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>
#include <cmath>
#include <cstdio>
#include <time.h>     // for clock()
using namespace std;

/*
1???2 ?????? n ?????§?????´??°???????????????
2??????????????£???????????°????????§???????°?????????????????????? p ??¨?????????????????????????????????????????°?????? p ??¨?????????
p ??\?????? p ????????°?????¨????¶????????????????
3???2??????????????°?????????????????????p ??? ???n ????¶??????????????????????????????????????????£?????????????´???°???
*/
// ??¨????????????????????????
int SieveOfEratosthenes(int num) {
	
	if (num < 2) {
		return 0;
	}
	// num???????´???°?????°
	int count = 0;
	bool fl[1000001];
	memset(fl, true, sizeof(fl));
	for (int i = 2; i <= sqrt(num); i++) {
		if (fl[i]) {
			for (int j = 2; j*i <= num; j++) {
				fl[i*j] = false;
			}
		}
	}
	for (int i = 2; i <= num; i++) {
		if (fl[i]) {
			count++;
		}
	}
	return count;
}

int main() {

	int num = 0;
	while (cin >> num) {
		//clock_t start = clock();    // ??????????????????
		cout << SieveOfEratosthenes(num) << endl;
		//clock_t end = clock();     // ????????????
		//std::cout << "duration = " << (double)(end - start) / CLOCKS_PER_SEC << "sec.\n";
	}
	return 0;
}

C++: #include <stdio.h>
  int main(){
   int i,j,n,hantei=0;
   int sum=0;
    while(scanf("%d",&n) != EOF){
    for(i=2;i<=n;i++){
      for(j=2;j*j<=i;j++){
        if(i%j != 0 || i == j){
            hantei += 0;
               }else{
                  hantei += 1;
                    }
}
if(hantei == 0){
sum++;
}
hantei=0;
}
printf("%d\n",sum);
sum=0;
hantei = 0;
}
return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <cmath>


using namespace std;
#define MAX_NUM 999999


//int do_count(bool *isPrime, int n);

int main(){
  int n;
  bool flag = false;
  int numPrime[MAX_NUM];
  int pivot = 1;
  
  //  for(int i=0;i<MAX_NUM;i++){
  //numPrime[i] = 0;
  //  }
  
  while(cin >> n){
    int cnt=0;
    if(n < pivot){
      //      cout << "n < pivot" << endl;
      cnt = numPrime[n];
      cout << cnt << endl;
      continue;
      
    }
    
    //    if(n >= pivot){
    //  }
    
    for(int i=pivot+1;i<=n;i++){
      flag = false;
      //?´???°?????????????????§??????
      for(int j=2;j<=sqrt(i);j++){
	if(i%j==0){
	  flag = true;
	  break;
	}
      }
      if(flag){
	numPrime[i] = numPrime[i-1];
	continue;
      }else{
	numPrime[i] = numPrime[i-1] + 1;
      }
    }
    cnt = numPrime[n];
    cout << cnt << endl;
  }
}

C++: #include <stdio.h>

int main(){
	int sosuu[100000] = { 2, 3, 5 }, i, x, y = 3,n,z;
	
	while (scanf("%d", &n)!=0){
		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 <math.h>
using namespace std;

int main(){
  int a;
  int i, j, k;
  int past=1, a_sqrt;
  int count;
  bool chk[1000000];

  for(i=1; i<1000000; i++) chk[i]=true;
  while(cin>>a){
    count=0;
    if(past>=a){
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
    else {
      a_sqrt=(int)sqrt((double)a);
      for(i=2; i<=a_sqrt; i++){
        for(j=i; j<=a/i; j++){
          chk[j*i]=false;
        }
      }
      past=a;
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
  }

  return 0;
}

C++: #include<iostream>
using namespace std;
#define rep(i,a) for(int i = 0 ; i < a ; i ++)
#define loop(i,a,b) for(int i = a ; i < b ; i ++)
const int M = (1e6);
bool p[M];
short cnt[M];
void isPrime(){
  rep(i,M)p[i] = true;
  p[0] = p[1] = false;
  cnt[0] = cnt[1] = 0;
  loop(i,2,M){
    cnt[i] = cnt[i-1] + p[i];
    if(p[i])
      for(int j = i + i ; j < M ; j += i)
	p[j] = false;
  }
}
int main(void){
  isPrime();
  int n;
  while(cin>>n)cout<<cnt[n]<<endl;
}

