Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

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

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

C++: #include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <cassert>
#include <iostream>
#include <cctype>
#include <sstream>
#include <string>
#include <list>
#include <vector>
#include <queue>
#include <set>
#include <stack>
#include <map>
#include <utility>
#include <numeric>
#include <algorithm>
#include <iterator>
#include <bitset>
#include <complex>
#include <fstream>
using namespace std;
typedef long long ll;
const double EPS = 1e-9;
typedef vector<int> vint;
typedef pair<int, int> pint;
#define rep(i, n) REP(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define MSG(a) cout << #a << " " << a << endl;
#define REP(i, x, n) for(int i = x; i < n; i++)
template<class T> T RoundOff(T a){ return int(a+.5-(a<0)); }
template<class T, class C> void chmax(T& a, C b){ if(a < b) a = b; }
template<class T, class C> void chmin(T& a, C b){ if(b < a) a = b; }
template<class T, class C> pair<T, C> mp(T a, C b){ return make_pair(a, b); }

// stl;
const int MAX = 10000;
vector<bool> count()
{
	vector<bool> res(MAX + 1, false);
	
	for(int i = 2; i <= MAX; i++)
	{
		res[i] = true;
		for(int j = 2; j * j <= i; j++)
		{
			if(i % j == 0)
			{
				res[i] = false;
				break;
			}
		}
	}	
	
	return res;
}

int main()
{
	vector<bool> table(count());
	
	int n;
	while(cin >> n)
	{
		cout << count(table.begin(), table.begin() + n + 1, true) << endl;
	}
}

C++: #include <stdio.h>
const int MAX_V = 1000000;
int prime[MAX_V + 1];
int sum[MAX_V + 1] = {0};
int main() {
  int i, k, v, n, max = 0;
  for(i = 2; i <= MAX_V; i++) {
    prime[i] = 1;
  }
  for(i = 2; i*i <= MAX_V; i++) {
    if(prime[i]){
      for(k = 2 * i; k <= MAX_V; k += i) {
        prime[k] = 0;
      }
    }
    sum[i] = prime[i - 1] + sum[i - 1];
  }
  while(scanf("%d", &n) != EOF){
    printf("%d\n", sum[i + 1]);
  }
  return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <iomanip>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <stack>
#include <utility>
#include <numeric>
#include <algorithm>
#include <functional>
#include <cctype>
#include <complex>
#include <string>
#include <sstream>

using namespace std;

#define all(c) c.begin(),c.end()
#define rall(c) c.rbegin(),c.rend()
#define rep(i,n) for(int i=0;i<(n);i++)
#define tr(it,container) for(typeof(container.begin()) it = container.begin(); \
                                                  it != container.end(); ++it)
#define mp(a,b) make_pair((a),(b))
#define eq ==


const int N = 1000010;
bool isPrime[N];

void sieve(){
    for(int i=2;i<N;i++) isPrime[i] = true;
    for(int i=2;i*i<N;i++){
        if(not isPrime[i]) continue;
        for(int j=i*i;j<N;j+=i){
            isPrime[j] = false;
        }
    }
}

int main(){
    vector<int> C(N);
    sieve();
    C[0] = 0;
    for(int i=1;i<N;i++){
        if(isPrime[i]){
            C[i] = C[i-1] + 1;
        }else{
            C[i] = C[i-1];
        }
    }
    int n;
    while(cin >> n){
        cout << C[n] << endl;
    }
    return 0;
}

C++: #include<iostream>

using namespace std;

int limit(int n) {
	int i = 1;
	while (i*i <= n) {
		i += 1;
	}
	return i - 1;
}

int main() {
	int n;
	while (std::cin >> n) {
		int *array = new int[n + 1];

		for (int i = 1; i <= n; i++) {
			array[i] = 0;
		}

		int num_max,count;
		num_max = limit(n);
		count = 0;
		for (int i = 2; i <= num_max; i++) {
			if (array[i] == 0) {
				for (int j = 2 * i; j <= n; j += i) {
					if (array[j] == 0) {
						array[j] = 1;
						count += 1;
					}
				}
			}
		}
		std::cout << n - (count+1) << std::endl;
	}
}

C++: import sys

def prime_calc(n):
    if n < 2:
        return False
    else:
        i = 2
        while n > i:
            if n % i == 0:
                return False
            else:
                i += 1
    return True

def prime(n):
    cnt = 0
    for i in range(0, n+1):
        ans = prime_calc(i)
        if ans is True:
            cnt = cnt + 1
    return cnt

def main():
    a = []

    for line in sys.stdin:
        a.append(int(line))

    for line in l:
        print(prime(line))

if __name__ == "__main__":
    main()

