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] = 1
        }
        primes[0] = 0
        primes[1] = 0
        Soe()
}

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

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

#define MAX 1000000

int main(){
	bool p[MAX+1];
	int num,answer;
	p[0] = false;
	p[1] = false;
	for(int i=2 ; i<=MAX ; i++){
		p[i] = true;
	}
	for(int i=2 ; i<=sqrt(MAX)+1 ; i++){
		if(p[i]==1){
			for(int j=i*2; j<=MAX; j=j+i){
				p[j] = false;
			}
		}
	}
		
	while( scanf("%d" , &num) != EOF ){	
		answer = 0;	

		for(int i=0; i<=num; i++){
			if(p[i]){
				answer++;
			}
		}
		printf("%d\n", answer);
	}
}

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

int main() {
	int n, ans; bool check;
	while (cin >> n) {
		ans = 0;
		for (int i = 1; i <= n; i++) {
			if (i <= 3) { if (i != 1) ans++; continue; }
			if (i % 2 == 0 || i % 3 == 0) continue;
			if (i % 6 != 1 && i % 6 != 5) continue;
			check = true;
			for (int j = 5; pow(j, 2) <= i; j += 6) {
				if (i % j == 0 || i % (j + 2) == 0) check = false;
				if (check == false) break;
			}
			if (check == true) ans++;
		}
		cout << ans << endl;
	}
	return 0;
}

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

int main() {
	
	int p[168],l=0,n;
	
	for(int i=2;i<1000;i++){
		int k=0;
		for(int j=2;j<=sqrt(i);j++)if(i%j==0&&i/j!=1)k=1;
		if(k==0){p[l]=i; l++;}
	}
	
	while(cin>>n){
		l=0;
		for(int i=2;i<=n;i++){
			int k=0;
			for(int j=0;j<168;j++)if(i%p[j]==0&&i/p[j]!=1)k=1;
			if(k==0)l++;
		}
		cout<<l<<endl;
	}
	
	return 0;
}

C++: #include <cstdio>
#define MAX 1000000

int main(){
	int n;
	bool c[MAX]={true};
	int t[MAX];t[0] = 0;
	for(int i = 2;i < MAX;i++){
		for(int j = i;i*j < MAX;j++){
			c[i*j-1] = true;
		}
	}
	for(int i = 1;i < 15;i++){
		if(c[i])t[i]=t[i-1];
		else t[i] = t[i-1]+1;
	}
	while(scanf("%d", &n)){
		printf("%d\n", t[n-1]);
	}
	return 0;
}

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

int main()
{
        long long max;
        vector<long> pn;
        bool npn[1000001];
        npn[0] = npn[1] = true;


        while(cin >> max){
                for(long j=2;j<=999999;j++) npn[j] = false;
                for(long i=2;i<=max;i++){
                        if(npn[i]) continue;
                        pn.push_back(i);
                        for(long j=i*1;j<=max;j+=i)npn[j] = true;
                }

                cout << pn.size() << "\n";
                pn.clear();
        }

        return 0;
}

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

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;

#define MOD 1000000007
#define REP(i, N) for (int i = 0; i < N; ++i)
#define REP1(i, N) for (int i = 1; i <= N; ++i)
#define RREP(i, N) for (int i = N - 1; i >= 0; --i)
#define ALL(a) a.begin(), a.end()

int main() {
  bool is_prime[1000000];
  REP1(i, 999999) is_prime[i] = true;
  is_prime[0] = is_prime[1] = false;
  for (int i = 2; i * i <= 999999; ++i) {
    if (is_prime[i]) {
      for (int j = 2; i * j <= 999999; ++j) is_prime[i * j] = false;
    }
  }
  int sum[1000000];
  sum[0] = 0;
  REP1(i, 999999) sum[i] = sum[i - 1] + is_prime[i];
  int n;
  while (cin >> n) cout << sum[n] << endl;
  return 0;
}

