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:
		return true
	case num < 2 || num%2 == 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 <iostream>
using namespace std;
 
int main(){
   bool is_prime[1000000];
   is_prime[0]=is_prime[1]=false;
   for(int i=2;i<1000000;i++)
      is_prime[i]=true;
   for(int i=2;i*i<1000000;i++){
      if(is_prime[i]){
         for(int j=i*i;j<1000000;j+=i){
            is_prime[j]=false;
         }
      }
   }
   int n[1000000];
   int count=0;
   for(int i=0;i<1000000;i++){
      if(is_prime[i])
         count++;
      n[i]=count;
   }
   while(true){
      int x;
      cin >> x;
      if(cin.eof())
         break;
      cout << n[x] << endl;
   }
    
   return 0;
}

C++: #include <iostream>

using namespace std;

int isprime(int x){
	if(x < 2){
		return false;
	}else if(x==2){
		return true;
	}else{
		for(int i=3;i<x/2;i=i+2){
			if(x%i==0){
				return false;
			}
		}
		return true;
	}
}

int main(){
	int n;
	while((cin >> n)){
	int a = 0;
	for(int i=2;i<=n;i++){
		if(isprime(i)){
			a++;
		}
	}
	cout << a << endl;
	}
	return 0;
}

C++: #include <stdio.h>

int main(){
    int e[30];

    int k = 0;
    while( scanf( "%d",&e[k] ) != EOF )k++;

    for( int i = 0;i < k;i++ ){
        int n = 10;
        int k = 1;
        while( true ){
            if( e[k] % n != 0 ){
                k++;
                n *= 10;
            } else {
                break;
            }
        }
        printf( "%d\n",k );
    }
    return 0;
}

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;
}