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++: #include <iostream>
#include <cstdio>
using namespace std;
const int MAX_N = 1000005;
bool is_prime[MAX_N];
int n;
int main(int argc, const char * argv[])
{
    while (~scanf("%d", &n)) {
        for (int i = 2; i <= n; i++) is_prime[i] = true;
        //エラストテネスのふるい
        int count = 0;
        for (int i = 2; i <= n; i++) {  //2からnまで確認
            if (is_prime[i]) {  //素数iを発見
                count++;
                for (int j = 2*i; j <= n; j += i) {  //iの倍数を消す
                    is_prime[j] = false;
                }
            }
        }
        printf("%d\n",count);
    }
    
    return 0;
}

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

using namespace std;

int main() {

	int n;
	int *k;
	
	while (cin >> n) {
		int counter = 0;
		k = (int *)malloc(sizeof(int) * n);
		if (k == NULL) {
			exit(0);
		}
		k[0] = 1;
		for (int i = 1; i < n; i++) {
			k[i] = 0;
		}

		for (int i = 1; i < n; i++) {
			if (k[i] == 0) {
				for (int j = 1; j < i; j++) {
					if (k[j] == 0) {
						if ((i+1) % (j+1) == 0) {
							k[i] = 1;
							break;
						}
					}
				}
				if (k[i] == 0) {
					for (int t = 2; t < n/(i+1); t++) {
						k[(i+1)*t-1] = 1;
					}
				}
			}
		}

		for (int i = 0; i < n; i++) {
			if (k[i] == 0) {
				counter++;
			}
		}

		cout << counter << endl;

		free(k);
	}

	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

#define rep(i, n) for (int i = 0; i < n; ++i)
#define pb push_back
#define all(v) (v).begin(), (v).end()
typedef long long lint;
typedef vector<int> vi;

const int mod = 1e9 + 7;

lint gcd(lint a, lint b) {
	if (b == 0) return a;
	return gcd(b, a % b);
}

lint lcm(lint a, lint b) {
	return a / gcd(a, b) * b;
}

// ?????????????????????????????????????????????
lint extgcd(lint a, lint b, lint& x, lint& y) {
	lint d = a;
	if (b != 0) {
		d = extgcd(b, a % b, y, x);
		y -= (a / b) * x;
	} else {
		x = 1; y = 0;
	}
	return d;
}

bool is_prime(lint n) {
	for (lint i = 2; i * i <= n; ++i) {
		if (n % i == 0) return false;
	}
	return true;
}

// n????´???° ???????????????unsorted
/*
vi divisor(lint n) {
	vi res;
	for (int i = 1; i * i <= n; ++i) {
		if (n % i == 0) {
			res.pb(i);
			if (n / i != i) res.pb(n / i);
		}
	}

	return res;
}*/
vector<lint> divisor(lint n) {
	vector<lint> res;
	deque<lint> d;

	lint i = sqrt(n);
	if (i * i == n) {
		d.push_back(i);
		i--;
	}
	for (; i > 0; --i) {
		if (n % i == 0){
			d.push_front(i);
			d.push_back(n / i);
		}
	}

	res.insert(res.begin(), d.begin(), d.end());
	return res;
}


// 2 * 2 * 3 * 5 * 5 * 5 = {2:2, 3:1,5:3} ???????????? map ????????£????????????
map<int, int> prime_factor(lint n) {
	map<int, int> res;
	for (int i = 2; i * i <= n; ++i) {
		while (n % i == 0) {
			++res[i];
			n /= i;
		}
	}

	if (n != 1) res[n] = 1;
	return res;
}

const int MAX_N = 202020;
int prime[MAX_N];
bool memo[MAX_N + 1];

// ?´???°?????¨?????? n??\???????´???°????????°?????????
int sieve(int n) {
	int p = 0;
	memset(memo, true, sizeof(memo));
	memo[0] = memo[1] = false;

	for (int i = 2; i <= n; ++i) {
		if (memo[i]) {
			prime[p++] = i;
			for (int j = i * 2; j <= n; j += i) {
				memo[j] = false;
			}
		}
	}

	return p;
}

const lint MAX_LEN = 1000000;
const lint MAX_SQRT_B = 1000000;
bool memo_all[MAX_LEN];         //i means i + a
bool memo_small[MAX_SQRT_B];

// [a, b)????´???°????????°???????????????
lint count_prime(lint a, lint b) {
	for (int i = 0; (lint)i * i < b; i++) memo_small[i] = true;
	for (int i = 0; i < b - a; i++)     memo_all[i] = true;

	for (int i = 2; (lint)i * i < b; i++) {
		if (memo_small[i]) {
			for (int j = 2; (lint)j * j < b; j += i) memo_small[j] = false;
			for (lint j = max(2LL, (a + i - 1) / i) * i; j < b; j += i) memo_all[j - a] = false;
		}
	}

	lint ret = 0;
	for (lint i = a; i < b; i++) {
		if (memo_all[i]) ret++;
	}
	return ret;
}

