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 <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include <algorithm>
#include <cmath>
#include <queue>
#include <set>
#include <map>
#include <vector>
#include <iomanip>
#include <sstream>

using namespace std;

#define ll long long
#define eps 1e-7
#define inf 0x1fffffff
#define maxn 1000010



bool is[maxn];
vector<int>p;



int main(int argc, char *argv[])
{
	ios::sync_with_stdio(false);
	int n;
	memset(is,true,sizeof(is));
	for(int i=2;i<maxn;i+=2)
		is[i] = false;
	is[0] = is[1] = false;
	p.push_back(2);
	for(int i=3;i<maxn;i+=2)
	{
		if(is[i])
		{
			p.push_back(i);
			for(int j=i+i;j<maxn;j+=i)
			{
				is[j] = false;
			}
		}
	}
	while(cin>>n)
	{
		cout<<upper_bound(p.begin(),p.end(),n)-p.begin()<<endl;
	}
	return 0;
}

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

int main() {
  constexpr int max = 1000000;
  vector<bool> isPrime(max, true);
  isPrime[0] = isPrime[1] = 0;
  for (int i = 2; i * i < max; i++) {
    if (isPrime[i]) {
      for (int j = 2; i * j < max; j++) {
        isPrime[i * j] = false;
      }
    }
  }
  int n;
  while (cin >> n) {
    int ans = 0;
    for (int i = 0; i <= n; i++) {
      if (isPrime[i]) ans++;
    }
    cout << ans << endl;
  }
  return 0;
}

C++: #include <iostream>

using namespace std;

int main(){
  int n,visited,cnt=0;
  
  while(1){
    int count =0;
    cnt++;
    if(cnt==30)break;
    cin >> n;
    if(n==0)count =0;
    for(int j=2;j<=n;j++){
      visited=0;
      for(int i = 2;i<=n;i++){
	
	if(j%i==0&&j!=i)visited=1;
	else if(j%i==0&&j==i&&visited==0)count++;
	
      }
    }
    cout << count << endl;
  }
  return 0;
}

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

#define Max 1000000
int Prime[Max] = {};

int main()
{
	//?????????
	for (int i = 2; i < Max; i++)
	{
		Prime[i] = 1;
	}
	//??¨??????????????????????????????
	for (int i = 2; i*i < Max; i++)
	{
		if (Prime[i] == 1)
		{
			for (int j = i * 2; j < Max; j += i)
			{
				Prime[j] = 0;
			}
		}
	}
	//?´???????????¨????
	for (int i = 2; i < Max; i++)
	{
		Prime[i] += Prime[i - 1];
	}

	int n;
	while (cin >> n)
	{
		cout << Prime[n] << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main()
{
	while (1)
	{
		int tmp;

		cin >> tmp;

		vector<int> prime;

		for (int i(2); i <= tmp; ++i)
		{
			for (vector<int>::iterator i2(prime.begin()), n2(prime.end()); i2 != n2; ++i2)
			{
				if ((i % *i2) == 0)
				{
					goto nx;
				}
			}

			prime.push_back(i);
		nx:;
		}

		cout << prime.size() << endl;

		if (cin.eof()) { break; }
	}

	return 0;
}

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

using namespace std;
typedef long long ll;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<int, int> PII;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define MP make_pair
#define PB push_back
#define MOD 1000000007
#define INF (1LL << 30)
#define LLINF (1LL << 60)
#define PI 3.14159265359
#define EPS 1e-12
#define int ll

int prime[1000000];
signed main(void) {
  REP(i, 1000000) prime[i] = 1;
  prime[0] = prime[1] = 0;
  for (int i = 2; i * i <= 1000000; i++) {
  	if (prime[i]) {
  		for (int j = 2 * i; j <= 1000000; j += i) {
  			prime[j] = 0;
  		}
  	}
  }
  //REP(i, 10) cout << prime[i] << " "; cout << endl;
  FOR(i, 1, 1000000) {
    prime[i] += prime[i-1];
    //cout << prime[i] << endl;
  }
  int n;
  while (cin >> n) {
    cout << prime[n] << endl;
  }
  return 0;
}

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

bool primeFlg[1000000];

void makePrime(){
  memset(primeFlg, true, sizeof(primeFlg));
  primeFlg[0] = primeFlg[1] = false;

  for(int i = 2; i * i <= 1000000; i++){
    if(primeFlg[i]){
      for(int j = i + i; j < 1000000; j += i){
        primeFlg[j] = false;
      }
    }
  }
}

int main(){
  int n;

  makePrime();

  while(cin >> n){
    int primeCount = 0;

    for(int i = 0; i <= n; i++){
      primeCount += primeFlg[i];
    }

    cout << primeCount << endl;
  }
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <string>

using namespace std;

int main (int argc, char * const argv[]) {
    // insert code here...
    int n, N=1000000;
	bool p[1000000]={0};

	for(int i=2;i<N;i++){
		if(!p[i]){
			for(int j=i+i;j<N;j+=i){
				p[j]=1;
			}
		}
	}
	
	while(cin >> n){
		int count =0;
		for(int i=2;i<=n;i++){
			if(!p[i])count++;
		}
		cout<<count<<endl;
	}
	
	return 0;
}

C++: #include<iostream>
#include<vector>
#include<algorithm>

using namespace std;

int main(){
  int k, n;
  vector<int> prime;
  vector<int>::iterator i, j;

  while(cin >> n){
    for(k=2; k<=n; ++k)
      prime.push_back(k);
    
    i = prime.begin();
    while(i != prime.end()){
      j = i + 1;
      while(j != prime.end()){
	if((*j)%(*i) == 0)
	  j = prime.erase(j);
	else
	  ++j;
      }
      ++i;
    }
    cout << prime.size() << endl;
    prime.clear();
  }

  return 0;
}

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

int main()
{
	int counter=1,n,flag;
	
	while(cin>>n)
	{
		if(n<=1) 
		{
			cout<<'0'<<endl;
			break;
		}

		for(int i=3;i<=n;i+=2)
		{
			flag=1;
			for(int j=3;j<=sqrt(n);j+=2)
				if(i%j==0)
				{
					flag=0;
					break;
				}
			if(flag==1)
				counter++;
		}
		cout<<counter<<endl;
	}
}

C++: #include<bits/stdc++.h>
using namespace std;
 
#define maxPN 1000000
#define PNs  1000100
int countPN = 0, countPNs = 0;
long long PrimeNum[maxPN], temp;
bool PN[PNs];
 
bool IsPrimeNumber(long long 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(long long 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(long long i = 0; PrimeNum[i] * PrimeNum[i] <= x; i++)
        if(x%PrimeNum[i] == 0)return false;
    return true;
}
 
 
int main(){
    clock_t start, end;
    long long 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;