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++: import java.util.*;

class Prime2 {
    public static void main(String[] args) {
      
        Scanner scan = new Scanner(System.in);
        int[] x = new int[30];
        int t=0,j,count=0;
        
        
        while(scan.hasNextInt()){
        	count = 0;
			int lim = scan.nextInt();
        	int ptr = 0; //?????????????´???°?????°
        	int[] prime = new int[500000]; //?´???°???????´?????????????
        	
        	if(lim >= 2){
        		prime[ptr++] = 2; 
        		count++;
        	}
        	if(lim >= 3){
				prime[ptr++] = 3; 
				count++;
			}
			
        	for (int n = 5; n <= lim; n += 2) { 
        	    boolean wflag = false; 
        	    for (int i = 1; prime[i] * prime[i] <= n; i++) {
              
            	    if (0 == n % prime[i]) {
                    	wflag = true;
                    	break;
                	}
            	}
            	if (!wflag) { 
                	prime[ptr++] = n; 
                	count++; 
            	}
        	}
        	x[t] = count;
        	t++;
        	
		}
        	for (int i : x)  
            	System.out.println(i);
    }
}

C++: /*
    ????????°??§???????????¨????????????
*/

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
#include <limits.h>
#include <iostream>
#include <algorithm>
#include <string>
#include <iterator>
#include <vector>
#include <map>
#include <utility>
#define LLU unsigned long long int
#define LL long long int
#define LD long double
using namespace std;
#define LLUmax ULLONG_MAX
#define LLmax LLONG_MAX
#define dmax INT_MAX
#define vd vector<int>
#define vLL vector<long long int>
#define vLLU vector<unsigned long long int> 
#define pb push_back
#define mp make_pair
#define fi first
#define sec second
#define debug printf("asd\n")
#define debugd(a) printf("--%d\n",a)
#define debugdd(a,b) printf("--%d %d\n",a,b)
#define debugarr(a,b) for(int i=0 ; i<b ; i++)printf("%d ",a[i]);printf("\n")
#define newline printf("\n")
#define forin(a,b,c) for(int a=b ; a<c ; a++)
#define forde(a,b,c) for(int a=b ; a<c ; a--)


inline void scd(int &a){scanf("%d",&a);}
inline void scdd(int &a,int &b){scanf("%d %d",&a,&b);}
inline void sctd(int &a,int &b,int &c){scanf("%d %d %d",&a,&b,&c);}
inline void scdlld(long int &a,long int &b){scanf("%I64d %I64d",&a,&b);}
inline void sclld(long long int &a){scanf("%I64d",&a);}

#define scarrd(a,b) for(int index=0 ; index<b ; index++)scanf("%d",&a[index])
#define prdlld(a,b) printf("%I64d %I64d",a,b)
#define prdd(a,b) printf("%d %d",a,b)
#define prd(a) printf("%d",a)
#define prarr(a,b) for(int i=0 ; i<b ; i++)printf("%d ",a[i])
#define iniarr(a,b) for(int i=0 ; i<b ;i++)a[i]=0
#define arr(a,b) int *a=(int *)malloc(b*sizeof(int))
#define llarr(a,b) long long int *a=(long long int *)malloc(b*sizeof(long long int))
#define st(a,b) char *a=(char *)malloc(b*sizeof(char))
#define grav 9.8
#define pi 3.14159265

int prime(int a)
{
	if(a<2)
	return 1;

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

int main()
{
	int a;
	int pm[200000];
	int z=0;
	forin(i,2,1000000)
	{
		if(prime(i)==1)
		{
			pm[z]=i;
			z++;
		}
	}
	
	
	while(scanf("%d",&a)!=EOF)
	{
		forin(i,0,z)
		{
			//debugdd(pm[i],a);
			if(pm[i]>a)
			{
				printf("%d\n",i);
				break;		
			}
		}
	}
	return 0;
}

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

int main(){
        int prime[999999];
        bool is_prime[1000000];
        int p =0;
        int n ;
        int i;
        while(cin >> n){
        for(i=0;i<=n;i++) is_prime[i] = true;
        is_prime[0] = is_prime[1] = false;
        for(i=2;i<=n;i++){
                if(is_prime[i]){
                        prime[p++] = i;
                        for(int j=2*i;j<=n;j+=i) is_prime[j] = false;
                }
        }
        cout << p << endl;
        p = 0;
        }
return 0;
}
[tebora@tebora cpp]$ cat  pr.cpp
#include <iostream>
using namespace std;

int main(){
        int prime[999999];
        bool is_prime[1000000];
        int p =0;
        int n ;
        int i;
        while(cin >> n){
        for(i=0;i<=n;i++) is_prime[i] = true;
        is_prime[0] = is_prime[1] = false;
        for(i=2;i<=n;i++){
                if(is_prime[i]){
                        prime[p++] = i;
                        for(int j=2*i;j<=n;j+=i) is_prime[j] = false;
                }
        }
        cout << p << endl;
        p = 0;
        }
return 0;
}

C++: #include <iostream>
using namespace std;
bool t[999999] = {false};
int main()
{
	int n, c, i, j;
	t[0] = true;
	for (i = 2; i <= 999999; i++)
	{
		if (!t[i-1])
		{
			for (j = i+i; j < 999999; j+=i)
				t[j-1] = true;
		}
	}
	while (cin >> n)
	{
		if (cin.eof()) break;
		for (i = c = 0; i < n; i++)
			if (!t[i]) c++;
		cout << c << endl;
	}
}

C++: #include<stdio.h>
const int size=1000000;
int main(){
  int i,k,n;
  int data[size];
  for(i=2;i<size;i++){
    data[i]=1;
  }
  for(i=2;i<size;i++){
    if(data[i]){
      for(k=2*i;k<=size;k+=i){
        data[k]=0;
      }
    }
  }
  while(scanf("%d",&n)!=EOF){
    int cnt=0;
    for(i=2;i<=n;i++){
      if(data[i])cnt++;
    }
    printf("%d\n",cnt);
  }
}