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<vector>
#include<cmath>
using namespace std;

int main(){
    int maxy = 100000;
    int prime[maxy];
    //ツエツδ可トツスツテツネツスツづ個づ督づゥツつ「
    
    for(int i = 0; i < maxy; i++){
        prime[i] = 1;
    }
    prime[0] = prime[1] = 0;
    for(int i = 2; i <= (int)sqrt(maxy); i++){
        if(prime[i] == 1){
            for(int j=i*2; j <= (int)sqrt(maxy); j+=i){
                prime[j] = 0;
            }        
        }
    }
    int n;
    while(cin >> n){
        int ans = 0;
        for(int i=0; i <= n; i++){
            if(prime[i] == 1){
                ans++;
            }
        }
        cout << ans << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
// have you mooed today?
bool isprime(int& num)
{
    static int c;
    static int max;
    if(num == 2) return true;
    if(num % 2 == 0) return false;
    max = ceil(sqrt(num));

    for(c=3; c < max+1; c+=2)
        if(num % c ==0) return false;
    return true;
}

main()
{
    int ans;
    int buff;
    while(cin >> buff)
    {
        ans  = 0;
        for(int d =2; d <= buff; d++)
        {
            if(isprime(d)) ans++;
        }
        cout << ans << endl;
    }
}

C++: #include <iostream>
using namespace std;
 
int main() {
	int N;
	bool is_prime[1000000];
	is_prime[0]=0; is_prime[1]=0;
	for(int i=2;i<1000000; i++){
		is_prime[i]=1;
	}
 
	for(long s=2; s<1000000; s++){
		if(is_prime[s]==1){	
		for(long long i=s*s; i<1000000; i+=s){
		is_prime[i]=0;	
 
		}
		}
	}
 
	while(cin >> N){
 
		int ans=0;
		for(int i=2; i<N+1; i++){
			ans+=is_prime[i];
		}
		cout << ans <<endl;
 
	}
return 0;
}

C++: #pragma  warning (disable:4996)
#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<string>
#include<cmath>
#include<algorithm>
#define MAX 1000000
using namespace std;
int amount[MAX];
int vis[MAX];
int prime[MAX];
int main(){
	//memset(prime, 1, sizeof(prime));
	for (int j = 2; j <= MAX; j++) prime[j] = 1;
	prime[0] = prime[1] = 0;
	for (int i = 2; i*i<= MAX; i++){
		if (prime[i]){
				for (int j = i *i; j <= MAX; j += i){
				prime[j] = 0;
			}
		}
	}
	
	int n, m;
	while (cin >> n>>m){
		if (n==m) cout <<0 << endl;
		else {

		}
	}
	return 0;
}

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

using namespace std;

int HowManyPrimeNumbers1(const int);
int HowManyPrimeNumbers2(const int);
int MySqrt(const int);

int main(){
	int n;

	while(cin >> n){
		printf("%d\n",HowManyPrimeNumbers2(n));
	}

	return 0;
}

int HowManyPrimeNumbers1(const int n){
	int i,j,buf,cnt = 1;

	for(i=3;i<=n;i+=2){
		buf = (int)sqrt((double)i);

		for(j=3;j<=buf;j+=2){
			if((i % j) == 0){
				cnt--;
				break;
			}
		}

		cnt++;
	}

	return cnt;
}

int HowManyPrimeNumbers2(const int n){
	int i,j,cnt,root,rt;

	// 3 から n までの奇数の個数 + 1 を n に代入
	cnt = ((int)((((double)(n - 2)) / 2) + 0.5)) + 1;

	// 最大処理数を求める
	root = (int)sqrt((double)n);

