Go: package main

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

func main() {
	scanner := bufio.NewScanner(os.Stdin)

	for scanner.Scan() {
		num, _ := strconv.Atoi(scanner.Text())
		flag := false
		count := 0

		for i := 2; i <= num; i++ { //1を除く入力値以下の数字
			for j := 2; j < i-1; j++ {
				if i%j == 0 && j != 1 {
					flag = true
					//fmt.Println(i, "/", j)
					break
				}
			}
			if !flag {
				count++
			} else {
				flag = false
			}
		}
		fmt.Println(count)
	}
}

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

int main(){
  int n,prime[1000001],key,lf=0;
  prime[0]=2;
  key=1;
  for(int i=3;i<=999999;i+=2)
    {
      for(int j=2;j<=sqrt(i);j++)
	{
	  if(i%j == 0)
	    {
	      lf=1;
	      break;
	    }
	}
      if(lf == 1)
	{
	  lf=0;
	}
      else 
	{
	  prime[key]=i;
	  key++;
	}
    }

  while(cin >> n)
{
  int cou=0;
  for(int i=0;;i++)
    {
      if(prime[i] <= n)
	{
	  cou++;
	}
      else break;
    }

  cout << cou << endl;
 }
}

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

using namespace std;

const int N_MAX = 1000000;
int table[N_MAX];

int main() {
  table[0] = table[1] = 1;
  int sq = sqrt(N_MAX);
  for (int i = 2; i < sq+1; i++) {
    if (table[i] == 0) {
      for (int j = i*2; j < N_MAX; j += i) {
        table[j] = 1;
      }
    }
  }
  int n;
  while (cin >> n) {

    int ans = 0;
    for (int i = 0; i <= n; i++) {
      if (table[i] == 0) ans++;
    }
    cout << ans << endl;

  }

  return 0;
}

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

C++: #include<iostream>

using namespace std;

int get_numprime( int num );
bool prime( int n );


int main(void)
{
	int num;

	while(1)
	{
		cin >> num;
		if( cin.eof() ) break;
		cout << get_numprime( num ) << endl;
	}

	return 0;
}

int get_numprime( int num )
{
	int numprime = 0;

	for( int i = 2 ; i <= num ; i++ )
	{
		if( prime(i) ) numprime++;
	}

	return numprime;
}

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

C++: #include <iostream>

int main() {
    int n;
    while (std::cin >> n) {
        int primes = 0; // n以下の素数の数
        for (int i = 2; i <= n; i++) {
            bool flag = false; // はずれの目印
            for (int j = 2; j < i; j++)
                if (!(i % j)) {
                    flag = true;
                    break;
                }   
            if (!flag)
                 primes++;
        }
        std::cout << primes << std::endl;
     }   
}

C++: #include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <ctime>
#define INF 999999999
#define mod 1000000007

#define rep(i,n) for(int i=0;i<int(n);i++)
#define rrep(i,n) for(int i=(n);i>=0;i--)
#define REP(i,a,b) for(int i=(a);i<int(b);i++)
#define all(x) (x).begin(),x.end()
#define pb push_back
#define MOD(x) (x%(mod))
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef pair<int,int> pi;

bool debug=false;
/*---------------------------------------------------*/

bool prime[1000000];
int ans[1000000];

void is_prime(){
  rep(i,1000000)prime[i]=true;
  prime[0]=prime[1]=false;
  REP(i,2,1000000){
    if(prime[i]==true){
      for(int j=i+i;j<1000000;j+=i)prime[j]=false;
    }
  }
  if(debug)rep(i,1000)if(prime[i])cout<<i<<endl;
}

void calc(){
  ans[0]=0;
  REP(i,1,1000000){
    ans[i]=ans[i-1]+prime[i];
  }
}

int main(){
  int n;
  is_prime();
  calc();
  while(cin>>n){
    cout<<ans[n]<<endl;
  }
  return 0;
}

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

using namespace std;

int main()
{
	int n;
	int head,fin;
	int temp;
	
