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++: #define _USE_MATH_DEFINES

#include <iostream>
#include <string>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <algorithm>
#include <utility>
#include <vector>
#include <numeric>

using namespace std;

bool is_prime(int n)
{
	if (n == 2)
	{
		return true;
	}
	else if ((n == 1) || (n % 2 == 0))
	{
		return false;
	}

	int p = (int)sqrt(n);
	for (int i = 3; i <= p; i += 2)
	{
		if (n % i == 0)
		{
			return false;
		}
	}

	return true;
}

int main()
{
	vector<int> p;
	for (int i = 1; i < 1000000; i++)
	{
		if (is_prime(i))
		{
			p.push_back(i);
		}
	}

	int n;
	for (; cin >> n;)
	{
		int i;
		for (i = 0; i < p.size(); i++)
		{
			if (p.at(i) > n)
			{
				break;
			}
		}

		cout << i << endl;
	}
}

C++: #include <math.h>
#include <iostream>
#include <ios>
#include <iomanip>
#include <algorithm>
#include <functional>
#include <string>

#define YES "YES"
#define NO "NO"
#define space ' '
#define not true
#define prime false

using namespace std;

void init();
void func();

int main(void)
{
	
	func();

	return 0;
}


#define MAXIMUM 1000000
bool table[MAXIMUM];
int cnt[MAXIMUM];
#define not true
#define prime false

void init()
{
		table[0] = not;
	table[1] = not;
	for (int i = 2; i < MAXIMUM; i++)
	{
		if (table[i] == prime)
		{
			for (int j = i * 2; j < MAXIMUM; j += i)
			{
				table[j] = not;
			}
		}
	}
	for (int i = 1; i < MAXIMUM; i++)
	{
		if (table[i] == prime)
		{
			cnt[i] = cnt[i - 1] + 1;
		}
		else
		{
			cnt[i] = cnt[i - 1];
		}
	}

}

void func()
{
	
	int n;

	init();

	while (cin >> n, !cin.eof())
	{
		cout << cnt[n] << endl;
	}

}

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

int main(){
  int n,m,p[999999];
  bool ip[1000000];
  for(int i=0;i<=1000000;i++) ip[i]=true;
  ip[0]=ip[1]=false;
  for(int i=2;i<=1000000;i++){
    if(ip[i]){
      p[m++]=i;
      for(int j=2*i;j<=1000000;j+=i) ip[j]=false;
    }
  }
  while(cin>>n){
    for(int i=0;i<=m;i++){
      if(p[i]>n){
	cout<<i<<endl;
	break;
      }
    }
  }
  return 0;
}

C++: #include<stdio.h>

int main(void)
{
	int data,i=0,j=2,k=0,l=0,kai[100]={0},s[10000]={0};

	while(scanf("%d",&data) != EOF)
	{
		if(data<=1)
		{
			kai[k]=0;
		}
		else
		{
			for(i=2;i<data;i++)
				s[i]=i;
		
			for(i=2;i*2<data;i++)		
			{
				for(j=2;j*i<data;j++)				
					s[j*i]=0;
			}
			for(i=2;i<data;i++)
			{	
				if(s[i]!=0)
				{
					l+=1;
				}
			}
		}
		kai[k]=l;
		k+=1;
	}

  for(j=0;j<k;j++)
    printf("%d\n",kai[j]);
  return 0;
}

C++: #include <stdio.h>
#include <string>
#include <iostream>

//using namespace std;

int sosuu(int n);

int main(){
    int n = 0;
    int temp = 0;
    while(scanf("%d",&n)==EOF){
        if(n==1){
            printf("%d",0);
        }else{
            for(int i=2; i<=n; i++ ){
                temp+=sosuu(i);
            }
            printf("%d",temp);
        }
    }
    
    
}

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

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

using namespace std; 

long int a,b,loop,hasil;
long int lo;
long int hi;



int main()
{
	while(scanf("%ld",&hi)!= EOF)
	{
	lo=2;
	hasil=0;
		while(lo<=hi)
		{
			for(a=2;a<=lo;a++)
			{
				if(a==lo)
				{
					hasil++;
					break;
				}
				else if(lo%a==0)
				{
					break; 
				}
				else 
				{
				}
			}
			lo=lo+1; 
		}
	printf("%d\n",hasil);
	}
	return 0; 
}