C++: #include<iostream>

#define LIM 1000000
using namespace std;

int main(){
  bool num_map[LIM];
  int in_num,prime_count;
  for(int i=0;i<LIM;i++){
    num_map[i]=1;
  }
  
  for(int i=2;i<LIM;i++){
    if(num_map[i]==1)
      for(int j=i*2;j<LIM;j+=i){
	num_map[j]=0;
      }   
  }
  while(cin>>in_num){
    prime_count = 0;
    //    cin>>in_num;
    //cout<<in_num<<endl<<endl;
    for(int i=2;i<=in_num;i++){
      if(num_map[i]==1)
	prime_count++;
    }
    cout<<prime_count<<endl;  
  }
  
}

C++: #include<stdio.h>
int main(){
	long int a,b,n;
	long int p[80000];
	p[0]=2;
	n=1;
	for(a=3;a<1000000;a++){
		for(b=0;b<n;b++){
			if(a%p[b]==0){break;}
			if(a<p[b]*p[b]){
				p[n]=a;
				n++;
				break;
			}
		}
	}
	for(;;){
		if(scanf("%ld",&a)==EOF){break;}
		n=0;
		while(a>=p[n]){
			n++;
		}
		if(a<2){n=0;}
		printf("%ld\n",n);
	}
	return 0;
}

C++: #include<iostream>
#include<vector>
using namespace std;
int main(){
	int time;
	vector<int> v;
	v.push_back(2);
	while(cin>>time){
		int abcd=0;
		if(time>v[v.size()-1]){
			for(int abc=1+v[v.size()-1];abc<=time;abc++){
				for(int bcd=0;bcd<(int)v.size() ;bcd++){
					if(abc%v[bcd]==0){
						goto end;
					}
				}
				v.push_back(abc);
				end:;
			}
		}
		for(int abc=0;v[abc]<=time;abc++){
			abcd++;
		}
		cout<<abcd<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <numeric>
#define MAX 1000000
using namespace std;

bool isPrime(int x) {
  if(x == 2) {
	return true;
  }
  
  if(x < 2 || x % 2 == 0) {
	return false;
  }
  for(int i = 3; i <= sqrt(x); i += 2) {
	if(x % i == 0) {
	  return false;
	}
  }
  return true;
}

int main() {
  int n;
  bool a[MAX] = {};

  for(int i = 1; i < MAX; ++i) {
	if(isPrime(i)) {
	  a[i] = true;
	}
  }
  /*
  for(int i = 1; i < MAX; ++i) {
	if(isPrime(i)) {
	  cout << i << " ";
	}
  }
  */
  while(cin >> n) {
	cout << accumulate(&a[1], &a[n + 1], 0) << endl;
  }
  return 0;
}

/*-- primitive algorithm --*/
/*
bool isPrime(int n) {
  for(int i = 2; i < n; ++i) {
	if(n % i == 0) return false;
  }
  return true;
}
*/

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

bool a[1000001] ;
int main(){
    a[0] = 1 ;
    a[1] = 1 ;
    vector<int> v ;
    for(int i=2;i<=1000000;i++){
        if ( a[i] == 0 ){
            v.push_back(i);
            for(int j=i+i;j<=1000000;j+=i){
                a[j] = 1 ;
            }
        }
    }

    int x ;
    while(cin >> x){
        long long c = 0 ;
        for(int i=0;i<v.size();i++){
            if ( v[i] > x ) break;
            c++;
        }
        cout << c << endl;
    }



    return 0 ;
}

C++: #include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <vector>
using namespace std;

constexpr int LIM = 1000000;

static char isprime[LIM];
static int numofprime[LIM];

void setprime(int N) {
    fill(isprime, isprime + N + 1, 1);
    isprime[0] = isprime[1] = 0;
    for(int x = 2, lim = (int)sqrt(N); x <= lim; ++x) {
        if(isprime[x]) {
            for(int y = x * x; y <= N; y += x) {
                isprime[y] = 0;
            }
        }
    }
    numofprime[0] = 0;
    for(int x = 0; x < N; ++x) {
        numofprime[x + 1] = numofprime[x] + isprime[x + 1];
    }
}

int main() {
    setprime(999999);
    int n;
    while(scanf("%d", &n) != EOF) {
        printf("%d\n", numofprime[n]);
    }
    return 0;
}

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

using namespace std;

int prime[10000];
bool is_prime[1000000];

int sosuu(int n){
    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++]=i;
            for (int j=i*2; j<=n; j+=i) {
                is_prime[j]=false;
            }
        }
    }
    return p;
}

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