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<cmath>
using namespace std;
bool isPrime(int x)
{
	if (x < 2)
		return false;
	if (x == 2)
		return true;
	if (x % 2 == 0)
		return true;
	for (int i = 3; i <= (int)sqrt(x); i+=2)
		if (x%i == 0)
			return false;
	return true;
}
int main()
{
	int n;
	cin >> n;
	int count = 0;
	for (int i = 1; i <= n; i++)
		if (isPrime(i))
			count++;
	cout << count << endl;
}

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

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

int main()
{
	long int n,x;
	int ans;
	while(cin>>n){
		if(n>=2) ans=1;
		else if(n<2) ans=0;
		
		for(x=3;x<=n;x+=2){
			if(is_sosu(x)==true) ans++;
		}
		cout<<ans<<endl;
	}
}

C++: #include <iostream>
using namespace std;
#define max_n 1000000

int main(void) {

  int x;
  while(cin >> x){


    bool array[max_n];

    for(int i = 0; i < max_n; i++) {
      array[i] = true;
    }

    array[0] = false;
    array[1] = false;
    for(int i = 2; i < max_n; i++) {
      for(int j = 2; j < max_n; j++) {
        if(i * j > max_n) break;
        array[i * j] = false;
      }
    }

    int count_sosu = 0;

    for(int i = 0; i <= x; i++) {
      if(array[i] == true) count_sosu++;
    }

    cout << count_sosu << endl;
  }
  return 0;
}

C++: #include<stdio.h>

int main(void)
{
	int data=0,i=0,j=2,k=0,l=0,kai[100]={0},s[10000]={0};

	while(scanf("%d",&data) != EOF)
	{
		if(data<=1)
		{
			kai[k]=0;
		}
		else
		{
			for(i=2;i<data;i++)
				s[i]=i;
		
			for(i=2;i*2<data;i++)		
			{
				for(j=2;j*i<data;j++)				
					s[j*i]=0;
			}
			for(i=2;i<data;i++)
			{	
				if(s[i]!=0)
				{
					l+=1;
				}
			}
		}
		kai[k]=l;
		k+=1;
	}

  for(j=0;j<k;j++)
    printf("%d\n",kai[j]);
  return 0;
}

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

int main() {
	int n, count;
	bool *data;

	while (cin >> n) {
		data = new bool[n+1];
		count = 0;
		data[0] = data[1] = false;
		for (int i = 2;i <= n;i++) {
			if (data[i]) {
				count++;
				for (int j = i + i;j <= n;j += j) {
					data[j] = false;
				}
			}
		}
		cout << count << endl;
		delete[] data;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int main(void){
    int n;
    while(cin>>n){
        int ans=0;
        for(int i=2;i<=n;i++){
            int sum=0;
            for(int j=1;j<=pow(i,0.5);j++){
                if(i%j==0)sum++;
                if(sum==2)break;
            }
            if(sum==1)ans++;
        }
        cout<<ans<<endl;
    }
    return 0;
}

C++: #include <stdio.h>

long MAX=999999;

int main(void){
    long int s[1000000]={0},k=0,i,j,n;
    
    s[0]=s[1]=1;
    
    for(i=2;i<=MAX;i++){
        if( !s[i] ){
            k++;
            for(j=2;j*i<=MAX;j++){
                s[j*i]=-1;
            }
        }
        s[i]=k;
    }
    
    
    while(EOF != scanf("%ld",&n)){
        printf("%ld\n",s[n]);
    }
    
    
    return 0;
}

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

int main(void) {
    int N=999999;
    int p[N];
    p[0]=0;
    for (int i=1; i<N; i++){
        p[i] = 1;
    }
    for (int i=1; i<N; i++){
        if (p[i]==1){
            int n=i+1 , m=2*n;
            while (m<=N){
                p[m-1] = 0;
                m += n;
            }
        }
    }
    int n , s;
    while (1){
        cin >> n;
        s=0;
        if (cin.eof()){
            for (int i=0; i<n; n++){
                if (p[i]==1)
                    s+=1;
            }
            cout << s << endl;
        }
    }
    return 0;
}

