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++: 
//  test.cpp

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <string>
#include <algorithm>

using namespace std;


int main(){
    long int j,s[1000000]={0},d=1,i,n,k;
    
    for(i=2;i<1000000;i++){
        if(s[i]==0){
            
            for(j=2;j*j<=i;j+=2){
                if(i%j==0)
                    break;
                if(j==2)
                    j=1;
            }
            
            
            if(j*j>i){
                d++;
                printf("%ld\n",i);
                if(i<1000){
                    for(k=2;k*i<1000000;k++){
                        s[k*i]=-1;
                    }
                }
            }
        }
        s[i] = d;
    }
    
    while(EOF != scanf("%ld",&n)){
        printf("%ld\n",s[n]-1);
    }
    
    
    return 0;
}

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

int main(){

  int n,i,j,ans;


  while(cin >> n){
    ans = 0;

    for(;n > 1 ; n--){
      if(n == 2 ) ans++;
      for(i = 2 ; i <= (int)sqrt((double)n) ; i++){
	if(n%i == 0){
	  break; 
	}
      }
      if(n%i != 0){
	ans++;
      }
    }
    cout << ans << endl;

  }
  return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<cmath>
using namespace std;
#define N 1000000
int p[N];
int main() {
	int i, j,n,cnt;
	for (i = 2; i <= sqrt(N); i++) if (!p[i]) for (j = i*i; j < N; j += i)p[j] = 1;
	while (cin >> n) {
		cnt = 0;
		for (i = 2; i <= n; i++) if (!p[i]) cnt++;
		cout << cnt << endl;
	}
	return 0;
}

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

int main(){
	int n;
	int a=0;
	cin >> n)
		for(int i = 0; i < n; ++i){
			for(int j = 2; j <= i; ++j){

				if(i%j == 0){
				if(i==j){
					++a;
				}else{
					break;
				}
				}
		}
	
	}
		cout << a << '\n';
}

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

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

int main() {
	// your code goes here
	int count = 0;
	int n;
	while(1){
		cin >> n;
		if(cin.eof()){
			break;
		}
	
		for(int i=2;i<=n;i++){
			if(IsPrime(i)){
				count++;
			}
		}
		cout << count << endl;
		count = 0;
		
	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cstring>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <ctime>
#include <list>
#include <numeric>
#include <utility>
#include <ctime>
#define INF 100000000
#define LINF 9000000000000000000
#define mod 1000000007
 
#define rep(i,n) for(int i=0;i<int(n);i++)
#define REP(i,a,b) for(int i=(a);i<int(b);i++)
#define all(x) (x).begin(),x.end()
#define pb push_back
#define mp make_pair
#define MOD(x) (x%(mod))
 
using namespace std;
 
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef pair<int,int> pi;
 
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};
bool debug=false;

/*---------------------------------------------------*/

int sieve(int n){
  int prime[1000000];
  bool is_prime[1000000];
  int p=0;
  for(int i=0;i<=n;i++)is_prime[i]=true;
  is_prime[0]=is_prime[1]=false;
  for(int i=2;i<=n;i++){
    if(is_prime[i]){
      prime[p++]=true;
      for(int j=2*i;j<=n;j+=i)is_prime[j]=false;
    }
  }
  return p;
}

int main(){
  int n;
  while(cin>>n){
    cout<<sieve(n)<<endl;
  }
  return 0;
}

C++: #include<stdio.h>
int main(void)
{
    int n,i,j,z,x;
	
    
	while(scanf("%d",&n)!=EOF){
		for(i=2;i<=n;i++){
			z=0;
			for(j=2;j<i;j++){
				if(i%j==0){
					z++;
					break;
				}
			}
			if(z==0){
				x++;
			}
		}
		printf("%d\n",x);
		x=0;
	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cmath>

static int GetPrimeNumberNum(int maxValue);

int main(void)
{
  int n;
  
  while (std::cin >> n) {
    std::cout << GetPrimeNumberNum(n) << std::endl;
  }
  
  return 0;
}

static int GetPrimeNumberNum(int maxValue)
{
  std::vector<int> searchList;
  std::vector<int> primeList;
  
