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++: // IncludingFiles
#include<iostream>
#include<vector>
#include<cmath>

// Defines
#define     MAX_NUMBER     (999999)
#define     MIN_NUMBER     (1)
// #define     __DEBUG__

// Using Namespace
using namespace std;

// Function
int main(){
	vector <int> PrimeNumbers;
	vector <int> input;
	int buf;
	int sz,maxn;
	int i,root;

	// 入力
	while(cin >> buf){
		// 入力値のチェック
		if((buf < ((int)(MIN_NUMBER))) || (buf > ((int)(MAX_NUMBER)))){
			cerr << "InputError!!" << endl;
			cerr << "The number has to more than " << (((int)(MIN_NUMBER)) - 1);
			cerr << " and less than " << (((int)(MAX_NUMBER)) + 1) << "." << endl;
		}else{
			// 問題がなければ入力値をリストに追加
			input.push_back(buf);
		}
	}

	// n の最大値を求めて maxn に代入
	sz = (int)input.size();
	maxn = input[0];
	for(i=1;i<sz;i++){
		if(maxn < input[i]){
			maxn = input[i];
		}
	}

	// 素数2 を 素数表PrimeNumbers に追加
	PrimeNumbers.push_back(2);

	// 3 から maxn までの整数が素数かどうかを調べ、素数であれば 素数表PrimeNumbers に追加
	for(i=3;i<=maxn;i+=2){
		// root に i の平方根を代入
		root = (int)sqrt(i);
		// sz に 素数表PrimeNumbers の大きさを代入
		sz = (int)PrimeNumbers.size();
		// i が 素数表PrimeNumbers に含まれている素数で割り切れないかを確認
		for(buf=0;(buf < sz) && (PrimeNumbers[buf] <= root);buf++){
			if((i % PrimeNumbers[buf]) == 0){						// もし割り切れたら
				goto NotPrime;										// i を素数表に追加しない
			}
		}
		// i は素数である ならば i を 素数表PrimeNumbers に追加
		PrimeNumbers.push_back(i);
		// ここにジャンプする
		NotPrime:;
	}

	// n 以下の素数の個数を計測
	sz = (int)input.size();
	for(i=0;i<sz;i++){
		// input[i] == 1  ならば  0 [個]を出力
		if(input[i] == 1){
			cout << 0 << endl;
			continue;
		}

		// a, b および flag の宣言・初期化
		int a = 0,b = (int)PrimeNumbers.size() - 1,m,n;
		bool flag = false;

		// PrimeNumbers の最大値 <= input[i] となっていないかどうかを調べる
		if(PrimeNumbers[b] <= input[i]){
			n = b;
			flag = true;
		}

		// 二分検索
		while (flag == false){
			m = (a + b) / 2;
			if(PrimeNumbers[m] > input[i]){
				b = m;
			}else if((PrimeNumbers[m] <= input[i]) && (PrimeNumbers[m+1] > input[i])){
				n = m;
				flag = true;
			}else if(PrimeNumbers[m] < input[i]){
				a = m;
			}else{
				cerr << "BinarySearchError!!" << endl << endl;
				cerr << "PrimeNumbers[" << m << "] = " << PrimeNumbers[m] << endl;
				cerr << "PrimeNumbers[" << (m + 1) << "] = " << PrimeNumbers[m+1] << endl;
				cerr << "input[" << i << "] = " << input[i] << endl;
				return -1;
			}
		}

		// 出力
		cout << (n + 1) << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <vector>
using namespace std;
int main( void )
{
	int n;
	cin >> n;
	
	vector <bool> p(n+1, true);
	p[0] = p[1] = false;
	for ( int i = 4; i <= n; i += 2 )
	{
		p[i] = false;
	}
	for ( int i = 3; i <= n; i += 2 )
	{
		if ( p[i] )
		{
			cout << i << endl;
			for ( int j = i+i; j <= n; j += i )
			{
				p[j] = false;
			}
		}
	}

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

	cout << kotae << endl;

	return 0;
}

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

bool check_prime_number (int n) {
    if ( n <= 1 ) return false;

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

    return true;
}

int count_prime_number (int n) {
    int count = 0;

