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>
#include <vector>
#include <cstdio>

using namespace std;

vector<int> a(1000000, 1);

int main(void)
{
	a[0] = a[1] = 0;
	for (int i = 2; i < 1000000; i++) {
		if (a[i]) {
			for (int j = i * 2; j < 1000000; j += i) {
				a[j] = 0;
			}
		}
	}
	for (int i = 1; i < 1000000; i++) {
		a[i] += a[i - 1];
	}

	int n;
	while (scanf("%d", &n) != EOF) {
		cout << a[n] << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <vector>
#include <set>
#include <cmath>
std::vector<int> PRIMES;

void init_primes(int max)
{
  if (max < 2) return;
  std::set<int> candidate;
  PRIMES.push_back(2);
  for (int i = 3; i < max; i+=2) {
    candidate.insert(i);
  }
  for (int i = 3; i < sqrt(max); i+=2) {
    auto it = candidate.find(i);
    if (it != candidate.end()) {
      PRIMES.push_back(i);
      int del_num = i;
      while (del_num < max) {
        it = candidate.find(del_num);
        if (it != candidate.end()) candidate.erase(it);
        del_num += i;
      }
    }
  }
  for (auto it = candidate.begin(); it != candidate.end(); ++it) {
    PRIMES.push_back(*it);
  }
}

int main()
{
  const int max = 1000000;
  init_primes(max);
  int i;
  while (std::cin >> i) {
    auto it = PRIMES.begin();
    int n = 0;
    while (*it <= i && it != PRIMES.end()) {
      n++;
      it++;
    }
    std::cout << n << std::endl;
  }
  return 0;
}

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

int main(){

	int n,count;
	while(cin>>n){
		count=0;
		for(int i=2; i<=n; i++){
			int num = 0;
			for(int j=1; j<=i ;j++){
				if(i%j==0) num++;
			}
			if(num==2){
				count++;
			}
		}
		cout << count << endl;
	}
    return 0;
}

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

bool dec(int i, int prime[]) {
  for (int j = 0; prime[j] != 0; j++) {
    if (i % prime[j] == 0) {return false;}
  }
  return true;
}

int main() {
  int n;
  while (cin >> n) {
    int count = 0;
    int prime[1000000];
    for (int i = 0; i < 1000000; i++) {
      prime[i]=0;
    }
    if (n >= 2) {
      prime[0] = 2;
      count++;
    }
    for (int i = 3; i <= n; i += 2) {
      if (dec(i, prime)) {
	prime[count] = i;
	count++;
      }
    }
    cout << count << endl;
  }
  return 0;
}

C++: #include<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);i++)
#define loop(i,x,n) for(int i=(x);i<(n);i++)
#define all(x) (x).begin(),(x).end()
#define pb push_back
#define mp make_pair
#define fir first
#define sec second
#define ub upper_bound
#define lb lower_bound
#define int long long
using namespace std;
const int MOD=1000000007;
const int INF=1000000009;

bool Is_Prime(int n){
  if(n==1) return false;
  for(int i=2;i*i<=n;i++){
    if(n%i==0){
      return false;
    }
  }
  return true;
}

signed main(){
  int p[100006];
  loop(i,2,100001){
    if(Is_Prime(i))p[i]=1;
  }
  rep(i,100001)p[i+1]+=p[i];
  int n;
  while(cin>>n){
    cout<<p[n]<<endl;
  }
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int N, i, j, H[1000001];
bool A[1000001];
int main () {
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
	memset(A,true,1000000*sizeof(bool));
	A[1]=false;
	for (i=2;i<1000;i++) {
		if (A[i]) {
			for (j=i;j*i<1000000;j++) {
				A[i*j]=false;
			}
		}
	}
	H[1]=0;
	for (i=2;i<1000000;i++) {
		H[i]=H[i-1]+A[i];
	}
	while (cin>>N) {
		cout<<H[N]<<endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
#define FOR(I,X,Y) for(long long (I)=(X);(I)<(Y);(I)++)
#define REP(I,X,Y) for(long long (I)=(Y)-1;(I)>=(X);(I)--)
#define ALL(X) (X).begin(),(X).end()
#define pb push_back
#define COUNT(V,X) upper_bound((V).begin(),(V).end(),X)-lower_bound((V).begin(),(V).end(),X)
#define debug(x) cerr<<#x<<':'<<x<<endl;
#define DEBUG(v) cerr<<#v<<':';for(auto x:v)cerr<<x<<' ';cerr<<endl;
#define INF 1000000007
#define LINF 1000000000000000007
#define EPS 1e-9
typedef long long ll;

