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<iostream>
#include<cstdio>
#include <cstring>
using namespace std;
int main()
{
	int n;
	while(scanf("%d",&n)!=EOF)
	{
		int count=0;
		bool a[1000001]; 
		memset()
		for(int i=2;i*i<=n;i++)
		{
			if(a[i]==false)
			{
				for(int j=i;j<=n;j+=i)
			{
				if(j==i)
				continue;
				a[j]=true;
				
				
			}
			}
		  
		}
		for(int k=2;k<=n;k++)
		{
			if(!a[k])
			count+=1;
		}
		cout<<count<<endl;
	
		
	}
}

C++: #include <iostream>
#include <vector>
using namespace std;
int main( void )
{
	int n;
	cin >> n;
	
	vector <bool> p(n+1, true);
	p[2] = true;
	for ( int i = 3; i * i < n; i += 2 )
	{
		if ( p[i] )
		{
			for ( int j = i+i; j <= n; j += i )
			{
				p[j] = false;
			}
		}
	}

	int kotae = 1;
	for ( int i = 3; i <= n; i += 2 )
	{
		if ( p[i] )
			kotae++;
	}

	cout << kotae << endl;

	return 0;
}

C++: #include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <limits>
#include <map>
#include <string>
using namespace std;

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

int main(){
int n;


while(~scanf("%d",&n)){
int* table =  new int[n+1];
fill((int*)table,(int*)table+n+1,1);
table[0]=0;
table[1]=0;

for(int i=2;i*i<=n;i++){
if(!is_prime(i)) {
for(int j=i;j<=n;j+=i){
table[j] = 0;
}
}
}
printf("%d\n",count(table,table+1000000,1));
}
}

C++: #include <bits/stdc++.h>

using namespace std;

bool p[100000000];
vector<int> prime;

void init(){
    memset(p,1,sizeof(p));
    p[0] = p[1] = false;
    for(int i = 2; i <= 1000000; i++){
        if(!p[i]) continue;
        for(long long j = 1LL*i*i; j <= 1000000; j += i)
            p[j] = false;
        prime.push_back(i);
    }
}

int CntPrime(int n){
    vector<int>::iterator it = lower_bound(prime.begin(), prime.end(), n);
    if( *it > n) it--;
    return (it - prime.begin() + 1);
}

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

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

int Era(int N) {
	int prime_num = 0;
	int* array = new int[N + 1];
	for (int i = 2; i < N + 1; i++) {
		array[i] = 1;
	}
	for (int i = 2; i < sqrt(N); i++) {
		if (array[i]) {
			for (int j = 0; i*(j + 2) <= N; j++) {
				array[i*(j + 2)] = 0;
			}
		}
	}
	for (int i = 2; i < N + 1; i++) {
		if (array[i])prime_num++;
	}
	return prime_num;
}

void add(int* primes, int& now,int prime) {
	primes[now] = prime;
	now++;
}

int main() {
	int num;
	int primes[30]; int now = 0;
	while (cin >> num) {
		add(primes, now, Era(num));
	}
	for (int i = 0; i < now; i++) {
		cout << primes[i] << endl;
	}
	return 0;
}

C++: #include <bits/stdc++.h>

#define times(n, i) uptil(0, n, i)
#define upto(f, t, i) for(auto _##i = (t), i = decltype(_##i)(f) poi  i <= _##i poi  i++)
#define uptil(f, t, i) for(auto _##i = (t), i = decltype(_##i)(f) poi  i < _##i poi  i++)
#define downto(f, t, i) for(auto i = (f), _##i = decltype(i)(t) poi  i >= _##i poi  i--)
#define downtil(f, t, i) for(auto i = (f), _##i = decltype(i)(t) poi  i > _##i poi  i--)

#define unless(c) if(!(c))
#define until(c) while(!(c))
#define loop while(true)

#define poi ;
#define poipoi << "\n"

using namespace std;

