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>

using namespace std;

int HowManyPrimeNumbers1(const int);
int HowManyPrimeNumbers2(const int);

int main(){
	int n;

	while(cin >> n){
		printf("%d\n",HowManyPrimeNumbers1(n));
	}

	return 0;
}

int HowManyPrimeNumbers1(const int n){
	int i,j,buf,cnt = 1;

	for(i=3;i<=n;i+=2){
		buf = (int)sqrt((double)i);

		for(j=3;j<=buf;j+=2){
			if((i % j) == 0){
				cnt--;
				break;
			}
		}

		cnt++;
	}

	return cnt;
}

int HowManyPrimeNumbers2(const int n){
	return n;
}

C++: #include <iostream>
using namespace std;
int main(){
	int n , i , j;
	while(cin >> n ){
		int counter = 0;
		for( int i=1 ; i<=n ; i++ ){
			int temple=0;
			for( int j=1 ; j<=n ; j++ ){
				if( i%j == 0 )
					temple++;
			}
			if( temple == 2 )
				counter ++;
		}
		cout << counter << endl;
	}
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <iomanip>
#include <string>
#include <set>
#include <functional>

class Mod{
	const int n;
	public:
		Mod(const int _n):n(_n){}
		bool operator()(const int _src){
			return ! (_src % n);
		}
};

const std::vector<int> Eratosthenes(std::vector<int>& src){
	std::vector<int> result;
	int cnt(0);
	while(!src.empty()){
		int n(src[0]);
		result.push_back(n);
		Mod mod(n);
		std::vector<int>::iterator it= std::remove_if( src.begin(), src.end(), mod );
		src.erase(it,src.end());
		if(cnt<10 || cnt%10==0){
			src.swap(src);
		}
	}
	return result;
}
static int const MAX(1000000);

int main(int,char**){
	std::vector<int> vec;
	int max_2(MAX-MAX/2-MAX/3-MAX/5+MAX/6+MAX/10+MAX/15-MAX/30+3);
	vec.reserve(max_2);
	for(int i(3);i<=MAX;i+2){
		if(i!=3 && i%3==0) continue;
		if(i!=5 && i%5==0) continue;
		vec.push_back(i);
	}
	std::vector<int> primes;
	primes.reserve(10000);
	primes =Eratosthenes(vec);
	int n;
	while(std::cin >>n){
		std::vector<int>::const_iterator const it= std::upper_bound(primes.begin(),primes.end(),n);
		int off=it -primes.begin();
		if(n>=2){ ++off; }
		std::cout << off <<std::endl;
	}
	return 0;
}

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(i%*p==0){
				isprime=false;
				break;
			}
		}
		if(isprime){
			prime.push_back(i);
		}
	}
}
int main(){
	int n;
	init();
	cerr<<"init"<<endl;
	while(cin>>n){
		cout<<lower_bound(prime.begin(),prime.end(),n)-prime.begin()+1<<endl;
	}
	return 0;
}

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

#define maxPN 1000000
#define PNs  1000100
int countPN = 0, countPNs = 0;
int PrimeNum[maxPN], temp;
bool PN[PNs];

bool IsPrimeNumber(int x);

void PrimeN(){
	int i, j;
	PrimeNum[0] = 2;
	PN[2] = true;PrimeNum[countPN++] = 2;
	PN[3] = true;PrimeNum[countPN++] = 3;
//	PN[5] = true;PrimeNum[countPN++] = 5;
	
	for(i = 6; i  < PNs;i+=6){
		if(IsPrimeNumber(i-1)){
			PrimeNum[countPN++] = i-1;
			PN[i-1] = true;//cout<<i-1<<endl;
		}
		if(IsPrimeNumber(i+1)){
			PrimeNum[countPN++] = i+1;
			PN[i+1] = true;//cout<<i+1<<endl;
		}
		countPNs = i+1;//cout<<i-1<<" "<<PN[i-1]<<endl;
	}
}

bool IsPrimeNumber(int x){
	//if(x == 2 || x == 3) return true;
	//if(x <= 4 || x%2 == 0 || x%3 == 0)   return false;
	//if(x < countPNs) return PN[x];
	for(int i = 0; PrimeNum[i] * PrimeNum[i] <= x; i++)
		if(x%PrimeNum[i] == 0)return false;
	return true;
}


int main(){
//	clock_t start, end;
	int n = true, counta;
//	start = clock();
	PrimeN();
	cout<<PrimeNum[countPN-1]<<" "<<countPNs<<endl;
//	cout<<clock()-start<<"ms"<<endl;
	
	while(cin>>n){
		counta = 1;
		