	
//	vector<int> isPrime;
	bool isPrime[1000000];
	
	while(cin>>n)
	{
		for(int i = 0; i < n+1; i++){
//			isPrime.push_back(1);
			isPrime[i] = 1;
		}
//		cout << "Corecced!" << endl;
		isPrime[0] = isPrime[1] = 0;
		temp = 0;
		
		for(;;)
		{
			//TõXgÌæªð©Â¯é
			for(head = temp+1; head < n+1; head++){
				if(isPrime[head] == 1){
					break;
				}
			}
			//æªðÜÜÈ¢{ÍSÄ¬
			for(int i = head; i*head < n+1; i++){
				isPrime[i*head] = 0;
			}
			//TõXgÌÅãÖ
			for(fin = n; fin > 0; fin--){
				if(isPrime[fin] == 1){
					break;
				}
			}
		//	printf("head:%d    fin:%d\n",head,fin);
			
			if(fin <= head*head){
				break;
			}
			temp = head;
		}
		
		int cont = 0;
		for(int i = 0; i < n+1; i++){
			if(isPrime[i] == 1){
				cont++;
		//		cout << "+"<<i << endl;
			}
		}
		printf("%d\n",cont);
	}
}

C++: #include<stdio.h>
#include<math.h>
main(){int i,j,n,p[1000000];scanf("%d",&n);for(i=2;i<=n;i++)p[i]=1;for(i=2;i<=sqrt(n);i++)if(p[i])for(j=2;i*j<=n;j++)p[i*j]=0;j=0;for(i=2;i<=n;i++)j+=p[i];printf("%d\n",j);}

C++: #include<iostream>

bool sieve[1000000];

void makesieve();

int main()
{
	makesieve();
	while(1)
	{
		int n,ans=0;
		std::cin>>n;
		if(std::cin.eof())break;
		for(int i=0;i<n;i++)
		{
			ans+=sieve[i]?1:0;
		}
		std::cout<<ans<<std::endl;
	}
	return 0;
}

void makesieve()
{
	memset(sieve,-1,sizeof(sieve));
	sieve[0]=false;sieve[1]=false;
	for(int i=2;i<1000000;i++)
	{
		for(int j=i*2;j<1000000;j+=i)
		{
			sieve[j]=false;
		}
	}
}

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

using namespace std;

int main(){
	int n, k, i, c;
	bool flag = 0;
	while(cin >> n){
		c = 0;
		for(i = 2; i <= n; i++){
			flag = 0;
			if(i == 2 || i == 3) c++;
			else{
				for(int j = 3; j <= sqrt(n); j+=2){
					if(i % 2 == 0){
						flag = 0;
						break;
					}
					else if(i % j == 0){
						flag = 0;
						break;
					}
					else flag = 1;
				}
				if(flag == 1) c++;
			}
		}
		cout << c << endl;
	}
	return 0;
}

C++: #include <stdio.h>

int main() {
    int i, k, v,scan,an;
    int MAX_V = 999999;
    int count[MAX_V+1];
    int prime[MAX_V+1];
    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;   
            } 
        }
    }
    count[2]=0;
    for(i=2;i<=MAX_V;i++){
        count[i+1]= count[i]+prime[i];
    }
    while(scanf("%d",&scan)!= EOF){
        printf("%d\n",count[scan+1]);
    }
    return 0; 
}

C++: #include <iostream>
#include <cmath>
//#include <ctime>
#define Even(n) n%2
using namespace std;

void count_Prime(int n);
bool IsPrime(int num);


int main()
{
	int Data = 0;
//	clock_t start, end;
	while (cin >> Data)
	{
//		start = clock();
		count_Prime(Data);
//		end = clock();

//		cout << (double)(end - start) / CLOCKS_PER_SEC << "sec" << endl;
	}

	return 0;
}

void count_Prime(int n)
{
	int counter = 0,  i = 0;
	for (i = 2; i <= n; ++i)
	{
		if (IsPrime(i))
			counter++;
	}

	cout << counter << endl;

}

bool IsPrime(int num)
{
	if (num < 2) return false;
	else if (num == 2) return true;
	else if (num % 2 == 0) return false; // ??¶??°????????????????????????

	double sqrtNum = sqrt(num);
	for (int i = 3; i <= sqrtNum; i += 2)
	{
		if (num % i == 0)
		{
			// ?´???°??§?????????
			return false;
		}
	}

	// ?´???°??§??????
	return true;
}