// k???????????§????????£??????????????°??????{a_0 : 0, a_1 : 1, a_2 : 1, ... }
lint Fib[101010];
int calcFib(int k) {
	Fib[0] = 0;
	Fib[1] = Fib[2] = 1;
	for (int i = 3; i <= k; ++i) {
		Fib[i] = Fib[i - 1] + Fib[i - 2];
		// Fib[i] %= mod;
	}

	return Fib[k];
}


int main() {
	lint n;
	
	while (cin >> n) {
		cout << sieve(n) << endl;
	}
}

C++: #include <cstdio>
#include <vector>

const int MAX_N = 999999;
std::vector<bool> primes(MAX_N, true);

void sieve() {
  primes[0] = false;
  primes[1] = false;
  for (int i = 2; i < primes.size(); ++i) {
    if (!primes[i]) { continue; }
    for (int j = 2, m = i * j; m < primes.size(); m = i * ++j) {
      primes[m] = false;
    }
  }
}

int main() {
  sieve();

  int n;
  while (true) {
    if (scanf("%d", &n) != 1) { break; }
    int count = 0;
    for (int i = 0; i <= n; ++i) { if (primes[i]) { ++count; } }
    printf("%d\n", count);
  }

  return 0;
}

C++: #include <cstdio>
#include <stdio.h>
#include <iostream>
#include <iomanip>
#include <string> 
#include <vector> 
using namespace std;

int main(){
	int max,n[1000],c = 0,check;
	while(cin >> max){
		c = 0;
		for(int i = 2;max >= i;i++){
			check = 0;
			for(int iii = 1;c >= iii;iii++){
				if(i % n[iii] == 0){check = 1;break;}
			}
			for(int ii = 2;i > ii and check == 0;ii++){
				if(i % ii == 0){check = 1;break;}
			}
			if(check == 0){c++;n[c] = i;}

		}
		cout << c << endl;
	}



}

C++: #include<stdio.h>



int main(){
	int n,i,j;
	bool c[100000];
	int data[100000];
	int count;
	int premaxin=1;


	for(i=0;i<100000;i++){
		c[i]=1;
		data[i]=0;
	}
	c[0]=c[1]=0;

	while(scanf("%d",&n)!=EOF){

		if(premaxin<n){
			for(i=0;i<=n;i++){
				if(c[i]==1){
					if(premaxin>i){
						j=premaxin;
					}else{
						j=i+1;
					}
					for(;j<=n;j++){
						if(j%i==0){
							c[j]=0;
						}
					}
				}
			}

			count=data[premaxin];
			for(i=premaxin+1;i<=n;i++){

				if(c[i]==1){
					count++;
				}
				data[i]=count;
			}
		}

		printf("%d",data[n]);
	}
	return 0;
}

C++: #include<stdio.h>
#include<math.h>
main(){int i,j,n,p[1000000];scanf("%d",&n);for(i=2;i<=n;i++)p[i]=1;for(i=2;i<=n;i++)if(p[i])for(j=2;i*j<=n;j++)p[i*j]=0;j=0;for(i=2;i<=n;i++)j+=p[i];printf("%d\n",j);}

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

bool CheckPrimeNumber(int number) {
	int max = (int)std::sqrt(number);
	for (int i=2; i<=max; i++)
		if (number % i == 0)
			return false;
	return true;
}

int main() {
	int n, cnt;
	while (std::cin >> n) {
		cnt = 0;
		for (int i=2; i<=n; i++)
			if (CheckPrimeNumber(i))
				cnt++;
		std::cout << cnt << std::endl;
	}
	return 0;
}

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

int main(){
 int i,j,n,count,flg;
 scanf("%d",&n);
 for(i=2,count=0;i<=n;i++){
  for(j=2,flg=1;j<=sqrt(i);j++)
   if(i%j==0){ printf("%d\n",i); flg=0; break; }
  count+=flg;
 }
 printf("%d\n",count);
 return 0;
}

C++: #include <stdio.h>

int main() {
    int i, k, v,an1,an2,suu;
    int MAX_V = 10000;
    int prime[MAX_V+1];
    for(i = 2; i <= MAX_V; i++) {
        prime[i] = 1;
    }
    for(i = 2; i*i <= MAX_V; i++) {
        if(prime[i]) {
            for(k = 2 * i; k <= MAX_V; k += i) {
                prime[k] = 0;   
            } 
        }
    }
   while(1){
        scanf("%d",&suu);
        if(suu != 0){
            for(k=2;k<=suu;k++){
            	if(prime[k-2]==1 && prime[k]==1){
        	    	an1=k-2;
        	    	an2=k;
            	}
            }
            printf("%d %d\n",an1,an2);
        }
        else{
    	    break;
        }
    }
    return 0; 
}