	// a
	for(i=3;i<=root;i+=2){
		rt = MySqrt(i);
		for(j=3;j<=rt;j+=2){
			if((i % j) == 0){
				cnt += ((int)(n / i));
				break;
			}
		}
		cnt -= ((int)(n / i));
	}

	return n;
}

int MySqrt(const int i){
/*	int n;

	n = i - 1;

	return (int)(1 + (1 / 2) * (double)(i - 1) - (1 / 8) * (double)(i - 1) * (i - 1) + (1 / 16) * (double)(i - 1) * (i - 1) * (i - 1)); */

	return (int)sqrt((double)i);
}

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

typedef long long ll;
 
ll mod_pow(ll x, ll n, ll mod){
  ll res = 1;
  while(n > 0){
    if(n & 1) res = res * x % mod;
    x = x*x % mod;
    n >>= 1;
  }
  return res;
}

ll mod_mul(ll x, ll n, ll mod){
  x %= mod;
  n %= mod;
  return x*n % mod;
}

bool millrr_rabin(ll n){
  if(n == 2) return true;
  if((n & 1) == 0 || n <= 1) return false;
  int a[] = {2, 3, 5, 7, 11, 13, 17, -1}, r;
  ll s = 0, d, x;
  d = n - 1;
  while((d & 1) == 0){
    s++;
    d = d >> 1;
  }
  
  for(int i = 0 ; a[i] != -1 && a[i] < n ; i++){
    if((x = mod_pow(a[i], d, n)) != 1){
      for(r = 0 ; r < s ; r++){
	if(x == n-1) break;
	x = mod_mul(x, x, n);
      }
      if(r == s) return false;
    }
  }
  return true;
}

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

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

C++: #include <iostream>
using namespace std;
#define MAX 10000000
int prime[MAX];

int main(){
	prime[1]=1;
	prime[2]=0;
	prime[3]=0;

	for(int i = 2;i<=MAX;i++)
		if(prime[i]==0)
			for(int j = 2 * i;j<=MAX;j+=i)
				prime[j]=1;
	
	int n;
	while(true){
	
	cin >> n;
	if(cin.eof())
		break;
		
	int num = 0;	
	for(int k=1;k<=n;k++)
		if(prime[k]==0)
			num++;

	cout << num << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
#define MAX 100

bool prime[MAX]={};

void run(){
  for(int i=2;i<MAX;i++){
    prime[i]=true;
  } 

  for(int i=2;i<MAX;i++){
    if(!prime[i])continue;
    for(int j=2;j*i<MAX;j++){//i*1 is prime number
      prime[i*j]=false;
    }
  }
}

int main(){

  run();
  
  int n;
  while(cin>>n){
    int sum=0;
    for(int i=2; i<=n; i++){
      if(prime[i])sum++;
    }
    cout<<sum<<endl;
  }

}

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

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    static bool isprime[1000001];
    isprime[2]=true;
    for(int i=3; i<1000000; i+=2) isprime[i]=true;
    for(int i=3; i<1000; i+=2){
        if(!isprime[i]) continue;
        int d=i+i;
        for(int j=i+d; j<1000000; j+=d)isprime[j]=false;
    }
    int n;
    while(cin>>n){
        int ans=0;
        for(int i=2; i<=n; ++i) if(isprime[i]) ++ans;
        cout<<ans<<'\n';
    }
    return 0;
}

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

int main()
{
	bool is_prime[1000000];
	
	is_prime[0]=is_prime[1]=false;
	for(int i=0; i<1000000; i++){
		is_prime[i]=true;
	}
	
	for(int i=0; i<1000000; i++){
		if(is_prime[i]){
			for(int j=i*i; j<1000000; j+=i){
				is_prime[j]=false;
			}
		}
	}

	int n_prime[1000000];

	int cnt=0;
	for(int i=0; i<1000000; i++){
		if(is_prime[i]){
			cnt++;
		}
		n_prime[i]=cnt;
	}

	while(true){
		int n;
		cin>>n;

		if(cin.eof())
			break;
	
		cout<<n_prime[n]<<endl;
	}
	
	return 0;
}

C++: #include<iostream>
 
using namespace std;
 
int main(){
  static bool cmp[1000000]={true};
  static int npr[1000000];
  for(int i=2;i<1000000;i++){
    if(!cmp[i]){
      for(int j=2;j*i<1000000;j++){
    cmp[i*j]=true;
      }
    }
    npr[i]=npr[i-1]+!cmp[i];
  }
  for(int n;cin>>n;n++){
    cout<<npr[n]<<endl;
  }
  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++){
        k=past/i;
        for(j=k; 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;
}