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 <cstdio>

int Eratos[1000000];

int main()
{
	Eratos[0] = Eratos[1] = 0;
	for (int i = 0; i < 1000000; i++){
		Eratos[i] = 1;
	}
	for (int i = 2; i < 1000; i++) {
		if (Eratos[i]) {
			for (int j = 2 * i; j < 1000000; j += i) {
				Eratos[j] = 0;
			}
		}
	}
	int n,res;
	res = 1;
	for (int i = 2; i < 1000000; i++) {
		if (Eratos[i])
			Eratos[i] = res;
		res++;
	}
	while (scanf("%d", &n)) {
		res = 0;
		while (!Eratos[n]) {
			n--;
		}
		printf("%d\n",Eratos[n]);
	}
	return 0;
}

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

inline bool prime(int n) {
	for(int i=2;i<=sqrt(n);++i) {
		if(n%i==0) { return false; }
	}
	return true;
}

int main() {
	int n;
	while(scanf("%d",&n) != EOF) {
		int cnt=0;
		for(int i=2;i<=n;++i) {
			if(prime(i)) { ++cnt; } 
		}
		printf("%d\n",cnt);
	}
	return 0;
}

C++: #include <iostream>

using namespace std;

#define M 10000000
bool prime[M];
int main()
{
	
	int ans, n;

	for (int i = 2; i * i < M; i++){
		if (prime[i] == false){
			for (int j = i; j < M; j += i){
				if (!(i == j)) {
					prime[j] = true;
				}
			}
		}
	}
	while (cin >> n){
	  ans = 0;
		for (int i = 2; i <= n; i++){
	 	 if (prime[i] == false) ans++;
	  }
		cout << ans << endl;
	}
	return (0);
}

C++: #include <stdio.h>

int main(void){
int n;
while(scanf("%d",&n)!=EOF){
int count=0;
for(int m=2;m<=n;m++){
bool check=true;
for(int i=2;i<m;i++){
if(i!=m && m%i==0){
check=false;
}
}
if(check==true){
count++;
}
}
}
return 0;
}

C++: #include<bits/stdc++.h>
#define rep(i,n) for(int i=0; i<n; i++)
using namespace std;
int main(){
  int n;
  while(cin >> n){
    int cnt=0,res=0;
    for(int i=2; i<=n; i++){
      for(int j=1; j<=i; j++){
        if(i%j == 0) cnt++;
        if(cnt > 2) break;
      }
      if(cnt==2) res++;
      cnt = 0;
    }
    cout << res << endl;
  }
  return 0;
}

C++: #include <stdio.h>

const int MAX_V = 1000000;
int prime[MAX_V+1];
int a[MAX_V] = {'0'};
int sum[MAX_V+1];
int main() {
  int i, k, v, j, m, n;
  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;
      }
    }
  }
  sum[0] = 0;
  for(m = 0; m < MAX_V; m++) {
    sum[m+1] = sum[m] + prime[m];
  }
  while( scanf( "%d", &n ) != EOF ) {
    for( j = 2; j <= n; j++ ) {
      int isprime = prime[j];
      int isprimee = prime[j-2];
    }
    printf( "%d\n", sum[n+1] );
  }
  return 0;
}

