Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
#include <numeric>
#include <functional>
#include <set>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cctype>
#include <climits>
#include <fstream>
#include <bitset>
#include <time.h>
#include <assert.h>
#include <random>

#define LL long long
#define VI vector<int>
#define VL vector<long long>
#define FOR(i,a,b) for(int i= (a); i<((int)b); ++i)
#define RFOR(i,a) for(int i=(a); i >= 0; --i)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define DUMP(x)  cerr << #x << " = " << (x) << endl;
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v,x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());
#define EPS 1e-14


using namespace std;

// nまでの素数のリストを作成(nを含む) O(N log long N)
vector<int> make_prime_list(int n) {
    vector<bool> is_prime(n + 1, true);
    is_prime[0] = false;
    is_prime[1] = false;

    vector<int> prime_list;
    for (int i = 2; i < n + 1; ++i) {
        if (!is_prime[i]) { continue; }
        prime_list.emplace_back(i);

        for (int j = i; j < n + 1; j += i) {
            is_prime[j] = false;
        }
    }
    return prime_list;
}

int main(int argc, char *argv[]) {
    cin.tie(0);
    ios::sync_with_stdio(false);

    vector<int> prime_list = make_prime_list(1000000);

    int n;
    while(cin >> n) {
        int ans = 0;
        for(int i=0;;++i)  {
            if(prime_list[i] <= n){
                ans++;
            }
            else {
                break;
            }
        }

        cout << ans << endl;
    }

    return 0;
}

C++: #include <iostream>
#include <vector>
using namespace std;
#define MAX 1000000
int main( void )
{
	vector <bool> prime( MAX+1, false );
	prime[2] = true;
	for ( int i = 3; i <= MAX; i += 2 )
	{
		prime[i] = true;
	}

	for ( int i = 3; i <= MAX; i += 2 )
	{
		if ( prime[i] )
		{
			for ( int j = i + i; j <= MAX; j += i )
			{
				prime[j] = false;
			}
		}
	}

	while ( !cin.eof() )
	{
		int n;
		cin >> n;
		if ( cin.eof() )
			break;

		int cnt = 1;
		for ( int i = 3; i <= n; i += 2 )
		{
			if ( prime[i] )
			{
				cnt++;
			}
		}

		if ( n == 2 )
			cnt = 1;
		else if( n <= 1 )
			cnt = 0;

		cout << cnt << endl;

	}

	return 0;
}

C++: #include<stdio.h>



int main(){
	int n,i,j;
	bool c[100000];
	int data[100000];
	int count;
	int premaxin=1;


	for(i=0;i<100000;i++){
		c[i]=1;
		data[i]=0;
	}
	c[0]=c[1]=0;

	while(scanf("%d",&n)!=EOF){

		if(premaxin<n){
			for(i=0;i<=n;i++){
				if(c[i]==1){
					if(premaxin>i){
						j=premaxin;
					}else{
						j=i+1;
					}
					for(;j<=n;j++){
						if(j%i==0){
							c[j]=0;
						}
					}
				}
			}

			count=data[premaxin];
			for(i=premaxin+1;i<=n;i++){

				if(c[i]==1){
					count++;
				}
				data[i]=count;
			}
		}

		printf("%d\n",data[n]);
	}
	return 0;
}

C++: #include <cctype>
#include <cmath>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define LL long long
#define pb(a) push_back(a)
#define INF 999999999
using namespace std;
typedef pair<int, int> P;
typedef pair<LL, LL> LP;
typedef pair<int, P> PP;
typedef pair<LL, LP> LPP;

int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};

vector<int> vec;

void makesosuu(void){
	FOR(i, 2, 999999){
		if(i == 2){
			vec.pb(i);
		}else if(i % 2 != 0){
			bool flag = true;
			int j = 3;
			while(j * j <= i){
				if(i % j == 0){
					flag = false;
					break;
				}
				j += 2;
			}
			if(flag) vec.pb(i);
		}
	}
	return;
}

int sosuu(int n){
	int i = 0;
	while(vec[i] <= n && i < vec.size()) ++i;

	return i;
}

int main(){
    int n;
    makesosuu();
    while(cin >> n){
        cout << sosuu(n) << endl;   
    }	
}

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

int is_it_primeNum(int n)
{
	int a = 1;
	if (n == 0 || n == 1)
		return 0;
	else if (n == 2)
		return 1;
	else if(n % 2 == 0)
		return 0;
	else{ 
		for(int i=3;i<n/2;i+=2){
			if (n%i==0){
				a = 0;
				break;}
		}
	}
	return a;
}
int how_much_primeNum(int n)
{
	int ans = 0;
	for(int i=2;i<=n;i++){
		ans += is_it_primeNum(i);
	}
	return ans;
}

int main()
{
	int n;
	while(cin>>n)
		cout << how_much_primeNum(n) << endl;
	return 0;
}

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

void sieve(int n, list<int> &primes)
{
    vector<bool> v(n + 1, true);

    // check
    for (int i = 3, end = sqrt(n); i <= end; i += 2) {
        if (!v[i])
            continue;
        for (int j = i * i, skip = i * 2; j <= n; j += skip)
            v[j] = false;
    }

    primes.push_back(2);
    for (int i = 3; i <= n; i += 2)
         if (v[i])
            primes.push_back(i);
}

int main()
{
    list<int> primes;
    vector<int> inputs;

    // read all inputs
    for (int x; cin >> x;)
        inputs.push_back(x);

    // init sieve
    sieve(*max_element(inputs.begin(), inputs.end()), primes);

    for (auto x : inputs) {
        int count = 0;
        for (auto p : primes) // find
            if (p == x) {
                count++;
                break;
            } else if (p > x)
                break;
            else
                count++;
        cout << count << endl;
    }

    return 0;
}

C++: #include<iostream>
using namespace std;
int d[1000000]={},p[1000000]={};
void calc(){
	for(int i=2;i<1000000;i++){
		if(d[i]==0){
			for(int j=2;j<1000000;j++){
				if(i*j>1000000) break;
					d[i*j]=1;
			}
		}
	}
	for(int i=2;i<1000000;i++){
		if(d[i]==0)
		p[i]=p[i-1]+1;
		else
		p[i]=p[i-1];
	}
}
int main(){
	calc();
	int n;
	while(cin>>n,n){
		cout<<p[n]<<endl;;
	}
	return 0;
}