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>
using namespace std;
int main(){
	int N,count=0;
	while(cin >> N){
		count = 0;
		if(N<0)	break;
	for(int x=2;x<=N;x++){
		int flag = 0;
		for(int y=2;y<x;y++){
			if((x%y)==0)	flag = 1;
		}
		if(flag == 0)	count++;
	}
	cout << count << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
bool prime[1000001];
using namespace std;
int main(){
	int i,j,n;

	for (i = 3; i <= 1000000; i += 2){
		prime[i] = true;
	}
	prime[2] = true;

	for (i = 3; i <= 1000; i += 2){
		if (prime[i] == 1){
			for (j = 3; j <= 1000000 / i; j += 2){
				prime[i * j] = false;
			}
		}
	}
	while(cin >> n && n){
		int count = 0;
		for(i=2;i<=n;i++){
			if(prime[i] == true) count++;
		}
		cout << count << endl;
	}
	return 0;
}

C++: #include<bits//stdc++.h>
using namespace std;
int main(){
	int a[1000500];
	a[0]=1;
	a[1]=1;
	for(int i=2;i*i<1000500;i++){
		if(a[i]==0){
			for(int j=i*2;j<1000500;j+=i){
				a[j]=1;
			}
		}
	}
	int b;
	while(cin>>b){
		int c=0;
		for(int i=0;i<=b;i++){
			if(a[i]==0) c++;
		}
		cout<<c<<endl;
	}
}

C++: #include<iostream>
const int MAX_N=1000000;
using namespace std;
bool value[MAX_N];
int main(){
	int n;
	while(cin>>n){
	  int count=0;
	for(int i=1;i<=n;++i){
	 value[i]=true;
	}
	for(int i=2;i<=n;++i){
		if(value[i]){
		for(int j=i+1;j<=n;++j){
		    if(j%i==0)
		      value[j]=false;
		  }
		}
	}
	for(int i=2;i<=n;++i){
	   if(value[i])
		   ++count;
	}
	cout<<count<<endl;
	}
return 0;
}

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

Int pn[1000002];

int main(){
int n, Count;
con.tie(0);
while(scanf("%d",&n)!=EOF){
Count=0;
memset(pn,0,n);
pn[0]=pn[1]=1;
for(int i=0;i<=n;i++) if(!pn[i]) for(int j=0;i*j<=n;j++) pn[i*j]=1;
Count=count(pn,pn+n,0);
count<<Count<<endl;
}
return 0;
}

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

int main() {
	const int MAX_V = 100000;
	int prime[MAX_V+1];
	int i, k, v;
	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;
			}
		}
	}
	scanf("%d", &v);
	while (v != 0){
		int count = 0;
		for(i=0;i<=v;i++){
			int isprime = prime[i];
			if(isprime) count ++;
		}
		printf("%d\n",count);
		scanf("%d", &v);
	}
	return 0;
}

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

int main() {
	int n,i,j,a=0,b=0;
	while (cin >> n) {
		if(n>=9) {
			for (i = 9; i <= n; i++) {
				a=0;
				for (j = 3; j < i; j++) {
					if (i%j == 0) {
						a++;
						b += a;
						break;
					}
					j++;
				}
				i++;
			}
		}
		if (n == 2)cout << "1" << endl;
		else if (n == 1)cout << "0" << endl;
		else if (n == 3)cout << "2" << endl;
		else if (n == 5) cout << "3" << endl;
		else if (n == 7)cout << "4" << endl;
		else if (n % 2 == 0) cout << n / 2 - b << endl;
		else cout << (n + 1) / 2 - b  << endl;
		b = 0;
	}
	return 0;
}

C++: #include <iostream>

auto is_prime (int x) -> bool
{
    for (auto i = 2; i * i <= x; ++ i)
    {
        if (x % i == 0)
        {
            return false;
        }
    }
    return true;
}

auto main () -> int
{
    int prime_num [1000000] = {0 , 0 , 1};
    for (auto i = 3; i < 1000000; ++ i)
    {
        prime_num [i] = prime_num [i - 1] + is_prime (i);
    }

    int n;
    while (std::cin >> n)
    {
        std::cout << prime_num [n] << std::endl;
    }
}

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

const int C = (int)1e7+10;

bool isprime[C];
int prime[C];

void init() {
  fill(isprime, isprime+C, true);
  isprime[0] = isprime[1] = false;
  for (auto i=2; i<C; i++) {
    if (isprime[i]) {
      for (auto j=2; i*j<C; j++) {
	isprime[i*j] = false;
      }
    }
  }
  prime[0] = 0;
  for(auto i=1; i<C; i++) {
    prime[i] = ((isprime[i]) ? prime[i-1]+1 : prime[i-1]);
  }
}

int main() {
  init();
  int n;
  while (cin >> n) {
    cout << prime[n] << endl;
  } 
}

C++: 
#include <iostream>
#include <algorithm>
#include <string.h>
#include <math.h>
#include <vector>

using namespace std;

#define pb push_back
#define mp make_pair
#define ll long long
#define ull unsigned ll
#define db double
#define INF 0x3f3f3f3f
#define MOD 1000000007
#define PII pair<int, int>

const int N=1000000;
bool p[N+10];
vector<int> primes;

int main()
{
    for (int i=2;i<=N;i++) {
        if (!p[i]) primes.pb(i);
        for (int j=0;j<primes.size()&&primes[j]*i<=N;j++) {
            p[primes[j]*i]=1;
            if (i%primes[j]==0) break;
        }
    }
    int n;
    while (~scanf("%d",&n)) {
        int ans=upper_bound(primes.begin(),primes.end(),n)-primes.begin();
        printf("%d\n",ans);
    }
}

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

int main(){
  int a;
  int i, j;
  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=0;
      for(i=2; i<=a_sqrt; i++){
        for(j=past+1; j<=a; j++){
          if(j%i==0) chk[j]=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;

int main(){
	int n , ans = 1;

	cin >> n;

	if(n <= 1){
		ans--;
	}

	for(int i = 2; i <= n; i++){
		if(i % 2 != 0 && 9 > i){
			//cout << i << endl;
			ans++;

		}if(i >= 9){
			if(i % 2 != 0 && i % 3 != 0 && i % 5 != 0 && i % 7 != 0){
				//cout << i << endl;
				ans++;
			}
		}
	}

	cout << ans << endl;

	return 0;
}

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

using namespace std;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  
  int n;
  while (cin >> n) {
    int prime_table[1000000] = {};
    for (int i = 2; i <= n; i ++) {
      if (prime_table[i] == 0) {
        for (int j = 2; j <= n / i; j ++) {
          prime_table[i * j] = 1;
        }
      }
    }
    int count = 0;
    for (int i = 2; i <= n; i ++) {
      if (prime_table[i] == 0) {
        count += 1;
      }
    }
    cout << count << endl;
  }
}

C++: #include <iostream>
#include <vector>
#include <cstdint>

int main()
{
	std::uint32_t n;
	while (std::cin >> n) {
		std::uint32_t num_prime = (n > 1) ? 1 : 0;
		std::vector<std::uint8_t> is_prime(n / 2, true);
		for (std::uint32_t i = 3; i < n + 1; i += 2) {
			if (is_prime[i / 2 - 1]) {
				++num_prime;
				for (std::uint32_t j = 3 * i; j < n + 1; j += i + i) {
					is_prime[j / 2 - 1] = false;
				}
			}
		}
		std::cout << num_prime << std::endl;
	}
	return 0;
}

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