Find the 10001st prime number (in C++)

I wrote this solution for project Euler #7, to find the 10001th prime number using sieve of eratosthenes. It's really slow though. Any suggestions to improve performance (without using more complex sieves like sieve of atkin)?

#include
#include
#include
#define max 500000
using namespace  std;

int main()
{
 int ctr = 0;
 int i=2,j,n=10001,p=2;
 int arr[max];
 memset(arr,0,sizeof(arr));
 while(1)
 {

 p=2;
 while(p*p<=i)
 {

  if(arr[p] == 0)
     for(j=p*p;j<=i;j+=p)
        arr[j] = 1;

        p++;

    }
   if(arr[i]==0)
    ctr++;
   if(ctr==n)
    break;
   i++;

  }

 cout<<i;

 }

• Why is everybody using #include? It is so hard to include ` to get memset working?






• Why do you need ?






• Don't use using namespace std;. It's a bad practice.






• Static array of ints with size 500000 is likely too big to fit into stack. Use int * arr = new int[max] and delete [] arr;` instead.

• Using whole integers to store boolean value is just a waste of memory.

-
It takes 42 seconds to find result. My version (without sieve) takes about real: 0.015s, user: 0.013s.

#include 

bool isPrime(int num) {
  for (int i = 2; i*i  1;
}

int main() {
  int i = 2;
  for (int primes = 0; ; ++i) {
    if (isPrime(i)) {
      if (++primes == 10001) {
        std::cout << i << "\n";
        break;
      }
    }
  }
}

StackExchange Code Review Q#136328, answer score: 6