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 <bits/stdc++.h>

const static double	de_PI	= 3.14159265358979323846;
const static int	de_MOD	= 1000000007;
const static int	de_MAX	= 999999999;
const static int	de_MIN = -999999999;

bool Judge_PrimeNumber(const int a) {

	if (a < 2) { return false; }
	if (a == 2) { return true; }
	if (a % 2 == 0) { return false; }

	for (int i = 3; i <= a / i; i += 2) {
		if (a%i == 0) {
			return false;
		}
	}
	return true;
}

int main(void) {

	//std::ifstream in("123.txt");	std::cin.rdbuf(in.rdbuf());

	int n = 0, ans = 0;
	std::vector<int> A;

	while (std::cin >> n) {
		A.push_back(n);
	}

	for (unsigned int i = 0; i < A.size(); i++) {
		ans = 0;
		for (int j = 2; j <= A[i]; j++) {
			if (Judge_PrimeNumber(j)) {
				ans++;
			}
		}
		std::cout << ans << std::endl;
	}

}

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

int main(){

	const int N_MAX=100000;
	int i,j,n[32],ni,soi,max;
	int so[N_MAX],count[N_MAX];
	
	for(i=0;i<N_MAX;i++)
		count[i]=so[i]=0;
	ni=0;
	max=0;
	while(cin>>n[ni])
		if(n[ni++]>max)
			max=n[ni-1];
	int sq_max = (int)sqrt(max);
	so[0]=so[1]=1;
	for(soi=0,i=2;i<=sq_max;i++){
		if(so[i]>0)continue;
		for(j=i*2;j<=max;j+=i)
			so[j]=1;
	}
	for(i=2;i<=max;i++){
		count[i] = count[i-1]+1-so[i];
	}
	for(i=0;i<ni;i++)
		cout << count[n[i]] << endl;

	return 0;
}

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

void count_Prime(int n);
int Euclid(int n, int m);

int main()
{
	int Data;

	while (cin >> Data)
	{
		count_Prime(Data);
	}

	return 0;
}

void count_Prime(int n)
{
	int counter = 0;
	for (int i = 2; i <= n; i++)
	{
		for (int j = 2; j <= i; j++)
		{
			int euclid = Euclid(j, i);
			// ?????§??¬?´???°???1(??????????´?)?????????????????¨????????¨??????
			if (euclid == i)
				counter++;     // ????????????????????§??????
			else if ( euclid != 1)               // ?????????????????¨??????Break????¨????????????\
				break;
		}
	}

	cout << counter << endl;

}

int Euclid(int n, int m)
{
	if (m % n == 0)
	{
		return n;
	}
	else
	{
		return Euclid(m%n, n);
	}
}

C++: #include <iostream>
#include <vector>
using namespace std;
int main(){
	vector<int> array;
	int n;
	bool f;
	cin >> n;
	for(int i=2;i<=n;i++){
		f=true;
		for(int j=0;j<array.size();j++)
		if(!(i%array[j])){
		f=false;
		continue;
	}
		if(f==true)array.push_back(i);
	}
	cout << array.size() << endl;
	return 0;
}

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

int is_prime(int n){
    int i;
    int flag = 0;
    for(i=2; i<n; i++){
        if(n%i==0) { flag=1; break; }
    }
    return (flag==1)?0:1;
}


int main(){
	int i, j, n, num = 0, count = 0;

	while(scanf("%d", &n) != EOF){
		count = 0;
		for(i = 1; i <= n; i++){
			if(is_prime(i) == 1 && i != 1) count ++;
		}
		cout << count << endl;
	}

	return 0;
}

C++: //
//  main.cpp
//  CppTest
//
//  Created by Ryu on 2017/02/08.
//  Copyright ?? 2017??´ Ryu. All rights reserved.
//

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cmath>
using namespace std;

int main(int argc, const char * argv[]) {
    // insert code here...
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    
    long in[30];
    long out[30];
    int n = 0;
    while(cin >> in[n]){
        
        long p[1000];
        //int length = sizeof(p) / sizeof(*p);
        int m = 0;
        //printf("length: %d", length);
        for (int i = 2; i <= in[n]; i++) {
            bool prime = true;
            for(int j = 0; j < m; j++){
                if (i % p[j] == 0) {
                    prime = false;
                }
            }
            if (prime) {
                p[m] = i;
                //printf("%ld\n", p[m]);
                m++;
            }
        }
        
        int s = 0;
        while (in[n] >= p[s]) {
            s++;
        }
        out[n] = s;
        n++;
    }
    
    for (int i = 0; i < n; i++) {
        printf("%ld\n", out[i]);
    }
    
    return 0;
}

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

bool Prime(int n);

int main(){

  int n;
  int sum;

  while(scanf("%d",&n)!=0){
    sum = 0;
    for(int i=3; i<=n; i+=2){
      sum += Prime(i);
    } 
    cout << sum+1 << endl;
  }

  return 0;
}


bool Prime(int n){

  if(n%2==0)
    return false;

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

  return true;
}

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

#define YES "YES"
#define NO "NO"
#define space ' '

using namespace std;

void init();
void func();

int main(void)
{
	
	func();

	return 0;
}


#define MAXIMUM 1000000
bool table[MAXIMUM];
int cnt[MAXIMUM];
bool non = true;
bool prime = false;

void init()
{
		table[0] = non;
	table[1] = non;
	for (int i = 2; i < MAXIMUM; i++)
	{
		if (table[i] == prime)
		{
			for (int j = i * 2; j < MAXIMUM; j += i)
			{
				table[j] = non;
			}
		}
	}
	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 <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b) - 1;i>=(a);i--)
#define REP(i,n) for(int i=0;i<(n);i++)
#define RREP(i,n) for(int i=n-1;i>=0;i--)

#define PB push_back
#define INF INT_MAX/3
#define ALL(a) (a).begin(),(a).end()
#define CLR(a) memset(a,0,sizeof(a))

typedef long long int ll;

using namespace std;

int main(){
    vector<int> primes;
    primes.PB(2);
    FOR(i,3,100000){
	bool flg = true;
	REP(j,primes.size()) if(i % primes[j] == 0 ) {
	    flg = false;
	    break;
	}
	if(flg) primes.PB(i);
    }

    int n;
    while(true){
	cin >> n;
	if(cin.eof())break;
	int res = upper_bound(ALL(primes),n) - primes.begin();
	cout << res << endl;
    }

    return 0;
}

C++: #include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<iostream>
#include<string>
using namespace std;



bool primeJudge(int num) {
	if (num == 1)return false;
	if (num == 2)return true;

	for (int i = 2; i * i <= num; i++) {
		if (num % i == 0)return false;
	}
	return true;
}

int main() {
	
	int n;
	int count;
	while (cin >> n) {
		count = 0;
		for (int i = 1; i <= n; i++) {
			if (primeJudge(i)) {
				count++;
			}
		}

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

	}

	//system("pause");
	return 0;
}