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++: import math
import sys

def prime_calc(n):
    rootN = int(n**0.5)
    prime = [2]
    data = [i + 1 for i in range(2,n,2)]
    while True:
        p = data[0]
        if rootN <= p:
            return len(prime + data)
        prime.append(p)
        data = [e for e in data if e % p != 0]

def main():
    l = []

    for line in sys.stdin:
        l.append(line)        

    for line in l:
        print(prime_calc(line))

if __name__ == "__main__":
    main()

C++: #include<stdio.h>
int flg[1000000];
int main(void)
{

	int a,i,j,cnt;
	while(scanf("%d",&a)!=EOF){	
		for(i=2;i<=a;i++){
		flg[i]=1;
		}
		for(i=2;i*i<=a;i++){
			if(flg[i]=1){
				j=2;
				while(i*j<=a){
				flg[i*j]=0;
//				printf("i=%d j=%d a=%d\n",i,j,a);
				j++;
				}
			}
		}
//		for(i=2;i<=10;i++)	printf("flg=%d\n",flg[i]);
		cnt=0;
		for(i=2;i<=a;i++){
//		printf("i=%d\n",i);
			if(flg[i]==1){
//			printf("i=%d\n",i);
			
			cnt++;
			}
		}
		printf("%d\n",cnt);
	}	
	return 0;
}

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

void AOJ0009()
{
	int n = 0;
	int prime[10000];
	int count = 0;
	for (int i = 2; i <= 10000; i++)
	{
		int j = 2;
		bool flag = false;
		for (j = 2; j <= i; j++)
		{
			if (i % j == 0)
				break;
		}
		if (i == j)
		{
			prime[count] = i;
			count++;
		}
	}

	while (cin >> n)
	{
		count = 0;
		while (prime[count] <= n)
		{
			count++;
		}
		cout << count << endl;
	}
}
int main()
{
	//AOJ0005();
	//AOJ0007();
	AOJ0009();
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include<string>
#include<vector>
int main() {
	int n;
	while (std::cin >> n)
	{
		std::vector<bool> v(n + 1 ,false);
		int c = 0;
		v[0] = true, v[1] = true;
		for (int i = 2; i * i <= n ; ++i) {
			if (!v[i]) {
				for (int j = i * i; j <= n; j += i) {
					v[j] = true;
					++c;
				}
			}
		}

		std::cout << n-c-1 << std::endl;
	}
}

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

int main(){
int n;
cin >> n;
int a[n+1];
int b;
int c;
b=1;
while(b<n+1){
a[b]=0;
b=b+1;}
a[1]=1;
int d;
d=0;
b=2;
while(b<n+1){
if(a[b]==0){d=d+1; c=2; while(1){if(c*b>n){break;} a[c*b]=1; c=c+1;}}
b=b+1;}


cout << d;


}

C++: #include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
#include<vector>
#include<string.h>
using namespace std;
const int N=11111;
vector<int> prime;
int sieve(long long n){
   bool arr[n+1];
   int cnt=0;
   memset(arr,true,sizeof(arr));
   for(int i=2;i<=n;++i){
    if(arr[i]){
        prime.push_back(i);
        cnt++;
    }
    for(int j=0;j<prime.size()&&i*prime[j]<n;j++){
        arr[i*prime[j]]=false;
        if(i%prime[j]==0)break;
    }
   }
   return cnt;
}

int main(){
long long num;
int a;
while(true){
    cin>>num;
    if(!num) break;
    a=sieve(num);
    printf("%d\n",a);
}
return 0;
}

C++: #include <iostream>
using namespace std;
int main()
{
	int prime[80000];
	int ptr=2;
	prime[0] = 2;
	prime[1] = 3;
	for(int num=5; num<1000000; num+=2){
		bool f = false;
		for(int i = 0; i<ptr; i++){
			if (prime[i]*prime[i] > num) break;
			if (num%prime[i]==0) {
				f = true;
				break;
			}
		}
		if(!f) prime[ptr++] = num;
	}
	int n;
	while(cin >> n){
		int k=0;
		for(;prime[k]<=n;k++);
		cout << k << endl;
	}
	return 0;
}

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

using namespace std;

int main(void){
  int n;
  while(cin >> n){
    if(n==1){
      printf("%d\n",0);
      break;
    }
    int p[100000];
    p[0]=2;
    int pi=1;
    int i=2;
    while(i<=n){
      int t=1;
      int j;
      for(j=0;j<pi;j++){
	if(i%p[j]==0){
	  t=0;
	  break;
	}
      }
      if(t==1){
	p[pi]=i;
	pi++;
      }
      i++;
    }
    printf("%d\n",pi);
  }
  return 0;
}