  for (int i = 2; i <= maxValue; i++) {
    searchList.push_back(i);
  }
  
  double maxValueSq = sqrt((double)maxValue);
  
  while (true) {
    int newPrime = searchList.at(0);
    
    if (newPrime > maxValueSq) {
      break;
    }
    
    primeList.push_back(newPrime);
    
    for (std::vector<int>::iterator it = searchList.begin();
         it != searchList.end();
         ) {
      if (*it % newPrime == 0) {
        it = searchList.erase(it);
      }
      else {
        ++it;
      }
    }
  }
  
  for (std::vector<int>::iterator it = searchList.begin();
       it != searchList.end();
       ++it) {
    primeList.push_back(*it);
  }
  
  searchList.clear();
  
  return primeList.size();
}

C++: #include <iostream>

using namespace std;

#define LN 1000001
 
int main()
{
    int n;
    static int pn[LN];
    
    for (int i = 0; i < LN; i++){
        pn[i] = 1;
    }
    
    pn[0] = pn[1] = 0;
     
    for (int i = 2; i * i < LN; i++){
        if (pn[i] == 1){
            for (int j = i * i; j < LN; j += i){
                pn[j] = 0;
            }
        }
    }
     
    for (int i = 1; i < LN - 1; i++){
        pn[i + 1] += pn[i];
    }
     
    while (cin >> n){
        cout << pn[n] << endl;
    }
     
    return (0);
}

C++: #include <stdio.h>
#include <iostream>
using namespace std;
 
int isPrime(int n){
	//sieve of Eratosthenes
	if(n==2) return 1;
	if(n%2==0 || n<2){
		return 0;
	}

	for(int i=3; i*i<=n; i+=2){
		if(n%i==0){
			return 0;
		}
	}
	return 1;

}

//0009
int main()
{
	int n;

	while(scanf("%d",&n)){
		
		int cnt = 0;
		for(; n>1; n--){
			if(isPrime(n))
				++cnt;
		}
		cout << cnt << endl;
	}
  return 0;
}

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

#define N 1000000L
bool num[N];

void furui(){
  long int i;
  int divide;
  int rt = 1000;
  for(i = 0L; i < N; i++){
    num[i] = true;
  }
  num[0] = false;
  num[1] = false;
  for(divide = 2; divide < rt; divide++){
    if(num[divide] == false) continue;
    for( i = divide; i < N; i+= divide){
      num[i] = false;
    }
    num[divide] = true;
  }
}

int main(void){
  int c, i, j;
  long int k;
  char buf[7];
  long int n;
  long int count;

  furui();

  for(j = 0; j < 30; j++){
    for(i = 0; ;i++){
      c = getchar();
      if(c == EOF || c == '\n'){
        buf[i] = '\0';
        break;
      }
      buf[i] = c;
    }
    n = atol(buf);
    count = 0L;
    for(k = 1L; k <= n; k++){
      if(num[k] == 1) count++;
    }
    std::cout << count << std::endl;
    if(c == EOF) break;
  }
  return 0;
}

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

bool sieve[1000000];
void makesieve();

int main()
{
	makesieve();
	while(1)
	{
		int n,ans=0;
		std::cin>>n;
		if(std::cin.eof())break;
		for(int i=2;i<=n;i++)
		{
			ans+=sieve[i]?1:0;
		}
		std::cout<<ans<<std::endl;
	}
	return 0;
}

void makesieve()
{
	memset(sieve,-1,sizeof(sieve));
	sieve[0]=false;sieve[1]=false;
	for(int i=2;i<1000000;i++)
	{
		for(int j=i*2;j<1000000;j+=i)
		{
			sieve[j]=false;
		}
	}
}

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

bool is_prime(int n){
for(int i=2;i*i<=n;i++){
if(n % i == 0) return false;
}
return n!=1;
}

int main(){
int n;

while(~scanf("%d",&n)){
int res=0;
for(int i=2;i<=n;i++){
if(is_prime(n)) res++;
}
printf("%d\n",res);
}
}