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++: #include <cstdio>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <climits>
#include <ctime>
#include <queue>
#include <stack>
#include <algorithm>
#include <list>
#include <vector>
#include <set>
#include <map>
#include <iostream>
#include <deque>
#include <complex>
#include <string>
#include <iomanip>
#include <sstream>
#include <bitset>
#include <valarray>
#include <iterator>
using namespace std;
typedef long long int ll;
typedef unsigned int uint;
typedef unsigned char uchar;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;

#define REP(i,x) for(int i=0;i<(int)(x);i++)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();i++)
#define RREP(i,x) for(int i=((int)(x)-1);i>=0;i--)
#define RFOR(i,c) for(__typeof((c).rbegin())i=(c).rbegin();i!=(c).rend();i++)
#define ALL(container) container.begin(), container.end()
#define RALL(container) container.rbegin(), container.rend()
#define SZ(container) ((int)container.size())
#define mp(a,b) make_pair(a, b)
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
os<<"["; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"]"; return os;
}
template<class T> ostream& operator<<(ostream &os, const set<T> &t) {
os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}

const int INF = 1<<28;
const double EPS = 1e-8;
const int MOD = 1000000007;

vi eratosthenes_sieve(int n){
	vi res(n+1);
	if(n<2) return res;
	res[2]=1;
	for(int i=3;i<=n;i+=2){
		if(i*i<=n && !res[i]) for(int j=i+i+i;j<=n;j+=i+i) res[j]=1;
		res[i]=!res[i];
	}
	return res;
}
vi prime_list(const vi& isprime){
	vi res;if(isprime.size()<=2)return res;
	res.push_back(2);
	for(unsigned int i=3;i<isprime.size();i+=2)
		if(isprime[i])res.push_back(i);
	return res;
}
vi prime_list(int n){
	return prime_list(eratosthenes_sieve(n));
}

int n;

main(){
vi pl = prime_list(1100000);
	while(cin >> n){
		cout << upper_bound(ALL(pl), n) - pl.begin() << endl;
	}
	return 0;
}

C++: #include<stdio.h>
const int MAX_V = 1000000;
int prime[MAX_V+1];
int main() {
  int i, k;
  for(i = 2; i <= MAX_V; i++) {
    prime[i] = 1;
  }
  for(i = 2; i*i <= MAX_V; i++) {
    if(prime[i]) {
      for(k = 2 * i; k <= MAX_V; k += i) {
        prime[k] = 0;
      }
    }
  }

  int right;
  int sum[MAX_V+1] = {0};
  sum[1] = 0;
  for(i = 1; i < MAX_V+1; i++){
    if(prime[i] == 1){
      sum[i + 1] = sum[i] + 1;
    }else{
      sum[i + 1] = sum[i];
    }
  }
  while(scanf("%d", &right) != EOF){
    printf("%d\n", sum[right + 1]);
  }
  return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<math.h>
using namespace std;
int n;
int main(){
	while(cin>>n){
		int sum=0;
		if(n<=10000){
			for(int i=2;i<=n;i++){
				for(int j=2;j<=sqrt(i);j++){
					if(i%j==0){
						goto loopa;
					}
				}
				sum++;
				loopa:;
			}
			cout<<sum<<endl;
		}
		else if(n<=100000){
			for(int i=10001;i<=n;i++){
				for(int j=2;j<=sqrt(i);j++){
					if(i%j==0){
						goto loopb;
					}
				}
				sum++;
				loopb:;
			}
			cout<<sum+1229<<endl;
		}
		else{
			for(int i=100001;i<=n;i++){
				for(int j=2;j<=sqrt(i);j++){
					if(i%j==0){
						goto loopc;
					}
				}
				sum++;
				loopc:;
			}
			cout<<sum+9592<<endl;
		}
	}
	return 0;
}