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

    return count;
}

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

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

using namespace std;

int A[1000000];
int Eratosthenes(int n)
{
	int mama = 0;
	for (int i = 0; i < n; i++)
	{
		A[i] = 1;
	}
	for (int i = 2; i < sqrt(n); i++)
	{
		if (A[i])
		{
			for (int j = 0; i*(j + 2) < n; j++)
			{
				A[i*(j + 2)] = 0;
			}
		}
	}

	for (int i = 2; i < n; i++)
	{
		if (A[i])
		{
			mama++;
		}
	}
	return mama;
}

int main()
{
	int n;

	while (scanf("%d",&n)!= -1)
	{
		printf("%d\n", Eratosthenes(n));
	}
	return 0;
}

C++: #include<utility>
#include<vector>
#include<cmath>
#include<cstdio>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb push_back
#define mp make_pair
#define it ::iterator
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
const double ESP=1e-10;
using namespace std;
#define MAX 1000000
bool sosu[MAX]={false};
int main(){
  for(int i=3;i<MAX;i+=2){
    if(sosu[i])continue;
    for(int j=i*3;j<MAX;j+=i*2)
      sosu[j]=true;
  }
  for(int i=4;i<MAX;i+=2)
    sosu[i]=true;
  int in;
  while(cin>>in){
    int co=0;
    loop(i,2,in+1)if(!sosu[i])co++;
    cout<<co<<endl;
  }


}

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

int main()
{
	bool is_prime[1000000];
	memset(is_prime,false,sizeof(is_prime));
	for(int i=2;i<1000000;i++){
		if(!is_prime[i]){
//			cout<<i<<endl;
			for(int m=i*2;m<1000000;m+=i){
				is_prime[m]=true;
			}
		}
	}
	int n;
	while(cin>>n){
		int ans=0;
		for(int i=2;i<=n;i++){
			if(!is_prime[i]) ans++;
		}
		cout<<ans<<endl;
	}
}

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

void count_Prime(int n);
int IsPrime(int num);


int main()
{
	int Data = 0;

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

	return 0;
}

void count_Prime(int n)
{
	int counter = 1,  i = 0;
	for (i = 2; i < n; ++i)
		counter += IsPrime(i);		

	cout << counter << endl;

}

int IsPrime(int num)
{
	if (num < 2) return 0;
	else if (num == 2) return 1;
	else if (num % 2 == 0) return 0; // ??¶??°????????????????????????

	double sqrtNum = sqrt(num);
	for (int i = 3; i <= sqrtNum; i += 2)
	{
		if (num % i == 0)
		{
			// ?´???°??§?????????
			return 0;
		}
	}

	// ?´???°??§??????
	return 1;
}

C++: #pragma  warning (disable:4996)
#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<string>
#include<cmath>
#include<algorithm>
#define MAX 1000000
using namespace std;
int amount[MAX];
int vis[MAX];
int prime[MAX];
int main(){
	memset(vis, 0, sizeof(vis));
	memset(prime, 0, sizeof(prime));
	prime[0] = prime[1] = 0;
	
	for (int i = 2; i*i <=MAX; i++){
		if (!vis[i]){
			prime[i] = 1;
			for (int j = i+i; j*j <=MAX; j += i){
				vis[j] = 1;
				prime[j] = 0;
			}
		}
	}
	/*int sum = 0;
	for (int i = 2; i <= MAX; i++){
		if (prime[i]){
			sum++;
		}
		amount[i] = sum;
	}*/
	int n;
	
	while (cin >> n){
		int sum = 0;
		for (int i = 2; i <= n; i++){
			if (prime[i]){
				sum++;
			}
		}
		cout << sum << endl;
	}
	return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <string>
#include <cmath>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <bitset>
#include <functional>
using namespace std;

#define repl(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define rep(i,n) repl(i,0,n)
#define mp(a,b) make_pair((a),(b))
#define pb(a) push_back((a))
#define all(x) (x).begin(),(x).end()
#define dbg(x) cout<<#x"="<<((x))<<endl
#define fi first
#define se second

#define INF 2147483600

#define MAX_PRIME 1000000

vector<int> primes;
vector<bool> isPrime(MAX_PRIME+1, true);

// sieve of Erastosthenes
inline void prime_list(){
  const int sz = MAX_PRIME;
  isPrime[0]=isPrime[1]=false;
  for(int i=4;i<=sz;i+=2) isPrime[i]=false;
  primes.pb(2);
  for(int i=3; 2*i<=sz; i+=2){
    if(isPrime[i]){
      primes.pb(i);
      for(long j=(long)i*i; j<=sz; j+=i) isPrime[j]=false;
    }
  }
}

int main(){
  prime_list();
  int d;
  while(cin>>d){
    cout << upper_bound(all(primes), d) - primes.begin() << endl;
  }

  return 0;
}