C++: #include<cstdio>
#include<cmath>
#include<cstring>
int prime(long long num)
{
    for(int i=2;i<=sqrt(num);i++)
	if(num%i==0)
	    return 0;
    return 1;
}
int main()
{
    long long n;
    while (scanf("%lld",&n)!=EOF)
    {
    	int sum=0;
    	for (int i=2;i<=n;i++)
    	  if (prime(i)) sum++;
    	printf("%lld\n",sum);
	}
    return 0;
}

C++: #include <cstdio>
#include <vector>
#include <ctime>
 
using namespace std;
 
int main()
{
    int n;
    vector<int> prime;
     
    //unsigned long t = time(NULL);
    prime.push_back(2);
	prime.push_back(3);
    while (scanf("%d", &n) != EOF){
		if (prime[prime.size() - 1] <= n){
			for (int i = prime[prime.size() - 1] + 2; i <= n; i += 2){
				bool p = true;
				for (int j = 0; prime[j] <= i / 2; j++){
					if (i % prime[j] == 0){
						p = false;
						break;
					}
				}
				if (p){
					prime.push_back(i);
				}
			}
		}
		
		int count;
		for (count = 0; count < prime.size() && prime[count] <= n; count++){
			;
		}
		printf("%d\n", count);
    }
     
    return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <math.h>
#pragma warning(disable:4996)

int main()
{
	
	int num, cnt;
	unsigned int i, j, k, data_num;
	int data[30];
	unsigned int sq_num;
	unsigned int *prime;

	for (data_num = 0; data_num <3; data_num++)
	{
		data[data_num] = -1;
		scanf("%d", &data[data_num]);
		if (data[data_num] < 0)
			break;
	}

	for (k = 0; k < data_num; k++)
	{

		sq_num = (int)sqrt((double)data[k]);
		prime = (unsigned int*)malloc(sizeof(unsigned int) * data[k]);

		prime[0] = 0;
		for (i = 1; i < data[k]; i++)
		{
			prime[i] = 1;
		}

		for (i = 1; i < sq_num; i++)
		{

			if (prime[i] == i)
			{
				for (j = (i + 1); (i + 1)* j <= data[k]; j++)
				{
					prime[(i + 1) * j - 1] = 0;
				}
			}
		}

		cnt = 0; 
		for (i = 0; i < data[k]; i++)
		{
			if (prime[i] == 1)
			{
				cnt++;
			}
		}
		printf("%d\n", cnt);
		free(prime);
	}

	return 0;
}

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

int main() {
	int n;

	while (cin >> n) {
		int count = 0;
		bool table[n + 1] = { false };
		int upper = sqrt(n);

		table[1] = true;

		for (int i = 1; i <= upper; i++) {
			if (table[i] == false) {
				table[i] = true;
				count++;
				for (int j = i; j <= n; j++) {
					if (j % i == 0) {
						table[j] = true;
					}
				}
			}
		}

		for (int i = 1; i <= n; i++) {
			if (table[i] == false) {
				count++;
			}
		}

		cout << count << '\n';
	}

	return 0;
}

C++: #include <utility>
#include <iostream>

using namespace std;

#define MAX_N 99999
#define MAX_D 30

bool prime_check(int n, int k = 2)
{
	if (n < k * k) { return true; }

	if (n % k == 0) { return false; }

	return prime_check(n, k + 1);
}

int main()
{
	int n;

	bool isprime[MAX_N + 1];

	for (int i = 2; i < MAX_N + 1; i++)
	{
		isprime[i] = prime_check(i);
	}

	while (cin >> n)
	{
		cout << count(isprime + 2, isprime + n + 1, true) << endl;
	}

	return 0;
}