C++: #include <stdio.h>

inline int IsPrime(int n);

int main(void){
    int n, cnt;
    int list[100000];

    cnt=0;
    for (long i=2; i<1000000; i++){
        if (i%10==0) list[i/10] = cnt;
        cnt += IsPrime(i);
    }
	printf("input : ");
    while (scanf("%d", &n) && n!=EOF){
        cnt = list[n/10];
        for (long i=1; i<n%10; i++){
            cnt += IsPrime(i);
        }
        printf("%d\n", cnt);
    }
    return 0;
}

inline int IsPrime(int n){
    int i;

    for (i=2; i*i<=n; i++){
        if (n%i==0) return 0;
    }
    return 1;
}

C++: #include<iostream>
using namespace std;
int main(){
	int n;
	bool array[11];
	while(cin>>n){
		int count=0;
		array[0]=array[1]=false;
		for(int i=1;i<=n;i++){
			if(array[i]){
				count++;
				for(int j=i*2;j<=n;j+=i){
					array[j]=false;
				}
			}
		}
		cout<<count<<endl;
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
main(){
	int n,p[10003];
	for(int i=0;i<=10000;++i)p[i]=1;
	p[0]=0;
	p[1]=0;
	for(int i=2;i<=10000,i*i<10000;++i){
		for(int j=2;j<=10000;++j){
			if(p[i]==0)break;
			if(j%i==0 && i!=j)p[j]=0;
		}
	}
	while(cin>>n){
		int sum=0;
		for(int i=0;i<=n;++i)if(p[i]==1)++sum;
		cout<<sum<<endl;
	}
}

C++: #include <iostream>
using namespace std;
#define n_size 1000000

int main(){
    bool isPrime[n_size];
    isPrime[0]=false; isPrime[1]=false;

    for(unsigned i=2;i<n_size;i++)
        isPrime[i] = true;

    for(unsigned i=2;i*i<n_size;i++){
        if(isPrime[i]){
            for(unsigned j=(i*i);j<n_size;j+=i)
                isPrime[j] = false;
        }
    }

    unsigned n_prime[n_size];

    unsigned cnt = 0;
    for(unsigned i=0;i<n_size;i++){
        if(isPrime[i])
            cnt++;
        n_prime[i] = cnt;
    }

    unsigned n;
    while(cin>>n){
        cout << n_prime[n] << endl;
    }

    return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <math.h>
#pragma warning(disable:4996)

int main()
{
	int DataSet,nCnt,nCnt2,In,Total,Count;

	Total = 0;
	DataSet = 0;

	for (DataSet = 0; DataSet <= 30; DataSet++)
	{
		scanf("%d", &In);
		Total = 1;

		if (In > 2)
		{
			for (nCnt = 3; nCnt <= In; nCnt += 2)
			{
				Count = 0;

				for (nCnt2 = 3; nCnt2 <= sqrt((double)nCnt); nCnt2+=2)
				{
					if (nCnt%nCnt2 == 0)
					{
						if (nCnt != nCnt2)
						{
							Count++;
							break;
						}
					}
				}
				if (Count == 0)
					Total++;
			}
		}

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


	return 0;
}

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

int main(void){
	
	int a[1000001],b[100000],s,t,x;
	int n,N,m;
	
	a[0]=0;
	b[0]=2;
	m=1;
	N=1;
	
	for(s=3;s<1000000;s=s+2){//?????°????´???°??????
		
		for(t=0;t<N;t++){//???????????§????´???°??§??????
			
			if(s%b[t]==0){//????????????????????????
				break;
			}
			
			if(t==N-1||b[t]>=sqrt((double)s)){//???????????§?????£??????
				
				for(;m<s;m++){//????????°????????????
					
					a[m]=N;
				}
				b[N]=s;//??°???????´???°???????´????
				N++;//?´???°????????°????¢???????
				a[s]=N;
				break;
			}
		}
		
	}
	a[1]=0;
	for(;m<1000001;m++){//????????°????????????
		
		a[m]=N;
	}
	/*for(int x=0;x<1001;x++){
		
		printf("%d %d\n",x,a[x]);
	}*/
	
	while(1){
		scanf("%d",&x);
		printf("%d\n",a[x]);
	}
	return 0;
}