Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

func next()string{
	return sc.Text()
}

func nextInt()int{
	a, _ := strconv.Atoi(next())
	return a
}

C++: #include <iostream>
#include <vector>
using namespace std;
int main(){
int n;
while(cin >> n,!cin.eof()){
vector<int> prime;
prime.push_back(2);//ôðÈ­½ß2ÍÅÉüêÄ¨­
bool flag;
for(int x=3;x<=n;x+=2){
flag=true;
for(int y=0;prime[y]*prime[y]<=x;y++){
if(!(x%prime[y])){
flag=!flag;
break;
}
}
if(flag)prime.push_back(x);
}
cout << prime.size() << endl;
prime.clear();
}
return 0;
}

C++: #include <cstdio>
#include <iostream>
#include <algorithm>
#include <string.h>
#include <stdbool.h>
#include <math.h>
#define ll long long int

using namespace std;

int cnt(int n)
{
    for(int i=2;i<=sqrt(n);i++)
    {
        if(n%i==0)
            return 0;
    }
    return 1;
}

int main()
{
    int n,i,j,c=0,x;

    while(scanf("%d",&n)!=EOF)
    {
        for(i=1;i<=n;i++)
        {
            if(i>2)
            {
                x=cnt(i);
                if(x==1)
                    c++;
            }
            else if(i==2)
                c++;
        }
        cout<<c<<endl;
        c=0;
    }







    return 0;
}

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

#define MAX 100000

using namespace std;

bool prime[MAX+1];
int T[MAX];

void eratos(int n) {
  for(int i = 0; i < n; i++) prime[i] = false;
  for(int i = 3; i <=n; i+=2)prime[i] = true;
  prime[2] = true;
  for(int i = 3, limit = (int)sqrt(n); i <= limit; i += 2) {
    if(!prime[i]) continue;
    for(int j = i*i, k = i*2; j <= n; j += k) prime[j] = false;
  }
}

void init() {
  eratos(MAX);

  for(int i = 0; i < MAX; i++) {
    T[i] = T[i-1];
    if(prime[i]) T[i]++;
  }
}

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

C++: #include <cstdio>
#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <cmath>
#define rep(i,a,n) for(int i=a;i<n;i++)
using namespace std;

int main(void){
	int i,j,k,n,a;
	while(cin >> n){
		a=1;
	if(n==1){
		cout << "0\n";
	}
	else{
		for(i=3;i<=n;i+=2){
			k=0;
			for(j=3;j<=sqrt(i);j+=2){
				if(i%j==0){
					k=1;
					break;
				}
			}
	
			if(k==0) a++;
		}
	}

	printf("%d\n",a);
	}
	return 0;
}

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

int main(void){
    int num;
    while(cin>>num){
        int ans=0;
        for(int i=2;i<=num;i++){
            int check=0;
            for(int j=2;j*j<=i;j++){
                if(i%j==0){check++;break;}
            }
            if(check==0)ans++;
        }
        cout<<ans<<endl;
    }
}

C++: #include <iostream>
#include<cstring>
#include<cmath>
#include<cstdio>
#include<algorithm>
#include<map>
#include<vector>
#include<stack>
#include <string>
#include<fstream>
#include<ctime>
#include<queue>

using namespace std;
#define LL long long
#define mxn 100010
#define mxe 1000010
#define eps 1e-4
#define inf 0x7fffffff

#include<iostream>
#include<cstdio>
#define INF 999999999;
#include<iostream>
using namespace std;

#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(vis, 0, sizeof(vis));
	memset(prime, 0, sizeof(prime));
	prime[0] = prime[1] = 0;
	for (int i = 2; i*i<= MAX; i++){
		if (!vis[i]){
			prime[i] = 1;
			for (int j = i + i; j <= MAX; j += i){
				vis[j] = 1;
				prime[j] = 0;
			}
		}
	}
	int sum = 0;
	for (int i = 0; i <= MAX; i++){
		if (prime[i]){
			sum++;
		}
		amount[i] = sum;
	}
	int n;
	while (cin >> n){
		cout << amount[n] << endl;
	}
	return 0;
}

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

#define rep(i, n) for(int i = 2; i <= n; i ++)

using namespace std;

int main(void){
	for(int n, count = 0; cin >> n; cout << count << endl, count = 0){
		vector<bool> numberList(n + 1);

		rep(i, n) numberList[i] = true;

		rep(i, n) for(int j = i; i * j <= n; j ++) numberList[i * j] = !(numberList[i * j]);
		
		rep(i, n) if(numberList[i]) count ++;
	}

	return 0;
}

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

#define rep(i, n) REP(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define MSG(a) cout << #a << " " << a << endl;
#define REP(i, x, n) for (int i = x; i < n; i++)
#define OP(m) cout << m << endl;
typedef long long ll;
typedef unsigned long long ull;

int main()
{
    int n;
    while (cin >> n)
    {
        int count = 0, p[n + 1] = {};
        for (int i = 2; i <= (int)sqrt(n); i++)
        {
            if (p[i] == 0)
            {
                for (int j = 2; i * j <= n; j++)
                {
                    p[i * j] = 1;
                }
            }
        }
        for (int i = 2; i <= n; i++)
        {
            if (p[i] == 0)
            {
                count++;
            }
        }
        OP(count)
    }

    return 0;
}

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

int main(){
  int n,prime[1000001],key,lf=0;
  prime[0]=2;
  key=1;
  for(int i=3;i<=2000000;i+=2)
    {
      for(int j=2;j<=sqrt(i);j++)
	{
	  if(i%j == 0)
	    {
	      lf=1;
	      break;
	    }
	}
      if(lf == 1)
	{
	  lf=0;
	}
      else 
	{
	  prime[key]=i;
	  key++;
	}
    }

  while(cin >> n)
{
  int cou=0;
  for(int i=0;;i++)
    {
      if(prime[i] <= n)
	{
	  cou++;
	}
      else break;
    }

  cout << cou << endl;
 }
}

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

int main()
{
	int a,i=0,m=1e6;bool p[m]={0};
	for(i=2;i<m;i++)
	{
		for(a=i+i;a<m;a+=i)p[a]=1;
	}
	
	for(;cin>>a;)
	{
		i=0;
		for(m=2;m<=a;i+=!p[m++]);
		cout<<i<<endl;
	}
	return 0;
}

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>
#include <cmath>

bool eratosthenes(int n);

int main()
{
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

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

bool eratosthenes(int n)
{
    if (n == 2) return true;
    if (n < 2 || n % 2 == 0) return false;
    int i = 3;
    double root_n = std::sqrt((double) n);
    while (i <= root_n) {
        if (n % i == 0) return false;
        i += 2;
    }
    return true;
}