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 <cstdio>
#include <cstdlib>
#include <cstdint>
#include <cstring>
#include <cctype>
#include <iostream>
#include <sstream>
#include <vector>
#include <list>
#include <deque>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>
#include <functional>
using namespace std;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long long ll;
const double PI = 3.141592653589793238462643383279502884L;
const int LARGE_INT = 1000000007;
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define tr(c, i) for(auto i = (c).begin(); i != (c).end(); i++)
#define rtr(c, i) for(auto i = (c).rbegin(); i != (c).rend(); i++)
#define repfn(i,a,cont,next) \
  for(auto i = (a); [=](){return(cont);}(); i = [=](){return(next);}())
#define repby(i,a,b,s) repfn(i,a,i<b,i+s)
#define repab(i,a,b) repby(i,a,b,1)
#define rep(i,b) repab(i,0,b)
#define pb push_back
#define sz(c) int((c).size())

int check(vector<bool> &v, int from, int to)
{
  int i;
  for (i = from + 1; i*i <= to && i*i < sz(v); i++) {
    if (v[i]) {
      for (int j = 2; i*j < sz(v); j++) {
        v[i*j] = false;
      }
    }
  }
  return i - 1;
}

int count(vector<bool> &v, int to)
{
  int res = 0;
  repab (i, 2, to+1) {
    if (i >= sz(v))
      break;
    if (v[i])
      res++;
  }
  return res;
}

int main(int argc, char **argv)
{
  const int prime_max=1000000;
  vector<bool> v(prime_max, true);
  int done;

  int n;
  done = 1;
  while (cin >> n) {
    done = check(v, done, n);
    cout << count(v, n) << endl;
  }
  return 0;
}

C++: #include <iostream>
#include <string>
#include <stack>
#include <math.h>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <vector>

using namespace std;
int sieve(int n);
int prime[1000000];
bool is_prime[1000000];

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

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;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
 
vector<bool> GetPrimeTable(int n)
{
	vector<bool> IsPrimeNumber(n+1, true);
	for(int i = 2; i <= n; i++){
		while(IsPrimeNumber[i] == false){
			i++;
		}
		for(int j = 2; i*j <= n; j++){
			IsPrimeNumber[i*j] = false;
		}
	}
	return IsPrimeNumber;
}

int GetPrimeCount(vector<bool>& IsPrimeNumber, int n)
{
	int Count = 0;
	for(int i = 2; i <= n; i++){
		if(IsPrimeNumber[i] == true){
			Count++;
		}
	}
	return Count;
}

int main()
{
	vector<int> Input;
	while(true){
		int Nubmer;
		cin >> Nubmer;
		if(cin.eof() == true) break;
		Input.push_back(Nubmer);
	}
	int MaxNumber = *std::max_element(Input.begin(), Input.end());
	vector<bool> IsPrimeNumber = GetPrimeTable(MaxNumber);
	for(int i = 0; i < Input.size(); i++){
		cout << GetPrimeCount(IsPrimeNumber, Input[i]) << endl;
	}
	return 0;
}

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

typedef int TInt;
typedef std::vector<TInt> TList;
bool IsPrime(TInt Value, const TList& Prime)
{
    TInt Max = std::sqrt(Value);
    for(TList::const_iterator It = Prime.begin();
        It != Prime.end() && *It <= Max;
        ++It)
    {
        if(Value % (*It) == 0) return false;
    }
    return true;
}
TList GetPrimeList(TInt Max)
{
    TList Prime;
    Prime.reserve(Max);
    Prime.push_back(2);
    for(TInt i = 3; i <= Max; i += 2){
        if(IsPrime(i, Prime)){
            Prime.push_back(i);
        }
    }
    return Prime;
}
int main()
{
    TList List = GetPrimeList(999999);
    while(true){
        int n;
        std::cin >> n;
        if(std::cin.eof()) break;
        TList::iterator It = std::upper_bound(List.begin(), List.end(), n);
        std::cout << std::distance(List.begin(), It) << std::endl;
        
    }
    return 0;
}

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

using namespace std;

inline void era(vector<bool> &prime, int num);

using namespace std;

int main(void){
  int num;

  while(cin >> num){
    vector<bool> prime((num+1), true);
    prime[0] = false;
    prime[1] = false;
    era(prime, num);
    cout << count(prime.begin(), prime.end(), true) << endl;
  }
  return 0;
}

void era(vector<bool> &prime, int num){
  int N = 2;
  while (N <= num){
    int count = 2;
    while ((count*N) <= num){
      prime[count*N] = false;
      ++count;
    }
    ++N;
  }
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
#include <stack>
#include <fstream>
#include <vector>
#include <map>
#include <set>
#include <cmath>
#include <algorithm>
#include <functional>
#define inf 0x3f3f3f3f
#define inff -30000000
using namespace std;
typedef long long ll;
const int MAXN=1e5+10;
const int MAX=1000000+10;
bool notprime[MAX];
int num=0;
void init(){
    memset(notprime,0,sizeof(notprime));
    for(int i=2;i*i<=MAX;i++){
        if(!notprime[i]){
            for(int j=i*i;j<=MAX;j+=i)
                notprime[j]=1;
        }
    }
}
int main(){
    #ifdef SIYU
    freopen("in.txt","r",stdin);
    #endif // SIYU
    int n;
    init();
    while(cin>>n){
        int ans=0;
        for(int i=2;i<=n;i++){
            if(!notprime[i])
                ans++;
        }
        cout<<ans<<endl;
    }
    return 0;
}

C++: #include <iostream>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <algorithm>
#include <functional>

using namespace std;

int gcd( int, int );

bool isPrime( int n )
{
	for ( int i = 2; i <= sqrt( n ); i++ )
	{
		if ( !( n % i ) )
		{
			return false;
		}
	}

	return true;
}

int main()
{
	int n, count = 0;

	while ( cin >> n )
	{
		for ( int i = 2; i <= n; i++ )
		{
			if ( isPrime( i ) )
			{
				count++;
			}
		}

		printf("%d\n", count );
	}

	return 0;
}

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

using namespace std;

int main(){
	int n,i,j,buf,cnt;

	while(cin >> n){
		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++;
		}

		cout << cnt << endl;
	}

	return 0;
}

C++: #include <iostream>

using namespace std;

int main()
{
	int N[3]; int C[3] = { 0, 0, 0 };

	int PRIME[168];

	int cnt = 1;

	PRIME[0] = 2;

	for (int i = 3; i <= 999; i++)
	{
		for (int j = 2; j <= (i - 1); j++)
		{
			if (i % j == 0)
			{
				goto Exit1;
			}
		}
		
		PRIME[cnt] = i; cnt++;

	Exit1:;
	}

	cin >> N[0] >> N[1] >> N[2];

	for (int i = 0; i < 3; i++)
	{
		for (int j = 2; j <= N[i]; j++)
		{
			for (int k = 0; k <= 167; k++)
			{
				if (j % PRIME[k] == 0 && j != PRIME[k])
				{
					goto Exit2;
				}
			}

			C[i]++;

		Exit2:;
		}
	}

	cout << C[0] << endl << C[1] << endl << C[2] << endl;

	return 0;
}