error: array bound is not an integer constant before ']' token. How to make the variable be the length. 

```
#include<iostream>
using namespace std;

int e;

int a()
{
  int b=3;
  return b;
}

e=a();

int d[e]={12,2,3};

int main()
{
  cout<<d[4]<<endl;
  return 0;
}

```