Go: package main

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

func main() {
	scanner := bufio.NewScanner(os.Stdin)

	for scanner.Scan() {
		num, _ := strconv.Atoi(scanner.Text())
		flag := false
		count := 0

		for i := 2; i <= num; i++ { //1を除く入力値以下の数字
			for j := i - 1; j >= 1; j-- {
				if i%j == 0 && j != 1 {
					flag = true
					break
				}
			}
			if !flag {
				count++
			} else {
				flag = false
			}
		}
		fmt.Println(count)
	}
}

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

int main(){
	const int max_num=5000000;
	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;
int main(){
	int n, cnt;
	int N = 1000000;
	bool prime[N] = false;
	prime[1] = true; //1???????????\
	
	while(cin >> n){
		// ??¨??????????????????????????????
		for(int j = 2; j*j <= n; j++){
			if(prime[j] == false){
				for(int k = j; k*j <= n; k++){
					prime[j*k] =true;
				}
			}
		}
		
		for(int i = 1; i <= n; i++){
			if(prime[i] == false) cnt++;
		}
		
		cout << cnt << endl;
	}
}

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

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

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

const int MAXN = 1000005;
int num[MAXN];

int main() {
  fill(num, num+MAXN, 1);
  num[0] = num[1] = 0;
  for(int i = 2; i < MAXN; ++i) {
    if(!num[i]) continue;
    for(int j = i*2; j < MAXN; j += i) num[j] = 0;
  }
  for(int i = 1; i < MAXN; ++i) {
    num[i] += num[i-1];
  }
  int n;
  while(cin >> n) {
    cout << num[n] << endl;
  }
  return 0;
}

C++: #include<iostream>
#include<string>
#include<cmath>
#include<time.h>
using namespace std;

int main()
{
	
	long long int k,p[100000];
	long long int n;
	time_t time1,time2;
	p[0]=2;
cin>>n;
		time(&time1);
		if(n<0){
			break;
		}
	k=0;
	int r=(int)sqrt((float)n);
	for(int i=3;i<=r;i++){
		for(int j=0;j<k+1;j++){
			if(i%p[j]==0){
				break;
			}else if(j==k){
				k++;
				cout<<i<<endl;
				p[k]=i;
			}
		}
	}
	long long int r_k=k;
	for(;r<n;r++){
		for(long int i=0;i<=r_k;i++){
			if(r%p[i]==0){
				break;
			}else if(i==r_k){
				k++;
				cout<<r<<endl;
			}
		}
	}
	time(&time2);
	if(k!=0&&k!=1){
		k+=1;
		cout<<k<<endl;
	}else{
		cout<<k<<endl;
	}
	
}

C++: #define _CRT_SECURE_NO_WARNINGS

#include<fstream>
#include<iostream>
#include<string>
#include<iomanip>
#include<list>
#include<math.h>
#include<stack>
#include<queue>
#include<vector>
#include<algorithm>
#include<utility>

using namespace std;

int data[999999] = { -1 };
bool notprime[999999] = { false };

int main(int argc, char **argv){
	int n;
	int cnt;
	int checkcnt = 0;
	while (cin >> n && checkcnt < 30) {
		cnt = 0;
		int i;
		for (i = 2; i < sqrt(n); i++) {
			if (!notprime[i]) {
				cnt++;
				for (int j = 2; j <= n / i; j++) {
					notprime[i * j] = true;
				}
			}
		}
		for (int k = i; k <= n; k++) {
			if (!notprime[k]) {
				cnt++;
			}
		}
		cout << cnt << endl;
		checkcnt++;
	}
	return 0;
}

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

int main();
int count_prime_number(int n);
std::vector<int> return_prime_number(int n);

//最初に素数リストを作成し、入力された値に対してリストを参照する。
int main(){
	std::vector<int> prime_numbers;		//素数リスト
	int input;
	std::vector<int> answer;

	prime_numbers = return_prime_number(999999);
	//for (int i = 0; i < 3; i++){
	//std::cin >> input;
	while (std::cin >> input){
		//素数の探索
		for (int i = 0; i < input; i++){
			if (input == prime_numbers[i]){
				answer.push_back(i + 1);
				break;
			}
			else if (input < prime_numbers[i]){
				answer.push_back(i);
				break;
			}
			else{
				//何もしない
			}
		}
	}

	for (int i = 0; i < answer.size(); i++){
		std::cout << answer[i] << std::endl;
	}

	//while (1);
	return 0;
}


//入力のたびに素数のチェックをする
/*
int main(){
	int input;
	std::vector<int> answer;
	
	//for (int i = 0; i < 3; i++){
		//std::cin >> input;
	while (std::cin >> input){
		answer.push_back(count_prime_number(input));
	}

	for (int i = 0; i < answer.size(); i++){
		std::cout << answer[i] << std::endl;
	}

	//while (1);
	return 0;
}
*/

/**
* nまでの素数の個数を出力する
* args:数値n
* return:素数の個数
* 参考:http://tsg.ne.jp/TT/tsg/c/mansaku/P01.html
*/
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;
}

/**
* nまでの素数を返す
* args:任意の数値n
* return:素数を格納したvector
*/
std::vector<int> return_prime_number(int n){
	std::vector<int> rtn;

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

	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.push_back(i);

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

	return rtn;
}

C++: #include <iostream>
using namespace std;
bool t[999999] = {false};
int main()
{
	int n, c, i, j;
	t[0] = true;
	for (i = 2; i * i <= 999999; i++)
	{
		if (!t[i-1])
		{
			for (j = i+i; j <= 999999; j+=i)
				t[j-1] = true;
		}
	}
	while (cin >> n)
	{
		if (cin.eof()) break;
		for (i = c = 0; i < n; i++)
			if (!t[i]) c++;
		cout << c << endl;
	}
}