Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

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

int main()
{
    const int NUM_OF_INPUT = 30;

    for(int k=0; k<NUM_OF_INPUT; k++)
    {
        long long limit;

        cin >> limit;

        long long count = 0;
        int p = 2, i, j;
        bool isPrime[limit];

        for(i=0; i<limit; i++)
        {
            isPrime[i] = true;
        }

        for(i=p-1; i<limit; )
        {
            for(j=2; (p * j) <=limit; j++)
            {
                isPrime[(p * j) - 1] = false;
            }

            while(++i < limit)
            {
                if(isPrime[i])
                {
                    p = i + 1;
                    break;
                }
            }
        }

        for(i=1; i<limit; i++)
        {
            count = (isPrime[i] ? count + 1 : count);
        }

        cout << count;
    }

    return 0;
}

C++: #include<iostream>

typedef int moe;
typedef long lolita;
typedef char cuffs;
typedef float frill;
typedef double ruffle;
typedef void voile;
typedef unsigned tasogare;

#define katyusya const
#define apron volatile
#define pleats static
#define fancy virtual

using namespace std;

#define YUU 1000000

voile make_prime(moe n_prime[]){
  pleats moe is_prime[YUU];

  for( moe ai = 0; ai < YUU; ++ai )
    is_prime[ai] = 1;

  is_prime[0] = 0;
  is_prime[1] = 1;
  is_prime[2] = 1;

  for( moe ai = 2; ai < YUU; ++ai )
    if( is_prime[ ai ] )
      for( moe mai = 2; ai * mai < YUU; ++mai)
	is_prime[ ai * mai ] = 0;

  for( moe ai = 0; ai < YUU; ++ai )
    n_prime[ ai ] = 0;

  for( moe ai = 2; ai < YUU; ++ai ){
    n_prime[ ai ] = n_prime[ ai - 1 ];
    if( is_prime[ ai ] )
      n_prime[ ai ]++;
  }
}

moe main(){
  tasogare yuu;
  pleats moe n_prime[YUU];

  make_prime( n_prime );
  while(true){
    cin >> yuu;
    if( yuu == 0 || cin.eof() ) break;
    cout << n_prime[ yuu ] << endl;
  }

  return 0;
}

C++: #include <cstdio>

using namespace std;

int main()
{
    long long n;

    while (scanf("%lld", &n) != EOF){
        int cnt = 0;
        for (long long i = n; i >= 2 ; i--){
            for (long long j = 2; j < i; j++){
                if (i % j == 0){
                    //printf("%d\n", i);
                    cnt++;
                    break;
                }
            }
        }
        printf("%d\n", n - 1 - cnt);
    }

    return (0);
}

C++: #include <iostream>

using namespace std;

int main() {
	int num=0,y=0, n;
	while(cin>>n){
		for(int i=2; i<=n; i++){
			y = 0;
			for(int j=2; j<i; j++){
				if(i%j==0){ y++;}
			}
			if(y==0){ num++; }
		}
		cout<<num<<endl;
	}
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;

vector<int> sieve;

void make_prime(){
  bool a[1000001];
  for(int i = 0; i < 1e6; i++)a[i] = false;
  a[0] = true;
  a[1] = true;
  for(int i = 2; i <= 1e6; i++){
    if(a[i])continue;
    sieve.push_back(i);
    for(int j = i; j <= 1e6; j += i){
      a[j] = true;
    }
  }
}

int main(){
  int n;
  make_prime();
  while(cin >> n){
    int cnt = 0;
    for(int i = 0; sieve[i] <= n; i++)cnt++;
    cout << cnt << endl;
  }
}