vector<long long> primelist(long long n){
  vector<bool> prime(n+1,1);
  for(long long i = 2;i <= n;i++){
    if(prime[i]){
      for(long long j = 2*i;j <= n;j += i){
        prime[j] = 0;
      }
    }
  }
  prime[0] = 0;
  prime[1] = 0;
  vector<long long> primelist;
  for(long long i = 0;i <= n;i++)if(prime[i])primelist.pb(i);
  return primelist;
}

signed main(){
  int n;
  while(cin >> n){
    cout << primelist(n).size() << endl;
  }
}

C++: #include <iostream>
#include <vector>
#include <iterator>
#include <cmath>

using namespace std;

typedef long long ll;

int main()
{
	ll n;
	vector<ll> pl;
	back_insert_iterator<vector<ll> > it(pl);
	
	while(cin>>n){
		//ú»
		pl.clear();
		//nª2ÈºÌÆ«Ì
		if(n < 2){
			cout<<0<<endl;
			continue;
		}
		//â¿Ìì¬
		*it = 2;
		for(ll i = 3; i <= sqrt((long double)n); i++){
			for(ll t = 0; t < pl.size(); t++){
				if(i % pl[t] == 0) break;
				if(t == pl.size() -1) *it = i;
			}
		}
		//â¿É©¯é
		int L = pl.size();
		for(ll i = sqrt((long double)n) +1; i <= n; i++){
			for(ll t = 0; t < L; t++){
				if(i % pl[t] == 0) break;
				if(t == L -1) *it = i;
			}
		}
		cout<< pl.size() <<endl;
	}
}

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

C++: #include <cstdio>

long MAX=999999;

int main(void){
    long int s[1000000]={0},k=0,i,j,n;
    
    for(i=2;i<=MAX;i++){
        if( !s[i] ){
            k++;
            for(j=2;j*i<=MAX;j++){
                s[j*i]=-1;
            }
        }
        s[i]=k;
    }
    
    
    while(EOF != scanf("%ld",&n)){
        printf("%ld\n",s[n]);
    }
    
    
    return 0;
}

C++: #include<stdio.h>
#include<string.h>

bool mark [1000000];
 void sieve ()
 {
    int N = 1000000;
    memset (mark, true, N + 2);

    mark [0] = mark [1] = false;

    for ( int i = 4; i < N + 2; i += 2 )
        mark [i] = false;

    for ( int i = 3; i * i <= N + 2; i += 2 ) {
        if ( mark [i] == true ) {
            for ( int j = i * i; j < N + 2; j += 2 * i )
                mark [j] = false;
        }
    }
 }
int main()
{
    int n;
    sieve();
    while(scanf("%d",&n)!=EOF){
        int count = 0,i;
        for(i=2;i<=n;++i)
        {
            if(mark[i]==true)
                count++;
        }
        printf("%d",count);
   }
}

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


int main() {

//変数の宣言

	int i,n,j,sum,judge=1;
	
	while(cin>>n){
		sum = 1;
		for(i=3;i<n+1;i+=2){
			for(j=3;j<=sqrt(i);j+=2){
				if(i%j==0){
					judge = 0;
					break;
				}
			}
			if (judge==1){
				sum++;
			}
			judge = 1;
		}
		
		cout << sum <<endl;
	}

	return 0;
}