C++: #include<cstdio>
#include<string.h>
using namespace std;
int pNum;
bool IsPrime[1000000];
int Prime[1000000];
void GetPrime(){
    memset(IsPrime,true,sizeof(IsPrime));
    IsPrime[0]=IsPrime[1]=false;
    pNum=0;
    for(int i=2;i<1000000;i++){
        if(IsPrime[i])Prime[++pNum]=i;
        for(int j=1;j<=pNum&&Prime[j]*i<1000000;j++){
            IsPrime[Prime[j]*i]=false;
            if(i%Prime[j]==0)break;
        }
    }
}
int main(){
    GetPrime();
    int n;
    while(scanf("%d",&n)){
        int count=0;
        for(int i=1;i<=pNum;i++){
            if(Prime[i]<n)count++;
            if(Prime[i]==n){
                count++;break;
            }
            if(Prime[i]>n)break;
        }
        printf("%d\n",count);
    }
    return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#define maxn 1000000
using namespace std;
int prime[maxn],number[maxn];
void getprime()
{
    int m=sqrt(maxn+0.5);
    for(int i=2;i<=m;i++)
    {
        if(!prime[i])
        {
            for(int j=i+i;j<maxn;j+=i)
            {
                prime[j]=1;
            }
        }
    }
    int sum = 0;
    for(int i=2;i<maxn;i++)
    {
        if(!prime[i])
        {
            sum++;
        }
        number[i]=sum;
    }
}
int main ()
{
    int n;
    getprime();
    while(scanf("%d",&n)!=EOF)
    {
        printf("%d\n",number[n]);
    }
}
/**
--------------------- 
???zhaiqiming2010 
???CSDN 
???https://blog.csdn.net/zhaiqiming2010/article/details/62281598 
?????????????????????????
**/

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

int main(){

	const int N_MAX=1000000;
	int i,j,n[32],ni,soi,max;
	int so[N_MAX],count[N_MAX];
	
	for(i=0;i<N_MAX;i++)
		count[i]=so[i]=0;
	ni=0;
	max=0;
	while(cin>>n[ni])
		if(n[ni++]>max)
			max=n[ni-1];
	int sq_max = (int)sqrt(max);
	so[0]=so[1]=1;
	for(soi=0,i=2;i<=sq_max;i++){
		if(so[i]>0)continue;
		for(j=i*2;j<=max;j+=i)
			so[j]=1;
	}
	for(i=2;i<=max;i++){
		count[i] = count[i-1]+1-so[i];
	}
	for(i=0;i<ni;i++)
		cout << count[n[i]] << endl;

	return 0;
}

C++: #include <cstdio>

int Eratos[1000000];

int main()
{
	Eratos[0] = Eratos[1] = 0;
	for (int i = 2; i < 1000000; i++){
		Eratos[i] = 1;
	}
	for (int i = 2; i < 1000; i++) {
		if (Eratos[i]) {
			for (int j = 2 * i; j < 1000000; j += i) {
				Eratos[j] = 0;
			}
		}
	}
	int n,res;
	res = 1;
	for (int i = 2; i < 1000000; i++) {
		if (Eratos[i]) {
			Eratos[i] = res;
			res++;
		}
	}
	while (scanf("%d", &n)) {
		res = 0;
		while (!Eratos[n]) {
			n--;
		}
		printf("%d\n",Eratos[n]);
	}
	return 0;
}

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

template<typename T>
struct Sieve{
    vector<bool> is_prime;
    vector<T> prime;
    T prime_count;
    Sieve(T n):is_prime(n+1,true),prime_count(0){
        is_prime[0] = false;
        is_prime[1] = false;
        for (T i=2;i<=n;i++){
            if (is_prime[i]){
                prime.push_back(i);
                prime_count++;
                for (T j=2*i; j<=n ;j+=i) is_prime[j]=false;
            }
        }
    };
};

int main(){
    ll max_n = 1000000;
    ll n;
    Sieve<ll> a(max_n);
    vector<ll> prime_count(max_n,0);
    ll count = 0;
    for (ll i=0;i<max_n;i++){
        if (a.is_prime[i]) count++;
        prime_count[i] = count;
    }
    while(scanf("%lld",&n)==1){
        ll c= prime_count[n];
        printf("%lld\n", c);
    }
}

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

using namespace std;

#define MAX_N 99999
#define MAX_D 30

bool prime_check(int n, int k = 2)
{
	if (n < k * k) { return true; }

	if (n % k == 0) { return false; }

	return prime_check(n, k + 1);
}

int main()
{
	int n;

	bool isprime[MAX_N + 1];

	for (int i = 2; i < MAX_N + 1; i++)
	{
		isprime[i] = prime_check(i);
	}

	while (cin >> n)
	{
		int c = 0;

		for (int i = 2; i <= n; i++)
		{
			if (isprime[i] == true)
			{
				c++;
			}
		}

		cout << c << endl;
	}

	return 0;
}