Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		input := nextInt()
		fmt.Println(IsPrime(input))
	}
}

func IsPrime(num int) int {
	eratos := make([]bool, num+1)
	c := 0
	for i := 2; i < len(eratos); i++ {
		if eratos[i] == false {
			c++
			eratosCalc(i, eratos)
		}
	}
	return c
}

func eratosCalc(primeNum int, eratos []bool) {
	for i := primeNum * primeNum; i < len(eratos); i += primeNum {
		eratos[i] = true
	}
}

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

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

C++: #include<iostream>
using namespace std;
int main(){
	int a,n[1000000],j,o=0;
	for(int i=2;i<=99999;i++){
		n[i]=1;
		
	}
	n[0]=n[1]=0;
		for( j=2;j<=99999;j++){
			if(n[j]){
				for(int k=j*2;k<=9999;k+=j){
				n[k]=0;
				}
			}
		}
		while(cin>>a){
	for(int k=2;k<=a;k++) if(n[k]) o++;
	cout<<o<<endl;
		}
}

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

int main()
{
	int n,i,j,count,pnum;
	while(scanf("%d",&n)!=EOF){
		count=0;
		pnum=0;
		
		
		for(i=n; i>=1; i--){
			for(j=1; j<=i; j++){
				if(i%j==0) count++;
			}
			if(count==2) pnum++;
			count=0;
		}
		cout << pnum << endl;
	}
	
	return 0;
}

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

using namespace std;

constexpr int ceil_sqrt(int x) {
  int ans = 0;
  while (ans*ans < x) { ++ans; }
  return ans;
}

template<int size>
struct Sieve {
  constexpr Sieve(): factor(), primes() {
    factor[0] = factor[1] = 1;
    for (int p = 2; p < size; ++p) {
      primes[p] = primes[p-1] + (factor[p]?0:1);
      if (factor[p]) { continue; }
      for (int n = 2*p; n < size; n += p) {
        factor[n] = p;
      }
    }
  }
  constexpr bool is_prime(int n) const {
    assert(n < size);
    return !(factor[n]);
  }
  int factor[size];
  int primes[size];
};

constexpr int NMAX = 1000010;

int main() {
  int N = 0;
  const auto sieve = Sieve<NMAX>();
  while (cin >> N) {
    cout << sieve.primes[N] << endl;
  }
  return 0;
}

C++: #include<stdio.h>

int main(void){
  int n;
  while(scanf("%d",&n)!=EOF){
    if(n==1){
      printf("0");
      break;
    }
    int p[100000];
    p[0]=2;
    int pi=1;
    int i=2;
    while(i<=n){
      int t=1;
      int j;
      for(j=0;j<pi;j++){
	if(i%p[j]==0){
	  t=0;
	  break;
	}
      }
      if(t==1){
	p[pi]=i;
	pi++;
      }
      i++;
    }
    printf("%d\n",pi);
  }
  return 0;
}

C++: /* AOJ0009 Prime Number */

#include <iostream>
#include <stdio.h>
#include <math.h>
#include <string.h>

using namespace std;

const int N = 1e6;
const int SQRTN = ceil(sqrt((double) N));
bool isPrime[N + 1];
int prefixsum[N + 1];

// Eratosthenes筛选法
void esieve(void)
{
    memset(isPrime, true, sizeof(isPrime));

    isPrime[0] = isPrime[1] = false;
    for(int i=2; i<=SQRTN; i++) {
        if(isPrime[i]) {
            for(int j=i*i; j<=N; j+=i)  //筛选
                isPrime[j] = false;
        }
    }
}

void maketable()
{
    prefixsum[0] = 0;
    for(int i = 1; i <= N; i++)
        prefixsum[i] = isPrime[i] ? prefixsum[i - 1] + 1 : prefixsum[i - 1];
}

int main()
{
    esieve();

    maketable();

    int n;
    while(~scanf("%d", &n))
        printf("%d\n", prefixsum[n]);

    return 0;
}

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

int Era(int N) {
	int prime_num = 0;
	int* array = new int[N + 1];
	for (int i = 2; i < N + 1; i++) {
		array[i] = 1;
	}
	for (int i = 2; i < sqrt(N); i++) {
		if (array[i]) {
			for (int j = 0; i*(j + 2) <= N; j++) {
				array[i*(j + 2)] = 0;
			}
		}
	}
	for (int i = 2; i < N + 1; i++) {
		if (array[i])prime_num++;
	}
	return prime_num;
}

void add(int* primes, int& now,int prime) {
	primes[now] = prime;
	now++;
}

int main() {
	int num;
	int primes[1000]; int now = 0;
	while (cin >> num) {
		add(primes, now, Era(num));
	}
	for (int i = 0; i < now; i++) {
		cout << primes[i] << endl;
	}
	return 0;
}

