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>
using namespace std;

int main(){
	const int max_num=3000000;
	int prime[max_num];
	int prime2[max_num];
	int min_prime[168]={2 ,3 ,5 ,7 ,11 ,13 ,17 ,19 ,23 ,29 ,31 ,37 ,41 ,43 ,47 ,53 ,59 ,61 ,67 ,71 ,73 ,79 ,83 ,89 ,97 ,101 ,103 ,107 ,109 ,113 ,127 ,131 ,137 ,139 ,149 ,151 ,157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233 ,239 ,241 ,251 ,257 ,263 ,269 ,271 ,277 ,281 ,283 ,293 ,307 ,311 ,313 ,317 ,331 ,337 ,347 ,349 ,353 ,359 ,367 ,373 ,379 ,383 ,389 ,397 ,401 ,409 ,419 ,421 ,431 ,433 ,439 ,443 ,449 ,457 ,461 ,463 ,467 ,479 ,487 ,491 ,499 ,503 ,509 ,521 ,523 ,541 ,547 ,557 ,563 ,569 ,571 ,577 ,587 ,593 ,599 ,601 ,607 ,613 ,617 ,619 ,631 ,641 ,643 ,647 ,653 ,659 ,661 ,673 ,677 ,683 ,691 ,701 ,709 ,719 ,727 ,733 ,739 ,743 ,751 ,757 ,761 ,769 ,773 ,787 ,797 ,809 ,811 ,821 ,823 ,827 ,829 ,839 ,853 ,857 ,859 ,863 ,877 ,881 ,883 ,887 ,907 ,911 ,919 ,929 ,937 ,941 ,947 ,953 ,967 ,971 ,977 ,983 ,991 ,997 };
	int sosuu=0;
	int k=168;

	for(int i=0;i<168;i++){
		prime[i]=min_prime[i];
	}
	for(int i=1001;i<max_num;i+=2){
		sosuu=0;
		for(int j=0;j<168;j++){
			if(i%min_prime[j]==0){
				sosuu++;
			}
		}
		if(sosuu==0){
			prime[k]=i;
			k++;
		}
	}
	int l=0;
	for(int i=0;i<max_num;i++){
		if(i>=prime[l]){
			l++;
		}
		prime2[i]=l;
	}

	int n;

	while(cin>>n){
		cout<<prime2[n]<<endl;
	}

	return 0;
}

C++: #include<iostream>
using namespace std;
#define REP(i,n) for(unsigned int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define MAX 1000001
 
bool N[MAX] = {};
 
int M[MAX] = {};
 
void SoE()
{
    REP(i, MAX)N[i] = true;
 
    N[1] = true;
    for (unsigned int i = 2; i*i <= MAX; i++)
    {
        for (unsigned int j = i+i; j < MAX; j += i)
        {
            N[j] = false;
        }
    }
}
 
void Count()
{
    M[1] = 0;
    FOR(i, 2, MAX - 1)
    {
        int add = 0;
        if (N[i] == true)
        {
            add = 1;
        }
        M[i] = M[i - 1] + add;
    }
}
 
int main()
{
    SoE();
    Count();
    int tmp;
    while (cin >> tmp)
    {
        cout << M[tmp] << endl;
    }
    return 0;
}

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

int count(int n);

int main(void){
	while (true){
		int n; cin >> n;
		if (!n){
			break;
		}
		cout << count(n) << endl;
	}
	return 0;
}

int count(int n){
	int ct = 0;
	bool *prime = new bool[n];
	//bool prime[999999];
	for (int i = 0; i <= n;i++){
		prime[i] = 0;
	}
	prime[2] = 1;
	for (int i = 3; i <= n; i = i + 2){
		prime[i] = 1;
	}
	for (int i = 3; i <= sqrt(n * 1.0); i = i + 2){
		if (prime[i] == 0) continue;
		for (int j = i * 2; j <= n; j = j + i){
			prime[j] = 0;
		}
	}
	for (int i = 3; i <= n; i = i + 2){
		if (prime[i] == 1) ct++;
	}
	ct++; //2を数えていない分
	delete[] prime;
	return ct;
}

