Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

C++: #include <stdio.h>

int main(void){
	bool a[1000000];
	int n = 0;
	int c = 0;
	while(scanf("%d", &n) != EOF){
		switch(n){
		case 1:
			c = 0;
			break;
		case 2:
			c = 1;
			break;
		default:
			c = 1;
			for(int i=3; i<=n; i+=2){
				a[i] = true;
			}
			for(int i=3; i<=n; i+=2){
				if(a[i] == false){ continue; }
				c++;
				for(int j=i; j<=n; j+=i){
					a[j] = false;
				}
			}
			break;
		}
		printf("%d\n", c);
	}
	return 0;
}

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

int main(){
  bool prime[1000000];
  prime[0]=false;prime[1]=false;
  for(int i=2;i<1000001;i++){
    prime[i]=true;
  }

  for(int i=2;i<=sqrt(999999);i++){
    if(prime[i]){
      for(int j=1;j*i<=999999;j++){
        prime[j*i]=false;
      }
    }
  }
  int n;
  while(cin>>n){
    int cnt=0;
    for(int i=2;i<=n;i++){
      if(prime[i])cnt++;
    }
    cout<<cnt<<endl;
  }
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <math.h>
#pragma warning(disable:4996)

int main()
{
	
	int num, cnt;
	unsigned int i, j, k, data_num;
	int data[30];
	unsigned int sq_num;
	unsigned int *prime;

	for (data_num = 0; data_num <30; data_num++)
	{
		data[data_num] = -1;
		scanf("%d", &data[data_num]);
		if (data[data_num] < 0)
			break;
	}

	for (k = 0; k < data_num; k++)
	{

		sq_num = (int)sqrt((double)data[k]);
		prime = (unsigned int*)malloc(sizeof(unsigned int) * data[k]);
		prime[0] = 0;

		for (i = 1; i < data[k]; i++)
		{
			prime[1] = 1;
		}

		for (i = 1; i < sq_num; i++)
		{

			if (prime[i] == i)
			{
				for (j = (i + 1); (i + 1)* j <= data[k]; j++)
				{
					prime[(i + 1) * j - 1] = 0;
				}
			}
		}

		cnt = 0; 
		for (i = 0; i < data[k]; i++)
		{
			if (prime[j] == 1)
			{
				cnt++;
			}
		}
		printf("%d\n", cnt);
		free(prime);
	}

	return 0;
}

C++: #include<iostream>

using namespace std;

int main(){
  int n,flag=1,sum = 0;

  while(cin >> n){

    for(int i=2; i <= n; i++){
      if(i != 2 && i%2 == 0) flag = 0;
      else if(i != 3 && i%3 == 0) flag = 0;
      else if(i != 5 && i%5 == 0) flag = 0;
      else{
	for(int j=3; j < i/2; j++){
	  if(i%j == 0 ){
	    flag = 0;
	    j = i/2;
	  }
	}
      }
      if(flag == 1) sum++;
      flag = 1;
    }
    cout << sum <<endl;
    sum = 0;
  }
}

C++: #include<stdio.h>
int num[1000001]={};
int main ()
{
int p,n,m,cou;m=0;cou=0;p=3;


for(int i=2;i<=10000001;i++)
	num[i]=1;
for(int i=2;i<=1001;i++){
for(int k=2;k*k<=i;k++){
if(i%k==0){num[i]=0;}}
}
while(scanf("%d",&n)!=EOF)
{
for(int i=2;i<=n;i++)
	cou+=num[i];

if(n==2)cou=1;
if(n==1)cou=0;
//else cou++;
printf("%d\n",cou);
cou=0;
}
return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
    int i,j,flag,n,c=0;
    scanf("%d",&n);
    for(i=2;i<=n;i++)
    {
        flag=0;
        for(j=2;j<=sqrt(i);j++)
        {
            if(i%j==0)
            {
                flag=1;
            }
        }
        if(flag==0)
        {
            c++;
        }
    }
    printf("%d",c);
    return 0;
}

