001;
		}
		if (140000<n) {
			cnt = 13010;
			i = 140001;
		}
		if (150000<n) {
			cnt = 13848;
			i = 150001;
		}
		if (160000<n) {
			cnt = 14683;
			i = 160001;
		}
		if (170000<n) {
			cnt = 15493;
			i = 170001;
		}
		if (180000<n) {
			cnt = 16342;
			i = 180001;
		}
		if (190000<n) {
			cnt = 17170;
			i = 190001;
		}



		for (; i <= n; i+=2) {
			for (int j = 3; j <= (int)sqrt(i); j += 2) {
				if (i%j == 0) {
					fg = 1;
					break;
				}
			}
			if (fg)
				fg = 0;
			else
				cnt++;
		}
		if (n == 1)
			cnt--;
		cout << cnt << endl;
	}

	return 0;
}

C++: import math
import sys

def prime_calc(n):
    rootN = int(n**0.5)
    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<stdio.h>
int flg[1000000];
int main(void)
{

	int a,i,j,cnt;
	while(scanf("%d",&a)!=EOF){	
		for(i=2;i<=a;i++){
		flg[i]=1;
		}
		for(i=2;i*i<=a;i++){
			if(flg[i]=1){
				j=2;
				while(i*j<=a){
				flg[i*j]=0;
//				printf("i=%d j=%d a=%d\n",i,j,a);
				j++;
				}
			}
		}
//		for(i=2;i<=10;i++)	printf("flg=%d\n",flg[i]);
		cnt=0;
		for(i=2;i<=a;i++){
//		printf("i=%d\n",i);
			if(flg[i]==1){
//			printf("i=%d\n",i);
			
			cnt++;
			}
		}
		printf("%d\n",cnt);
	}	
	return 0;
}