C++: #include <iostream>

int PrimeNumberCount(int*, int);

const unsigned int MAX_NATURAL_NUMBER = 1000000;

int main(){
	int BoolPrimeNumber[MAX_NATURAL_NUMBER] = { 1, 1 };
	int NaturalNumber;
	for (int i = 2; i * i < MAX_NATURAL_NUMBER; ++i){
		if (!BoolPrimeNumber[i]){
			for (int j = i * 2; j < MAX_NATURAL_NUMBER; j += i){
				BoolPrimeNumber[j] = 1;
			}
		}
	}
	while (std::cin >> NaturalNumber){
		std::cout << PrimeNumberCount(BoolPrimeNumber, NaturalNumber) << std::endl;
	}
}

int PrimeNumberCount(int* BoolPrimeNumber, int NaturalNumber){
	int count = 0;
	for (int i = 2; i < NaturalNumber + 1; ++i){
		if (BoolPrimeNumber[i] == 0){
			++count;
		}
	}
	return count;
}

C++: #include <stdio.h>
#define false 0
#define true 1
 
int main(){
  char is_prime[1000000];
  int n;
  int n_prime[1000000];
 
  is_prime[0]=is_prime[1]=false;
  for(int i=2;i<1000000;i++){
    is_prime[i] = true;
  }
 
  for(int i=2;i*i<1000000;i++){
    if(is_prime[i]){
      for(int j=i*i;j<1000000;j=j+i){
        is_prime[j] = false;
      }
    }
  }
  int cnt = 0;
  for(int i=0;i<1000000;i++){
    if(is_prime[i]){
      cnt++;
    }
    n_prime[i] = cnt;
  }
 
  while(scanf("%d",&n) != EOF){
    printf("%d\n",n_prime[n]);
  }
 
  return 0;
}

C++: #include <stdio.h>

int main(){
	int sosuu[100000] = { 2, 3, 5 }, i, x, y = 3,n,z;
	
	while (scanf("%d", &n)!=EOF){
		z = 0; 
		for (i = 3; i < n + 1; i += 2){
			for (x = 0; x < y; x++){
				if (i % sosuu[x] == 0){ break; }
				if (x + 1 == y){ sosuu[y] = i, y++;}
			}
		}
		for (i = 0; i < y; i++){ if (sosuu[i] > n)break; z++; }
		printf("%d\n", z);
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <math.h>
#include <string>
#include <cmath>

int main(void);
int count_prime_number(int n);

int main(){
	int n;
	int cnt = 0;
	int result[30];

	//while (cnt < 3){
		//std::cin >> n;
	while (std::cin >> n){
		//result[cnt] = count_prime_number(n);
		
		int rtn = 0;

		//2だけ個別チェック
		if (n >= 2){
			rtn++;
		}

		for (int i = 3; i <= n; i += 2)
		{
			bool is_prime_num = true;
			for (int j = 3; j <= sqrt(i); j += 2)		//ルートi以上のiの約数はルートi以下のiの約数と対になっているので範囲はルートiまででいい
			{
				if (i % j == 0)
				{
					is_prime_num = false;
					break;
				}
			}

			if (is_prime_num){
				rtn++;

				//デバッグ
				//std::cout << i << std::endl;
			}
		}

		result[cnt] = rtn;

		cnt++;
	}

	for (int i = 0; i < cnt; i++){
		std::cout << result[i] << std::endl;
	}

	//while (1);
	return 0;
}


//0..nまでの素数の個数を出力する
//args:数値
//return:素数の個数
int count_prime_number(int n){
	int rtn = 0;

	//2だけ個別チェック
	if (n >= 2){
		rtn++;
	}

	for (int i = 3; i <= n; i+= 2)
	{
		bool is_prime_num = true;
		for (int j = 3; j <= sqrt(i); j+= 2)		//ルートi以上のiの約数はルートi以下のiの約数と対になっているので範囲はルートiまででいい
		{
			if (i % j == 0)
			{
				is_prime_num = false;
				break;
			}
		}

		if (is_prime_num){
			rtn++;

			//デバッグ
			//std::cout << i << std::endl;
		}
	}

	return rtn;
}

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

/*
10
3
11

4
2
5
*/

const int MAX_N = 1e6 + 10;
int N = 0;

bool is_prime[MAX_N + 1];
void sieve(int n) {
    fill(is_prime, is_prime + n + 1, true);
    is_prime[0] = is_prime[1] = false;
    for (int i = 2; i * i <= n; i++) {
        if (is_prime[i]) {
            for (int j = 2 * i; j <= n; j += i) {
                is_prime[j] = false;
            }
        }
    }
}

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

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

int main() {
    sieve(MAX_N);
    while (scanf("%d", &N) != EOF) {
        solve();
    }

    return 0;
}