C++: ///bismillah_hir_rahmanir_rahim

#include<bits/stdc++.h>
using namespace std;
#define n 999999


int i,j,p,f;

int arr [n];

void seive ()
{

    //arr[0]=arr[1]=1;
    arr[2]=0;

    for (i=4; i<=n; i=i+2)
    {
        arr[i]=1;
    }

    for(i=3; i<=sqrt(n); i=i+2)
    {
        if(arr[i]==0)
        {
            for( j=i*i; j<=n; j=j+(2*i) )
            {
                arr[j]=1;

            }
        }
    }

//printf("%d\n",arr[5]);

//    for( p=2; p<=100; p++)
//    {
//        if(arr[p]==0)
//        {
//            printf("%d ",p);
//        }
//    }


}

int main()
{

    seive();

    int num,count,p;

    while ( scanf("%d",&num)==1)
    {
        count = 0;

        for( p=2; p<=num; p++)
        {
            if(arr[p]==0)
            {
                count++;
            }
        }
        printf("%d\n",count);
    }
    return 0;
}

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

typedef unsigned long long ull;

ull qtde[1000000] = { 0, 0, 1, 2 };

bool isPrime(ull x) {
  ull i = 6;
  ull root = (ull)sqrt(x) + 2;
	if (x <= 1)
		return false;
  if (x == 2 || x == 3)
    return true;
  if (x % 2 == 0 || x % 3 == 0)
    return false;

  while (i <= root) {
    if (x % (i - 1) == 0)
      return false;
    if (x % (i + 1) == 0)
      return false;
    i += 6;
  }

  return true;
}

int main(int argc, char const *argv[]) {
    ull n;
    ull i;
    
    for (int i = 3; i < 1000000; i++)
      qtde[i] = qtde[i-1] + isPrime(i);

    while (cin >> n)
      cout << qtde[n] << endl;

    return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <vector>
using namespace std;

const int MAX = 1000000;

vector<bool> sieve(int n = MAX) {
	vector<bool> prime(n, true);
	prime[0] = prime[1] = false;
	for(int i = 2; i < n; ++i)
	if(prime[i])
		for(int j = i * 2; j < n; j += i)
			prime[j] = false;

	return prime;
}

int main() {
	vector<bool> prime = sieve();
	vector<int> num(MAX, 0);

	for(int i = 0; i < MAX - 1; ++i)
		num[i + 1] = num[i] + prime[i + 1];

	for(int n; scanf("%d", &n) == 1;)
		printf("%d\n", num[n]);

	return EXIT_SUCCESS;
}

C++: #include <iostream>

using namespace std;
int main(){
	bool n[1000000];
	for(int i=0; i<1000000; i++){ n[i]=1; }
	n[0]=0;
	n[1]=0;

	for(int i=2; i<1000000; i++){
		if(n[i]==0){ break; }
		if(n[i]==1){
			for(int j=i*2; j<1000000; j+=i){ n[j]=0;	}
		}
	}

	int num;
	int count;
	while(cin >> num){
		count = 0;
		for(int i=2; i<=num; i++){
			count += n[i];
		}
		cout << count << endl;
	}

	return 0;
}

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

bool isprime(int &num)
{
    static int i;
    i = 2;
    for(;i< num; i++)
        if(num%i==0) return false;
    return true;
}

int countprime(int &num)
{
    static int i;
    static int count;
    count = 0;
    i = 2;
    for(;i<=num;i++)
        if(isprime(i)) count++;
    return count;
}

int main()
{
    int num[30];
    for(int i = 0; i < 30; i++)
        cin >> num[i];

    for(int i = 0; i < 30; i++)
        cout << countprime(int num[i]) << endl;
}

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(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;
}