請問有人可以給我點提示嗎??有的話謝謝了...

//a007-2
#include <iostream>
#include <cmath>
using namespace std;

int main()
{                                            
    int x;
    do {
          cin >> x;
         
          int a = sqrt(x);
          bool prime[a];
          int b = 0;
         
          for (int i = 0; i < a + 1; i++)
              prime[i] = true;
          prime[0] = prime[1] = false;
          for (int i = 2; i < a + 1; i++) {
              if (prime[i]) {
                  for (int j = i + i; j < a + 1; j += i)
                      prime[j] = false;
                  if (x % i == 0) { b = 1; break; }
              }
          }

          (b == 0) ? cout << "質數\n" : cout << "非質數\n";
    } while (x >= 2 && x <= 2147483647);
   
    return 0;
}