Go: package main

import (
        "fmt"
)

const MAX int = 1000000
var primes [MAX]int

func main() {
        Init()
        var n, cnt int
        for {
                _, err := fmt.Scanf("%d", &n)
                if err != nil {
                        break
                }
                cnt = 0
                for i := 2; i <= n; i++ {
                        cnt += primes[i]
                }
                fmt.Printf("%d\n", cnt)
        }
}

func Init() {
        for i := 0; i < MAX; i++ {
                primes[i] = i
        }

        Soe()

        for i := 0; i < MAX; i++ {
                primes[i] %= 2
        }
        primes[1] = 0
        primes[2] = 1
}

func Soe() {
        for i := 2; i < MAX; i++ {
                if primes[i] > 0 {
                        for j := 2; j < MAX/i; j++ {
                                if i * j < MAX {
                                        primes[i*j] = 0
                                }
                        }
                }
        }
}

C++: #include<iostream>
#define K 1000
#define M 1000000
using namespace std;

int main(){
	int i, j, n, pnum[M]={0}, cnt=0;
	bool notp[M]={true, true, 0};
	i=2;
	j=0;
	while(j<K){
		for(j=i*i;j<M;j+=i){
			notp[j] = true;
		}
		for(j=i+1;j<=K;j++){
			if(notp[j] == false){
				i=j;
				break;
			}
		}
	}
	for(i=0;i<M;i++){
		if(!notp[i]) cnt++;
		pnum[i]=cnt;
	}
	while(cin>>n){
		cout << pnum[n] <<endl;
	}
	return 0;
}

C++: #include<iostream>
#include<cmath>
#include<stdio.h>
using namespace std;
int main()
{int n;
 while(scanf("%d",&n))
 {int sum=0;
  int i;
  for(int m=2;m<=n;m++)
  {int k=int (sqrt(double(m)));
	  i=2;
   while(m%i&&i<=k)
	   i++;
   if(i>k)sum++;
  }
  printf("%d\n",sum);
 }
}

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

int main(){
	int n[1000000]={0};
	int x, ans=0;
	
	for(int i=1; i<=999999; i++){
		if(i==1) n[i]=-1;
		else if(n[i]==0){
			for(int k=2; k*i<=999999; k++){
				n[k*i]=-1;
			}
		}
	}
	
	while(cin>>x){
		ans=x;
		for(int j=1; j<=x; j++) ans+=n[j];
		cout << ans << endl;
	}
	
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <ctime>
#define INF 999999999
#define mod 1000000007

#define rep(i,n) for(int i=0;i<int(n);i++)
#define rrep(i,n) for(int i=(n);i>=0;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 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;

bool debug=false;
/*---------------------------------------------------*/

bool prime[1000000];
int ans[1000000];

void is_prime(){
  rep(i,1000000)prime[i]=true;
  prime[0]=prime[1]=false;
  REP(i,2,1000000){
    if(prime[i]==true){
      for(int j=i+i;j<1000000;j+=i)prime[j]=false;
    }
  }
  if(debug)rep(i,1000)if(prime[i])cout<<i<<endl;
}

void calc(){
  ans[0]=0;
  REP(i,1,1000000){
    ans[i]=ans[i-1]+prime[i];
  }
}

int main(){
  int n;
  is_prime();
  calc();
  while(cin>>n){
    cout<<ans[n]<<endl;
  }
  return 0;
}

C++: #include <iostream>
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 <stdlib.h>
#include <iostream>
const int MAX_V = 1000000; // 100????????§?´???°??¨?????????
int prime[MAX_V+1]; // 1????´???°???0????´???°??§?????? 
int pcount[MAX_V+1];
int main() {
    int i, k,a,sum=0;
    for(i = 2; i <= MAX_V; i++) {
        prime[i] = 1; // 2??\????´???°??¨??????
    } 
    for(i = 2; i*i <= MAX_V; i++) {
        if(prime[i]) {
            for(k = 2 * i; k <= MAX_V; k += i) {
                prime[k] = 0; // ?´???°2?????\????????°?´???°??§?????? 
            }
            
        } 
        
    }
    pcount[0]=0;
    pcount[1]=0;
    for(i=2;i<MAX_V;i++){
        if(prime[i]){
            sum++;
        }
        pcount[i]=sum;
    }
    for(i=0;i<30;i++){
        scanf("%d",&a);
        printf("%d\n",pcount[a]);
    } 

    return 0; 
    
}

C++: #include <iostream>
#define max 999999

using namespace std;

int n[max];

int isprime(void){
  for( int i = 0 ; i < max ; i++ )n[i] = 1;
  n[0] = n[1] = 0; 

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

int main(void){
  
  isprime();

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

C++: #include<stdio.h>



int main(){
	int n,i,j;
	bool c[100000];
	int data[100000];
	int count;
	c[0]=c[1]=0;
	for(i=2;i<100000;i++){
		c[i]=1;
	}

	while((scanf("%d",&n)!=EOF)&&(n!=0)){
		
		count=0;
		for(i=0;i<=n;i++){
			if(c[i]==1){
				count++;
				for(j=i+1;j<=n;j++){
					if(j%i==0){
						c[j]=0;
					}
				}
			}
			data[i]=count;
		}

		printf("%d",data[n]);
	}
	return 0;
}

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

int main(){
	int i,j,n;
	bool prime[1000001];
	for (i = 3; i <= 1000000; i += 2){
		prime[i] = true;
	}
	prime[2] = true;

	for (i = 3; i <= 1000; i += 2){
		if (prime[i] == 1){
			for (j = 3; j <= 1000000 / i; j += 2){
				prime[i * j] = false;
			}
		}
	}
	while(cin >> n){
		int count = 0;
		for(i=2;i<=n;i++){
			if(prime[i] == true) count++;
		}
		cout << count << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <string>
#include <stack>
#include <math.h>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <vector>

using namespace std;
int sieve(int n);
int prime[1000000];
bool is_prime[1000000];

int main()
{

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

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

bool Eratos[1000000];

int main()
{
	Eratos[0] = Eratos[1] = false;
	for (int i = 0; i < 1000000; i++){
		Eratos[i] = true;
	}
	for (int i = 2; i < 1000; i++) {
		if (Eratos[i]) {
			for (int j = 2 * i; j < 1000000; j += i) {
				Eratos[j] = false;
			}
		}
	}
	int n,res;
	res = 1;
	for (int i = 2; i < 1000000; i++) {
		if (Eratos[i])
			Eratos[i] = res;
		res++;
	}
	while (scanf("%d", &n)) {
		res = 0;
		while (!Eratos[n]) {
			n--;
		}
		printf("%d\n",Eratos[n]);
	}
	return 0;
}