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

class SOSU{
   public:
      int sosu(int);
};
int SOSU::sosu(int n){
   if(n==2)
      return 1;
   
   int i,j,z=1,flag;
   int ans = 1;
   int s[9999];
   s[0]=2;
   for(i=3;i<=n;i++){
      flag = 0;
      for(j=0;j<z;j++){
         if(i%s[j]==0){
            flag++;
            break;
         }
      }
      if(flag == 0){
         s[z] = i;
         z++;
         ans++;
      }
   }
   return ans;
}
int main(){
   SOSU Sosu;
   int n,ans;
   while(cin>>n){
      ans = Sosu.sosu(n);
      printf("%d\n",ans);
   }
   return 0;
}

C++: #include<iostream>
using namespace  std;
int primes[1000010];
bool isPrime(int p)
{
	bool flag = true;
	for (int i = 2; i * i <= p; i++)
	{
		if (p % i == 0)
		{
			flag = false;
			break;
		}
	}
	return flag;
}
int main()
{
	int n;
	for (int i = 0; i <= 999999; i++)primes[i] = 0;
	for (int i = 2; i <= 999999; i++)if (isPrime(i))primes[i] = 1;
	for (int i = 0; i <= 999999; i++)primes[i + 1] += primes[i];
	while (cin >> n)cout << primes[n] << endl;
	return 0;
}

C++: #include <stdio.h>

#define MAX 2000000
#define UNCHECKED 0
#define PRIME 1
#define COMPO 2


bool isPrime(int n);

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

bool isPrime(int n){
    static int table[MAX]={0};
    static int minPrime=2;
    
    if (n<2) return 0;
    
    do{
        if (table[n] != UNCHECKED){
            return table[n] == PRIME ? 1 : 0;
        }
        table[minPrime] = PRIME;
        for (int i=2; i*minPrime<=MAX; i++){
            table[i*minPrime] = COMPO;
        }
        while (minPrime<=MAX && table[minPrime]!=UNCHECKED) minPrime++;
    }while(minPrime<=MAX);
}

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

void MakePrimeArray(bool A[],int n){
	A[0] = A[1] = false;
	for(int i=2;i<n;i++){
		A[i] = true;
	}
	
	for(int	i=2;i*i<n;i++){
		if(A[i]){
			for(int j=i*i;j<n;j+=i){
				A[j] = false;
			}
		}
	}
}

int main(){
	
	int n=1000000;

	bool primearray[n];
	
	MakePrimeArray(primearray,n);
	
	int NumPrimeArray[n];
	
	int count=0;
	for(int i=0;i<n;i++){
		if(primearray[i]){
			count++;
		}
		NumPrimeArray[i] = count;
	}
	
	while(true){
		int innum;
		cin >> innum;
		
		if(cin.eof()) break;
		
		cout << NumPrimeArray[innum] << endl;
	}
	
	return 0;
}

C++: #include<iostream>
using namespace std;
 
int isPrime(int n){
	//sieve of Eratosthenes
	if(n==2) return 1;
	if(n%2==0 || n<2){
		return 0;
	}

	for(int i=3; i*i<=n; i+=2){
		if(n%i==0){
			return 0;
		}
	}
	return 1;

}

//0009
int main()
{
	int n;
 
	while(cin >> n){
		int cnt = 0;
		for(; n>1; n--){
			if(isPrime(n))
				++cnt;
		}
		cout << cnt << endl;
	}
  return 0;
}

C++: #include<stdio.h>
#include<string.h>
#include<iostream>
using namespace std;
int prime[1000010];
bool is_prime[1000010];
int sieve(int n)
{
    int p(0);
    for(int i=0;i<=n;i++)
        is_prime[i]=1;
    is_prime[0]=is_prime[1]=0;
    for(int i=2;i<=n;i++)
    {
        if(is_prime[i])
        {
            prime[p++]=i;
            for(int j=2*i;j<=n;j+=i)
                is_prime[j]=0;
        }
    }
    return p;
}
int main()
{
    //freopen("input.txt","r",stdin);
    int n;
    while(cin>>n)
    {
        cout<<sieve(n)<<"\n";
    }




	return 0;
}