		
		//cout<<(IsPrimeNumber(n)?"yes":"no")<<endl;
	//	start = clock();
		//cout<<IsPrimeNumber(n)<<endl;
		for(int i = 1; i <= n; i+=2){
			if(PN[i])counta++;
		//	cout<<i<<" "<<PN[i]<<endl;
		}
		if(n == 1) counta = 0;
		cout<<counta<<endl;
	//	cout<<clock()-start<<"ms"<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <math.h>
#include <iomanip>
#include <vector>
#include <map>

int main() {
	int a;
	while (std::cin >> a) {
		int nSum = 0;
		for (int i = 2;i <= a;i++) {
			int flag = 0;
		for (int j = 2;j < i;j++) {
			if (i%j == 0) {
				flag = 1;
				break;
			}
		}
		if (flag == 0) {
			nSum += 1;
		}
	}
		std::cout << nSum << std::endl;
	}
	return 0;
}

C++: #include <stdio.h>
#include <string.h>
int main(void){
    int n=999999,a,b,c;
    int x[1000000];
    memset(x,0,sizeof(x));
    for(a=2,c=0;a<=n;a++){
        if(x[a]){x[a]=c;continue;}
        x[a]=++c;
        for(b=a*2;b<=n;b+=a)x[b]=-1;
    }
    while(~scanf("%d",&n)){
        printf("%d\n",x[n]);
    }
    return 0;
}

C++: #include <sstream>
#include <string>
#include <vector>
#include <map>
#include <algorithm>
#include <iostream>
#include <utility>
#include <set>
#include <cctype>
#include <queue>
#include <stack>
#include <cstdio>
#include <cstdlib>
#include <cmath>
using namespace std;
#define NUM 10000000
bool is_prime[NUM];
int dp[NUM];
int main(void) {
    for (int i = 0; i < NUM; i++) {
        is_prime[i] = true;
        dp[i] = 0;
    }
    is_prime[0] = is_prime[1] = false;
    for (int i = 2; i * i < NUM; i++) {
        if (is_prime[i]) {
            for (int j = 2; i * j < NUM; j++) {
                is_prime[i*j] = false;
            }
        }
    }
    for (int i = 1; i < NUM; i++) {
        if (is_prime[i]) dp[i] = dp[i-1] + 1;
        else dp[i] = dp[i-1];
    }
    long long n;
    while (cin >> n) {
        cout << dp[n] << endl;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define repb(i, a, b) for(int i = a; i >= b; i--)
#define all(a) a.begin(), a.end()
#define int long long
using namespace std;
typedef pair<int, int> P;

int prime[1000010];
bool isPrime[1000010];
int sum[1000010];
void sieve(int n){
    rep(i, 0, n) isPrime[i] = true;
    int now = 0;
    isPrime[0] = isPrime[1] = false;
    rep(i, 2, n){
        if(isPrime[i]){
            prime[now++] = i;
            for(int j = 2 * i; j < n; j += i) isPrime[j] = false;
        }
    }
    return;
}

signed main(){
    sieve(1000000);
    rep(i, 1, 1000000){
        sum[i] += sum[i - 1];
        if(isPrime[i]) sum[i]++;
    }
    int n;
    while(cin >> n){
        cout << sum[n] << endl;
    }
}

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

int main(){

	int n,count=0;
	while(cin>>n){

		for(int i=2; i<=n; i++){
			int num = 0;
			for(int j=1; j<=i ;j++){
				if(i%j==0) num++;
			}
			if(num==2){
				count++;
			}
		}
		cout << count << endl;
	}
    return 0;
}

C++: #include <iostream>
using namespace std;
const int MAX = 1000000;

int main(){
	bool b[MAX] = {true};
	static int a[MAX] = {0,0};
	int n;
	for(int i = 2;i < MAX;i++){
		if(!b[i]){
			for(int j = 2;i*j < MAX;j++){
				b[i*j] = true;
			}
		}
		a[i] = a[i-1] + !b[i];
	}
	while(cin >> n){
		cout << a[n] << endl;
	}
	return 0;
}

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

int main(){
    vector<int> isprime;
    int n;
    while (cin >> n){
	for (int i = 0; i <= n; i++) isprime.push_back(1);
	
	int prime = 0;
	for (int i = 2; i <= n; i++){
	    if (isprime[i]){
		prime++;
		for (int j = 2; i * j <= n; j++) isprime[i*j] = 0;
	    }
	}
	cout << prime << endl;
	isprime.clear();
    }
    return 0;
}