#define fuwafuwa(n, c) dokidoki((n) + (c) - 1, c)
#define dokidoki(n, c) ((n) / (c) * (c))

template<class T> bool read(T& t){
    return cin >> t poi
}
template<class T, class...U> bool read(T& t, U&... u){
    unless(cin >> t) return false poi
    return read(u...) poi
}

template<class T> T lov_yet(T t){
    if(t) return t poi
    else return 0 poi
}

template<class T> T gcd(T a, T b){
    if(b) return gcd(b, a % b) poi
    else return a poi
}

int main(){
    vector<int> count(1000000, 1) poi
    count[0] = count[1] = 0 poi

    times(1000000, i)
        if(count[i])
            upto(2, 1000000 / i, j)
                count[i * j] = 0 poi

    upto(1, 1000000, i)
        count[i] += count[i - 1] poi

    int n poi
    while(read(n)){
        cout << count[n] poipoi poi
    }

    return 0 poi
}

C++: #include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
using namespace std;
const int maxn=10000001;
int is_prime[maxn],prime[maxn];
int p;
void judge_prime()
{
    p=0;
    for(int i=0;i<maxn;i++)
        is_prime[i]=1;
    is_prime[0]=is_prime[1]=0;
    for(int i=2;i<maxn;i++)
    {
        if(is_prime[i])
        {
            prime[++p]=i;
            for(int j=i*2;j<maxn;j+=i)
                is_prime[j]=0;
        }
    }
}
int search(int n)
{
    int left=1,r=p;
    while((r-left)>1)
    {
        int mid=(left+r)/2;
        if(prime[mid]==n) return mid;
        else if(prime[mid]>n) r=mid;
        else left=mid;
    }
    return left;
}
int main()
{
    int n;
    judge_prime();
    while(scanf("%d",&n)!=EOF)
    {
        if(n<=1)
        {
            int nn=0;
            printf("%d\n",nn);
        }
        else printf("%d\n",search(n));
    }
}

C++: #include <iostream>

using namespace std;

int isprime(int x){
	if(x < 2){
		return false;
	}else{
		for(int i=2;i<x;i++){
			if(x%i==0){
				cout << x << " " << i << endl;
				return false;
			}
		}
		return true;
	}
}
int main(){
	int n;
	while((cin >> n)){
	int a = 0;
	for(int i=2;i<=n;i++){
		if(isprime(i)){
			a++;
		}
	}
	cout << a << endl;
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
 
int main(){
  static bool cmp[1000000]={true};
  static int npr[1000000];
  for(int i=2;i<1000000;i++){
    if(!cmp[i]){
      for(int j=2;j*i<1000000;j++){
    	cmp[i*j]=true;
      }
    }
    npr[i]=npr[i-1]+!cmp[i];
  }
  for(int n;cin>>n;n++){
    cout<<npr[n]<<endl;
  }
}

C++: #include<iostream>
#include<string>
using namespace std;
int main(){
	int n;
	bool a[1000000]={false};
	for(int i=2;i<1000;i++){
		bool t=false;
		for(int j=2;j*j<=i;j++){
			if(i%j==0)
				t=true;
		}
		if(i==2)
			t=false;
		if(t==false){
			for(int j=i*2;j<1000000;j+=i)
			    a[j]=true;
			a[i]=false;
		}
	}
	while(cin>>n){
		int count=0;
		for(int i=0;i<=n;i++){
			if(a[i]==false)
				count++;
		}
		cout<<count-2<<endl;
	}
	return 0;
}

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

int main(){
  int n,i,j;
  while(cin >> n) {
    
    int k = 0, a[n];
    a[0] = 2;
  
    for (i = 3; i <= n; i++) {
      for (j = 0; j <= k; j++) {
        if(i % a[j] == 0 ) break;
        if(i > a[j] / 2) {
          k++;
          a[k] = i;
          break;
        }
      }
      if(j == k + 1) {
        k++;
        a[k] = i;
      }
    }
    for(i = 0; i <= k; i++) {
      cout << a[i] << endl;
    }
  }
  return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
  int a,t[1000000],u[1000000];
  for(int i=0;i<1000000;i++){
    t[i]=1;
  }
  t[0]=t[1]=u[0]=u[1]=0;
  for(int i=2;i<=1000000;i++){
    if(t[i]==1){
      for(int j=i+i;j<1000000;j+=i)t[j]=0;
      u[i]=u[i-1]+1;
    }else{
      u[i]=u[i-1];
    }
  }
  while(cin>>a){
    cout<<u[a]<<endl;
  }
  return 0;
}