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

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

C++: #include<iostream>
#include<vector>
using namespace std;
int main(){
	int n;
	vector<int> pri(1000000);
	for(int i=0;i<1000000;i++)
		pri[i]=1;
	pri[0]=0;
	pri[1]=0;
	for(int i=0;i*i<n;i++){
		if(pri[i]){
			for(int j=i*2;j<1000000;j+=i)
				pri[j]=0;
		}
	}
	
	
	while(cin>>n){
	int i=0,k=0;
	while(k<=n)
	if(pri[k++])
	i++;
	
		cout<<i<<endl;
	}
	return 0;
}

C++:  #include<iostream>
 #include<stdio.h>
 #include<string>
 #include<math.h>
 #include<iomanip>
 #include<algorithm>
 #include<string.h>
 #include<cctype>
 #include<map>
 #include<set>
 #include<vector>
 #include<sstream>
 
 using namespace std;
 
 int kk[10*10];
 
 bool isPrime(int x)
 {
   for(int i=2;i<=(int)(sqrt(x));i++) 
   {
     if(x%i==0) return false;        
   }    
   return true;
 }
 
 int main()
 {
   int n;
   while(cin>>n)
   {
      int count=0;
      for(int i=2;i<=n;i++) if(isPrime(i)) count++;
      cout<<count<<endl; 
   }
   //while(1);      
   return 0;    
 }

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

int a[1000000];

int sosuu(){
	a[0] = 0;
	a[1] = 0; 
	int k;
	for(int i = 2;i<1000000;i++){
		k = 0;
		a[i] = a[i - 1];
		for(int j=2;j<=sqrt(i);j++){
			if(i%j == 0){
				k++;
				break;
			}
		}
		if(k==0){
			a[i]++;
		}
	}
	return 0;

}

int main(){
	int n;
	sosuu();
	while (true)
	{
		cin >> n;
		if (cin.eof()) { break; }
		cout << a[n] << endl;
	}
	return 0;
}

C++: #include <iostream>
#include<stdio.h>
#include<string>
#include<map>
#include<vector>
#include<stdlib.h>
typedef long long ll;
using namespace std;
#define INF 100000000000
#include<algorithm>
#include<string.h>
bool num[1000000];
int main(){
    int n;
    while(cin >> n){
        memset(num,true,sizeof(num));
        num[0] = num[1] = false;
        for(int i = 2;i<=n;i++){
            if(num[i] == true){
                for(int j = 2*i;j<=n;j+=i){
                    num[j] = false;
                }
            }

        }
        int ans = 0;
        for(int i =2;i<=n;i++){
            if(num[i]){ans++;}
        }
        cout<<ans<<endl;
    }

}

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

using namespace std;

int HowManyPrimeNumbers1(const int);
int HowManyPrimeNumbers2(const int);
int MySqrt(const int);

int main(){
	int n;

	while(cin >> n){
		printf("%d\n",HowManyPrimeNumbers2(n));
	}

	return 0;
}

int HowManyPrimeNumbers1(const int n){
	int i,j,buf,cnt = 1;

	for(i=3;i<=n;i+=2){
		buf = (int)sqrt((double)i);

		for(j=3;j<=buf;j+=2){
			if((i % j) == 0){
				cnt--;
				break;
			}
		}

		cnt++;
	}

	return cnt;
}

int HowManyPrimeNumbers2(const int n){
	int i,j,cnt,root,rt;

	// 3 から n までの奇数の個数 + 1 を n に代入
	cnt = ((int)((((double)(n - 2)) / 2) + 0.5)) + 1;

	// 最大処理数を求める
	root = (int)sqrt((double)n);

	// a
	for(i=3;i<=root;i+=2){
		rt = MySqrt(i);
		for(j=3;j<=rt;j+=2){
			if((i % j) == 0){
				cnt += ((int)(n / i));
				break;
			}
		}
		cnt -= ((int)(n / i));
	}

	return n;
}

int MySqrt(const int i){
	int n;

	n = i - 1;

	return (int)(1 + (1 / 2) * (double)(i - 1) - (1 / 8) * (double)(i - 1) * (i - 1) + (1 / 16) * (double)(i - 1) * (i - 1) * (i - 1));
}