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

int main(){

	const int N_MAX=1000000;
	int i,j,n[32],ni,max;
	int count[N_MAX];
	bool so[N_MAX];
	
	for(i=0;i<N_MAX;i++)
		so[i]=true;
	ni=0;
	max=0;
	while(cin>>n[ni])
		if(n[ni++]>max)
			max=n[ni-1];
	so[0]=so[1]=0;
	for(i=2;i<=1000;i++){
		if(so[i]==0)continue;
		for(j=i*2;j<=max;j+=i)
			so[j]=false;
	}
	for(i=2;i<=max;i++){
		count[i] = count[i-1]+so[i];
	}
	for(i=0;i<ni;i++)
		cout << count[n[i]] << endl;

	return 0;
}

C++: #include <bits/stdc++.h>
#include <iomanip>
#include <math.h>
//#include <unistd.h>
//#include <stdio.h>
#define rep(i,x) for(ll i = 0;i<x;i++)
#define all(a) (a).begin(),(a).end()
using ll = long long;
using ld = long double;
using namespace std;
using dou = double;
const ll INF = 1LL << 60;
const ll mod = 1000000007;
typedef pair<ll,ll> P;
using graph = vector<vector<ll>>;
template<class T> inline bool chmax(T& a, T b){if(a < b){a = b; return true;}return false;}
template<class T> inline bool chmin(T& a, T b){if(a > b){a = b; return true;}return false;}
ll gcd(ll a,ll b){
    if(a < b)swap(a,b);
    if(a % b == 0)return b;
    else return gcd(b,a%b);
}
bool isp(ll n){
    bool res = true;
    if(n == 1)return false;
    else{
        for(ll i = 2;i * i <= n;i++){
            if(n % i == 0){
                res = false;
                break;
            }
        }
        return res;
    }
}
ll fac[510000], finv[510000], inv[510000];

// テーブルを作る前処理
void COMinit() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (ll i = 2; i < 510000; i++){
        fac[i] = fac[i - 1] * i % mod;
        inv[i] = mod - inv[mod%i] * (mod / i) % mod;
        finv[i] = finv[i - 1] * inv[i] % mod;
    }
}


// 二項係数計算
long long COM(int n, int k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % mod) % mod;
}
/*
int main() {
    // 前処理
    COMinit();

    // 計算例
    cout << COM(100000, 50000) << endl;
}*/

//printf("%.10f\n", N);
/*
ll fibo(ll n){
    if(memo[n] != 0)return memo[n];
    if(n <= 1)return n;
    else return memo[n] = fibo(n - 1)+ fibo(n - 2);
}*/
/*  vector<ll> vec = {1,2,5,6,8,9,10};
    cout << binary_search(all(vec),5) << endl; -> true*/
/*
    vector<ll> vec = {1,2,5,6,8,9,10};
    auto n = upper_bound(all(vec),4);
    cout << *n << endl; -> 5*/
/*
    cout << fixed << setprecision(15);
*/

int main(){
    ll N;
    vector<bool> pr(1000001,true);
    pr[1] = false;
    pr[0] = false;
    rep(i,1001){
        if(pr[i]){
            for(ll j = i * 2;j <= 1000000;j += i){
                pr[j] = false;
            }
        }
    }
    vector<ll> num(1000001);
    for(ll i = 1;i <= 1000001;i++){
        if(pr[i])num[i] += num[i - 1] + 1;
        else num[i] += num[i - 1];
    }
   /*rep(i,20){
        cout << i << ":" << num[i] << endl;
    }*/
    while(cin >> N){
        cout << num[N] << endl;
    }
}

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


int main(){
	bool work[MAX];
	int num;

	// 初期化
	for(int i = 0; i < MAX; i++){
		if(i < 2){
			work[i] = false;
		}else{
			work[i] = true;
		}
	}

	// エラトネスのふるい
	for(int i = 2; i < sqrt((double)MAX) + 1; i++){
		if(work[i] == true){
			for(int j = i * 2; j < MAX; j += i){
				work[j] = false;
			}
		}
	}

	// 入力
	while( cin >> num){
		int ans = 0;

		// 計算
		for(int i = 0; i <= num; i++){
			if(work[i] == true){
				ans++;
			}
		}
		// 出力
		cout << ans << endl;

	}
	return 0;
}

C++: #include <stdio.h>

int main(){
	long sosuu[] = { 2, 3, 5 }, i, x, y = 3,n=0,z,d=0;
	while (scanf("%d", &n) != EOF){
		z = 0;
		if(d < n)
		{for (i = 3; i < n; i += 2){
			for (x = 0; x < y; x++){
				if (i % sosuu[x] == 0){ break; }
				if (x + 1 == y){ sosuu[y] = i, y++; d = i; }
			}
		}
	}
		for (i = 0; i < n; i++){ if (sosuu[i] > n + 1)break; z++; }printf("%d\n", z);
	}
	return 0;
}