C++: #include<iostream>
#include<cstdio>
#include<string>
#include<cstring>
#include<cstdlib>
#include<sstream>
#include<fstream>
#include<vector>
#include<list>
#include<deque>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<cmath>
#include<utility>
#include<numeric>
#include<iterator>
#include<algorithm>
#include<functional>
#include<ctime>
#include<cassert>
using std::cin;
using std::cout;
using std::endl;
typedef long long ll;
typedef unsigned long long ull;
typedef std::pair<int,int> P;
#define FOR(i,init,len) for(int i=(init);i<(len);++i)
#define For(i,init,len) for(int i=(init);i<=(len);++i)
#define fi first
#define se second
#define pb push_back
#define is insert
namespace IO {
	inline char getchar() {
		static const int BUFSIZE=5201314;
		static char buf[BUFSIZE],*begin,*end;
		if(begin==end) {
			begin=buf;
			end=buf+fread(buf,1,BUFSIZE,stdin);
			if(begin==end) return -1;
		}
		return *begin++;
	}
}
inline void read(int &in) {
	int c,symbol=1;
	while(isspace(c=IO::getchar()));
	if(c=='-') { in=0;symbol=-1; }
	else in=c-'0';
	while(isdigit(c=IO::getchar())) { in*=10;in+=c-'0'; }
	in*=symbol;
}
inline int read() { static int x;read(x);return x; }
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a; }
ll lcm(ll a,ll b) { return a/gcd(a,b)*b; }
#define PA(name,init,len) cout<<#name"["<<(len-init)<<"]=";FOR(_,init,len) cout<<name[_]<<" \n"[_==(len-1)];
#define Pa(name,init,len) cout<<#name"["<<(len-init+1)<<"]=";For(_,init,len) cout<<name[_]<<" \n"[_==(len)];
#define PV(name) cout<<#name"="<<name<<'\n';

const int maxn=1e6+10;
int pr[maxn];
bool vis[maxn];
int n;

int main() {
#ifdef MengLan
	int Beginning=clock();
	//freopen("in","r",stdin);
	//freopen("out","w",stdout);
#endif // MengLan

    memset(vis,1,sizeof(vis));
    vis[0]=vis[1]=false;
    For(i,2,maxn) if(vis[i]){
        int j=i*2;
        while(j<=maxn) vis[j]=false,j+=i;
    }
    For(i,2,maxn) pr[i]=pr[i-1]+vis[i];
    while(scanf("%d",&n)==1) printf("%d\n",pr[n]);

#ifdef MengLan
	printf("Time: %d\n",clock()-Beginning);
    //system("pause");
#endif // MengLan
	return 0;
}

C++: #include <iostream>
#define N 100000
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;

int main(void)
{
    bool a[N+1];
    int prime[N], n;
    rep(i,N)
    {
        a[i] = true;
    }
    a[0] = a[1] = false;
    for(int i=2;i*i<=N;i++)
    {
        if(a[i])
        {
            prime[i] = prime[i-1] + 1;
            for(int j=i*i;j<N;j+=i)
                a[j] = false;
        }
        else
            prime[i] = prime[i-1];
    }

    while(scanf("%d", &n)!=EOF)
        printf("%d\n", prime[n]);
    
    return 0;
}

C++: #include<stdio.h>
#include<float.h>
#include<vector>



int makePrimenumbers(int a, int ref_max, vector<int> primes){
	int temp;
	
	if(a<ref_max)return 0;

	temp = ref_max;
	ref_max=a;
	int i,k,isp=1;

	for(k=temp;k<ref_max;k++){
		for(i=0;i<primes.size();i++){
			if(k%primes[i]==0){
				isp=0;
				break;
			}
		}
		if(isp==1){
			primes.push_back(k);
		}
		isp=1;
	}
	return 0;
}

int countprime(int a, voctor<int> primes){
	int i;
	for(i=0;i<prime.size();i++)
		if(a<prime[i])
			break;
	return (i);
}

int main(void)
{
	vector<int> primes;
	int ref_max=2;
	int result;
	int a;
	while(scanf("%d",&a)!=EOF){
		makePrimenumbers(a, max_ref, primes);
		result = countprime(a, primes);
		printf("%d\n",result);
	}
  return 0;
}

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

int main(void){
  int n;
  short int p[1000000];
  p[0] = p[1] = 0;
  p[2] = 1;
  int ans = 0;
  bool flag;
  for(int i = 3; i <= 1000000; i ++){
    if(i % 2 != 0){
      flag = true;
      for(int j = 3; j * j <= i; j += 2){
	if(i % j == 0){
	  flag = false;
	}
      }
      if(flag)
	ans++;
    }
    p[i] = ans;
  }
  // cout << "end" << endl;
  while(cin >> n){
    cout << p[n] << endl;
  }
  return 0;
}