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

using namespace std;

int main() {

	int n,count;

	while (cin >> n) {
		count = 0;
		
		//?´???°????????????????¨???¶?????????????????????????¢????
		bool *is_prime;
		is_prime = (bool *)malloc(n+1);
		if (is_prime == NULL) {
			cout << "malloc error" << endl;
			break;
		}

		//????????¨????´???°??§????????¨??????
		for (int i = 0; i <= n; i++) {
			is_prime[i] = true;
		}

		for (int i = 2; i <= sqrt(n); i++) {
			//?´???°???????????§????????£???????????????????????§??????
			if (is_prime[i] == true) {
				//????????°???????????§????????°????????£?????????
				for (int j = i+1; j <= n; j++) {
					//cout << "i=" << i << " j=" << j << " is_prime(i)=" << is_prime[i] << " is_prime(j)=" << is_prime[j] << endl;
					if (is_prime[j] == true) {
						if (j%i == 0) {
							//????????????????????????????´???°??§?????????
							is_prime[j] = false;
						}
					}
				}
			}
		}

		//?´???°??¨?¢????????????°????????°????????????
		for (int i = 2; i <= n; i++) {
			if (is_prime[i] == true) {
				count++;
				//cout << " " << i;
			}
		}

		//??¨???
		cout << count << endl;
	}
	return 0;
}

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

int main(){
  int a;
  int i, j;
  int past=1, a_sqrt;
  int count;
  bool chk[1000000];

  for(i=1; i<1000000; i++) chk[i]=true;
  while(cin>>a){
    count=0;
    if(past>=a){
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
    else {
      a_sqrt=(int)sqrt(a);
      for(i=2; i<=a_sqrt; i++){
        for(j=past+1; j<=a; j++){
          if(j%i==0) chk[j]=false;
        }
      }
      past=a;
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
  }

  return 0;
}

C++: #include<iostream>
using namespace std;
int PrimeNum(int n){
  for(int i = 2;i <= n;i++){
    if(i == n){
      return 1;
    }
    if(n % i == 0){
      return 0;
    }
  }
}
int main(){
  int n,num;
  for(cin >> n; ;cin >> n){
    num = 0;
     for(int k = 2;k <= n;k++){
       if(PrimeNum(k) == 1){
	 num = num +1;
       } 
     }
     cout << num;
  }
}

C++: #include <iostream>

using namespace std;

bool isPrime(int);

int main() {
	int n, p;
	while (cin >> n) {
		p = 0;
		if (cin.eof())
			return 0;
		//O(n)
		for (int i = 2; i <= n; i++) {
			if (isPrime(i))
				p++;
		}
		cout << p << endl;
	}
	return 0;
}

//O(sqrt(n))
bool isPrime(int x) {
	if (x % 2 == 0)
		return true;
	for (int j = 3; j * j <= x; j += 2) {
		if (x % j == 0)
			return false;
	}
	return true;
}

//SÌÆµÄAO(n*sqrt(n))
//ÚÀÆµÄAOÌàÌ®ª1000ÈºÈçÜ¸ÔÉ¤A1­Â´¦éÆÜ¸ÔÉíÈ¢
//O(100*1000)=10­A

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];
bool not_prime_flag[MAX];

int main()
{
    int n;

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

C++: #include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b) - 1;i>=(a);i--)
#define REP(i,n) for(int i=0;i<(n);i++)
#define RREP(i,n) for(int i=n-1;i>=0;i--)

#define PB push_back
#define INF INT_MAX/3
#define ALL(a) (a).begin(),(a).end()
#define CLR(a) memset(a,0,sizeof(a))

typedef long long int ll;

using namespace std;

int main(){
    vector<int> primes;
    primes.PB(2);
    FOR(i,3,1000000){
	bool flg = true;
	REP(j,primes.size()) if(i % primes[j] == 0 ) {
	    flg = false;
	    break;
	}
	if(flg) primes.PB(i);
    }

    int n;
    while(true){
	cin >> n;
	if(cin.eof())break;
	int res = upper_bound(ALL(primes),n) - primes.begin();
	cout << res << endl;
    }

    return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <set>
#include <map>
#include <cmath>
//ranker
using namespace std;

#define REPS(i, a, n) for (int i = (a); i < (n); ++i)
#define REP(i, n) REPS(i, 0, n)
#define RREP(i, n) REPS(i, 1, n + 1)
#define DEPS(i, a, n) for (int i = (a); i >= n; --i)
#define DEP(i, n) DEPS(i, n, 0)

using ll = long long;
using ull = unsigned long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using pii = pair<int, int>;
using pis = pair<int, string>;
using psi = pair<string, int>;
using D = double;


const int Max=10000000;
bool prime[Max];
int main(void){
    int in;
    for(int i=0;i<Max;i++)prime[i]=true;
    prime[1]=false;
    for(int i=2;i<Max;i++){
        if(prime[i]){
            for(int s=i*2;s<=Max;s+=i)prime[s]=false;
        }
    }
    while(cin>>in){
        int cnt=0;
        for(int i=1;i<=in;i++){
            if(prime[i])cnt++;
        }
        cout << cnt << "\n";
    }
    return 0;
}

C++: #include<iostream>
#include<vector>
#include<string>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<cstdio>
using namespace std;
#define MAX 1000000
int prime[MAX];
bool is_prime[MAX+1];

int solve(int n){
  int p = 0; 
  //fill(prime,prime+MAX,true);
  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++] = i;
      for(int j = i + i ; j <= n ; j += i){
	is_prime[j] = false;
      }
    }
  }
  return p;
}
int main(void){ 
  
  long long int n;
  while(cin>>n){
    cout<<solve(n)<<endl;
  }
  
}

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

int main(){
int unti[100002] = {0};
fill(unti,unti + 100001,0);
for(int n = 2;n < 100001;n++)
{
if(unti[n] == 0)
{
for(int a = 2;(a * n) < 100001;a++)
{
	if((a * n  ) < 100001 ) unti[a * n]++;
}
}
}
int b;
while(cin >> b)
{
int cnt = (count(unti,unti+b+1,0) - 2);
cout << cnt << endl;
}

}

C++: #include <algorithm>
#include <map>
#include <vector>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cstring>
#include <cctype>
#include <cstdlib>
#include <cmath>
#include <string>
#include <queue>
using namespace std;

#define REP(i,n) for(int i=0; i<n; ++i)
#define IREP(it,array) for(vector<int>::iterator it=array.begin(); it!=array.end(); ++it)
#define SREP(it,array) for(vector<string>::iterator it=array.begin(); it!=array.end(); ++it)
#define MP make_pair
typedef long long LL;


int main()
{
  cout.setf(ios::fixed, ios::floatfield);
  cout.precision(6);

  int INF = 1000000;

  int n;
  while(cin>>n){
    int res=0;
    bool notPrime[n+1];
    memset(notPrime,0,sizeof(notPrime));
    
    notPrime[0] = notPrime[1] = true;

    for(int i=2; i<=n; i++){
      if(!notPrime[i]){
	res++;
	for(int j=2*i; j<=n; j+=i){
	  notPrime[j] = true;
	}
      }
    }
    cout << res << endl;
  }


  return 0;
}

C++: #include <iostream>

using namespace std;

#define MAX 1000000

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