Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	var primeCount int
	for sc.Scan() {
		input := nextInt()
		for i := 0; i <= input; i++ {
			if IsPrime(i) {
				primeCount++
			}
		}
		fmt.Printf("%d\n", primeCount)
		primeCount = 0
	}
}

func IsPrime(num int) bool {
	switch {
	case num == 2 || num == 5:
		return true
	case num < 2 || num%2 == 0 || num%5 == 0:
		return false
	}
	sqrt := math.Sqrt(float64(num))
	for i := 3; i <= int(sqrt); i += 2 {
		if num%i == 0 {
			return false
		}
	}
	return true
}

func next() string {
	return sc.Text()
}

func nextInt() int {
	a, _ := strconv.Atoi(next())
	return a
}

C++: #include <bits/stdc++.h>

using namespace std;

#define all(x) x.begin(), x.end()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define reps(i, m, n) for (int i = m; i <= n; ++i)

using i64 = long long;
using pii = pair<i64, i64>;

template<class A, class B>inline bool chmax(A &a, const B &b){return b > a ? a = b,1 : 0;}
template<class A, class B>inline bool chmin(A &a, const B &b){return b < a ? a = b,1 : 0;}

constexpr int INF = 0x3f3f3f3f;
constexpr i64 LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr int MOD = int(1e9) + 7;

vector<bool> isPrime;

void sieve(int MAX) {
  if (MAX + 1 > isPrime.size()) isPrime.resize(MAX + 1, true);
  isPrime[0] = false;
  isPrime[1] = false;
  for (int i = 2; i * i <= MAX; ++i) {
    if (isPrime[i]) {
      for (int j = 2; i * j <= MAX; ++j) isPrime[i*j] = false;
    }
  }
}

signed main()
{
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  sieve(10000010);

  vector<int> cum(10000015);
  for (int i = 1; i <= 10000010; ++i) {
    cum[i] = cum[i-1];
    if (isPrime[i]) ++cum[i];
  }

  i64 n;
  while (cin >> n) {
    cout << cum[n] << "\n";
  }

  return 0;
}

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

int prime(int data){
	int i;
	for(i=3;i<=sqrt(data);i+=2){
		if(data%i==0){
			return 0;
		}
	}
	return 1;
}

int main(void){
	int list[1000000]={0,1};
	int i,data,cnt;
	for(i = 3;i <= 999999;i++){
		if(i%2==0){
			list[i-1]=0;
		}else{
			list[i-1] = prime(i);
		}
	}
	while(std::cin >> data){
		cnt = 0;
		for(i=0;i<data;i++){
			if(list[i] == 1){
				cnt++;
			}
		}
		printf("%d\n",cnt);
	}
	return 0;
}

C++: #include<stdio.h>
int main(void)
{
    int n,i,j,z,x;
	
    scanf("%d",&n);
	for(n!=1){
		for(i=2;i<=n;i++){
			z=0;
			for(j=2;j<=i;j++){
				if(i%j==0 && i!=j){
					z++;
					break;
				}
			}
			if(z==0){
				x++;
			}
		}
		printf("%d\n",x);
		scanf("%d",&n);
	}
	return 0;
}

C++: #include <stdio.h>

int main(){
	long sosuu[200000] = { 2 }, i, x, y = 1, z = 0;


	while(scanf("%d", &z)!=EOF){//入力した数以下の素数を表示

		for (i = 3; i < z + 1; i += 2){
			for (x = 0; x < y; x++){
				if (i % sosuu[x] == 0){ break; }
				if (x + 1 == y){ sosuu[y] = i, y++; }
			}
		}
		if (z < 2){ y = 0; }
	printf("%d\n", y);//最後に素数の合計数を表示

	for (i = 1; i < y; i++){
		sosuu[i] = 0;
	}y = 1; z = 0;
	}

	return 0;
}

C++: #include <iostream>
using namespace std;
 
bool IsPrime(int n){
    for(int i=2;i*i<=n;i++){
        if(n%i == 0){return false;}
    }
    return true;
}
 
int main(void) {
	int n;
 while(cin >> n){	
    int count = 1;
    
    for(int i=3;i<=n;i=i+2){
        if(IsPrime(i)) count++;
    }
    cout << count << endl;
}
    return 0;
}

C++: #include<iostream>

int main(){
  int prime[20000];
  int k,n;
  prime[0]=2;
  while(std::cin >> n){
    if(n==1){
      std::cout << 0 << std::endl;
      continue;
    }
    k=0;
    for(int i=3;i<=n;i++){
      for(int j=0;j<=k;j++){
        if(i%prime[j]==0){
          break;
        }
        if(j==k){
          ++k;
          prime[k]=i;
        }
      }
    }
    std::cout << k+1 << std::endl;
  }
}

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;
}