Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	var primeCount int
	for sc.Scan() {
		input := nextInt()
		for i := 0; i <= input; i++ {
			if IsPrime(i) {
				primeCount++
			}
		}
		fmt.Printf("%d\n", primeCount)
		primeCount = 0
	}
}

func IsPrime(num int) bool {
	switch {
	case num == 2:
		return true
	case num < 2 || num%2 == 0:
		return false
	}
	sqrt := math.Sqrt(float64(num))
	for i := 3; i <= int(sqrt); i += 2 {
		if num%i == 0 {
			return false
		}
	}
	return true
}

func next() string {
	return sc.Text()
}

func nextInt() int {
	a, _ := strconv.Atoi(next())
	return a
}

C++: #define _USE_MATH_DEFINES
#define llint long long
#include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;

int pn = 0;
int p[1000000];
void getsosu(int Max) {
    for(int i = 2; i <= Max; i++) {
        bool flg = true;
        for(int j = 0; j < pn; j++) {
            if(p[j] * p[j] > i) break;
            if(i % p[j] == 0) {
                flg = false;
                break;
            }
        }
        if(flg) p[pn++] = i;
    }
}

int main() {
    getsosu(999999);

    int n;
    while(cin >> n) {
        int i = 0;
        while(p[i] <= n) i++;
        cout << i << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
using namespace std;
int main()
{
    vector<int> primes;
    primes.push_back(2);
    int Testcase,Counter;
    while(cin >> Testcase)
    {

        Counter = 0;
        if(Testcase >= 2)
        {
            Counter++;
        }
        for(int i=3; i <= Testcase; i+=2)
        {
            bool prime=true;
            for(int j=0;j<primes.size() && primes[j]*primes[j] <= i;j++)
            {
                if(i % primes[j] == 0)
                {
                    prime=false;
                    break;
                }
            }
            if(prime)
            {
                primes.push_back(i);
                Counter++;
            }

        }
        cout << Counter << endl;
    }

}

C++: #include <iostream>

using namespace std;

int w=0;
int HEY(int i){

	w=0;
	while(1){
		if(w*w>i){break;}
		w++;
	};
	return w;
}


int main(){
	int n=0,m=0,o=0,f=0,c=0;

	while(cin>>n){
		c=0;

	for(o=2;o<=n;o++)
	{	f=0;
		for(m=2;m<HEY(o);m++){
			if(o%m == 0){f++;}

		};
		if(f==0){c++;};

	};
	cout<<c<<endl;
	};
 return 0;

}

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=1e6+10;
int prime[MAX];
bool notprime[MAX];
int num=0;
void init(){
    memset(notprime,0,sizeof(notprime));
    for(int i=2;i*i<=MAX;i++){
        if(!notprime[i]){
            prime[num++]=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 *p=upper_bound(prime,prime+num,n);
        int ans=p-prime;
        cout<<ans<<endl;
    }
    return 0;
}

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

using namespace std;