C++: /*http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0009&lang=jp*/

#include <iostream>

#define MAX 999999

using namespace std;

int num_prime[MAX+1];
bool not_prime_flag[MAX+1];

int main()
{
    int n;

    for (int i = 2; i < MAX+1; ++i) {
        if(!not_prime_flag[i]){
            for (int j = 2; i*j < MAX+1; ++j) {
                not_prime_flag[i*j] = true;
            }
        }
        num_prime[i]=num_prime[i-1]+!not_prime_flag[i];
    }
    while (cin>>n){
        cout<<num_prime[n]<<endl;
    }
    return 0;
}

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

using namespace std;
int c = 0;
int i = 1;
unsigned long long a = 0;
unsigned long long b = 0;
unsigned long long sosuu[1000000] = { 0 };
int pattara = 0;

int sub(int c) {

	
	
	
	for (int i = 2; i<=c; i++){
		
		for (int q = 1; q <= sqrt(i); q++) {
			sosuu[pattara] = i;
			if (sosuu[pattara] % 2 == 0 && sosuu[pattara]!= 2) {
				break;
			}
			if (sosuu[pattara] % q == 0) {
				b += 1;
			}

		}

		if (b == 1) {
			
			pattara++;
			
		}
		b = 0;
		
	}

	
	return 0;
}


int main(){
	while (scanf("%d", &c) != EOF){

		sub(c);
		cout << pattara << endl;
		pattara = 0;
		
	}

	return 0;
}

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

using namespace std;

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

	for (int i = 2; i < n; i++)
	{
		if (A[i])
		{
			mama++;
		}
	}
	return mama;
}

int main()
{
	int n;
	cin >> n;
	while (n != -1)
	{
		cout << Eratosthenes(n) << endl;
		cin >> n;
	}
	return 0;
}

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

int main(){
  bool prime[1000000];
  prime[0]=false;prime[1]=false;
  for(int i=2;i<=999999;i++){
    prime[i]=true;
  }

  for(int i=2;i<=sqrt(999999);i++){
    if(prime[i]){
      for(int j=2;j*i<=999999;j++){
        prime[j*i]=false;
      }
    }
  }
  int n;
  while(cin>>n){
    int cnt=0;
    for(int i=2;i<=n;i++){
      if(prime[i])cnt++;
    }
    cout<<cnt<<endl;
  }
}

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

#ifdef _MSC_VER
#define _CRT_SECURE_NO_WARNINGS
#define __typeof__ decltype
#endif

#define ITER(c) __typeof__((c).begin())
#define FOREACH(it, c) for (ITER(c) it=(c).begin(); it != (c).end(); ++it)

const int limit = 1000000;
std::vector<int> q;

int main(){
    q.push_back(2);
    for(int i = 3; i < limit; i += 2){
        bool f = true;
        FOREACH(it, q){
            int p = *it;
            if(i < p*p) break;
            if(i % p == 0){ f = false; break; }
        }
        if(f) q.push_back(i);
    }
    int n;
    while(std::cin >> n){
        int l = 0, r = q.size()-1, c = -1;
        while(l <= r){
            c = (l + r)/2;
            if(q[c] < n){ l = c + 1; continue; }
            if(q[c] > n){ r = c - 1; continue; }
            break;
        }
        if(l > r) c = r;
        std::cout << c + 1 << std::endl;
    }
}

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

int main(){
	char data[999999];
	
	int i,j,num,check,count = 0;
	
	data[0] = 0;
	data[1] = 1;
	
	for(i=3;i<1000000;i++){
		check = 1;
		if(i%2==0){
			data[i-1]=0;
		}
		if(i%2==1){
			for(j=3;j*j<=i;j+=2){
				if(i%j==0){
					check = 0;
					break;
				}
			}
			if(check == 1){
				data[i-1] = 1;
			}
			else{
				data[i-1] = 0;
			}
		}
	}
	/*
	for(i=0;i<20;i++){
		if(data[i] == 1){
			printf("%d\n",i+1);
		}
	}
	*/
	
	
	while(cin>>num){
		count = 0;
		for(i=0;i<num;i++){
			if(data[i] == 1){
				count++;
			}
		}
		printf("%d\n",count);
	}
	
	
	
	return 0;
}