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

typedef int TInt;
typedef std::vector<TInt> TList;
bool IsPrime(TInt Value, const TList& Prime)
{
    TInt Max = std::sqrt(Value);
    for(TList::const_iterator It = Prime.begin();
        It != Prime.end() && *It <= Max;
        ++It)
    {
        if(Value % (*It) == 0) return false;
    }
    return true;
}
TList GetPrimeList(TInt Max)
{
    TList Prime;
    Prime.reserve(Max);
    Prime.push_back(2);
    for(TInt i = 3; i <= Max; i += 2){
        if(IsPrime(i, Prime)){
            Prime.push_back(i);
        }
    }
    return Prime;
}
int main()
{
    TList List = GetPrimeList(999999);
    while(true){
        int n;
        std::cin >> n;
        if(std::cin.eof()) break;
        TList::iterator It = std::upper_bound(List.begin(), List.end(), n);
        std::cout << std::distance(List.begin(), It) << std::endl;
        
    }
    return 0;
}

C++: //*-*-coding:utf-8-*-*/
#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <iterator>
#include <cmath>
#include <stack>
#include <functional>
#include <iomanip>
using namespace std;

int main()
{
  int n;

  while(cin >> n, !cin.eof()) {
    int ans = 0;
    if(n >= 2) ans++;    
    for(int i = 3; i <= n; i+=2) {
      int flag = 0;
      for(int j = 3; j <= sqrt(i); j+=2) {
	if(i % j == 0) {
	  flag = 1;
	  break;
	}
      }
      if(flag == 0) ans++;
    }    
    cout << ans << endl;
  }
  
  return 0;
}

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

using namespace std;

int setPrimeNumberCount(int n) {
    int count = 0;
    vector<int> primeList;
    vector<int> searchList;
    
    if(n >= 2){
        for(int i = 2; i <= n; i++) {
            searchList.push_back(i);
        }
        
        vector<int>::iterator end_it;
        
        for(int i = 0; i <= n; i++) {
            primeList.push_back(searchList[0]);
            
            for(vector< int >::iterator it = searchList.begin(); it != searchList.end();) {
                if(*it % primeList[count] == 0) {
                    it = searchList.erase(it);
                }
                else {
                    ++it;
                }
            }
            count++;
            if(pow(primeList[i],2) > searchList.back()) {
                for(int i = 0; i < searchList.size(); i++) {
                    primeList.push_back(searchList[i]);
                    count++;
                }
                break;
            }
        }
    }
    return count;
}

int main(void) {
    int n;
    int count = 0;
    
    while(cin >> n) {
        count = setPrimeNumberCount(n);
        cout << count << endl;
        count = 0;
    }
    return 0;
}

C++: #include <Stdio.h>

int main()
{
    int n,i,j,flag=0,cnt,h,k;
    while(scanf("%d",&n) !=EOF)
    {
        if(n==2)
            printf("1");
        else if(n==3)
            printf("2");
        else
        {
            cnt=2;
            h=n/2;
            for(k=4; k<=n; k++)
            {
                for(j=2; j<=h; j+=2)
              {
                if(k%j==0)
                {
                    flag=1;
                    break;
                }
              }
               if(flag==0)
                cnt++;
               else
               flag=0;
            }
            printf("%d",cnt-1);
        }

    }

    return 0;
}

C++: #include<iostream>
#include<queue>
#include<string>
#include<stack>
#include<cstdio>
#include<algorithm>

using namespace std;

int main(void)
{
  int i=0;
  int j=0;
  int n=0;
  int a[1000000]={};
  int sum=0;

  while(scanf("%d",&n)!=EOF)
    {
      for(i=0;i<1000000;i++)
        a[i]=1;
      sum=0;

      for(i=2;i<=n;i++)
        {
          if(a[i-1]==0)
            continue;
	  for(j=2;j<=n/i;j++)
            a[i*j-1]=0;
	}

      for(i=1;i<n;i++)
	sum+=a[i];

      cout<<sum<<endl;

    }


  return 0;

}