int main(void){
  const int n = 1000000;
  bool* check_A = (bool*)malloc(sizeof(bool)*n);
  int*  num_A   = (int*)malloc(sizeof(int)*n);
  int N = 0, temp = 2;
  memset(check_A, true, n);
  check_A[0] = false;
  check_A[1] = false;
  num_A[0] = 0;
  num_A[1] = 0;
  for(int i = 0; i < n; i++){
    if(check_A[i]){
      for(;temp < i;temp++){
	num_A[temp] = N;
      }
      temp++;
      num_A[i] = ++N;
      for(int j = 2; i*j < n; j++){
	check_A[i*j] = false;
      }
    }
  }
  int s;
  while(cin >> s){
    cout << num_A[s] << endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
/*{{{*/  //template
#define rep(i,n) for(int i=0;i<n;i++)
constexpr int INF = numeric_limits<int>::max()/2;
constexpr long long LINF = numeric_limits<long long>::max()/3;
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define sz(x) (int)(x).size()
#define debug(x) cerr<<#x<<":"<<x<<endl
#define debug2(x,y) cerr<<#x<<","<<#y":"<<x<<","<<y<<endl
//struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double{ double d; explicit Double(double x) : d(x){} };
ostream& operator<<(ostream& os,const Double x){ os << fixed << setprecision(20) << x.d; return os; }
template<typename T> ostream& operator<<(ostream& os,const vector<T>& vec){ os << "["; for(const auto& v : vec){ os << v << ","; } os << "]"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os,const pair<T,U>& p){ os << "(" << p.first << ","<< p.second <<")"; return os; }
template<typename T> ostream& operator<<(ostream& os,const set<T>& st){ os<<"{"; for(T v:st) os<<v<<","; os <<"}"; return os; }
template<typename T,typename U> inline void chmax(T &x,U y){ if(y>x) x = y; }
template<typename T,typename U> inline void chmin(T &x,U y){ if(y<x) x = y; }
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a,ll b){ if(b==0) return a; else return gcd(b,a%b); }
//constexpr double eps = 1e-14; 
constexpr double eps = 1e-10; 
constexpr ll mod = 1e9+7;
const int dx[]={1,0,-1,0} ,dy[] = {0,1,0,-1};
/*}}}*/

ll N;

constexpr ll max_n = 1000006;
bool is_prime[max_n];

void solve(){
    int cnt=0;
    for(int i=2;i<=N;i++) if(is_prime[i]) cnt++;
    cout << cnt << endl;
}

int main(){
    rep(i,max_n) is_prime[i]=true;

    for(int i=2;i<max_n;i++){
        if(!is_prime[i]) continue;
        for(int j=2;i*j<max_n;j++) is_prime[i*j]=0;
    }
    while(cin>>N){
        solve();
    }
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <string>
#include <map>
#include <bitset>
#include <vector>
#include <queue>

using namespace std;

typedef long long ll;
#define FOR(i,a,b)   for(ll i = (a); i < (b); i++ )
#define REP(i, n)    FOR(i,0,n)
typedef pair< ll, ll >   cp2;
typedef pair< ll, cp2 >   cp3;
#define fi first
#define se second
#define sec  se.fi
#define thr  se.se
const ll mod = 1000000007;
//              123456789

 
///////////////////////////////////////////////
//?????§??¬?´???°??¨????°???¬?????°
//O(1)?????????
///////////////////////////////////////////////

////////////////////////////////////////////////
////////////////////////////////////////////////

#define NUM_MAX   10000000
bool flag[NUM_MAX];
vector< int >prm;

int N;
int ans = 0;
int K;

void eratosthenes(){
	if( N < 2 ) return;
	fill( flag, flag+NUM_MAX, 0 );
	int idx = 2;
	while( idx <= K ){
		while( flag[idx] ) idx++;
		if( idx > N ) break;
		prm.push_back( idx );
		for( int i = idx; i <= N; i += idx ) flag[i] = 1;
		ans++;
	}
	while( idx < N ){
		idx++;
		if( !flag[idx] ){
			prm.push_back( idx );
			ans++;
		}
	}
}


int main(){
	
	while( cin>>N, N != EOF ){
		K = sqrt( N );
		K++;
		
		eratosthenes();
		
		cout<<ans<<endl;
		/*
		REP( i, prm.size() ){
			cout<<prm[i]<<endl;
		}*/
		prm.clear();
		ans = 0;
	}	
	return 0;
	
}

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

int main(){

  int n;
  int a,b,i;
  bool count[1000000];
  count[2] = true;
  for(a=3;a<1000000;a++){
    i = 0;
    if(a%2!=0){
      for(b=3,i=1;b<=a/3;b++){
	if(b%2!=0){
	  if(a%b == 0){
	    i = 0;
	    break;
	  }
	}
      }
    }
    if(i)
      count[a] = true;
    else
      count[a] = false;
  }

  while(cin >> n){
    for(a=2,b=0;a<n;a++)
      if(count[a] == true)
	b++;
    cout << b << endl;
  }
  return 0;
}