[MOAB-dev] IDs to index?

[Nico Schlömer]

Hi everyone,

I just checked out the elem values for some mesh,
```
  // Get regions, by dimension, so we stay generic to entity type
  Range elems;
  rval = mb->get_entities_by_dimension(0, 3, elems);MB_CHK_ERR(rval);
  cout << "Number of elements is " << elems.size() << endl;

  for (size_t k = 0; k < elems.size(); k++) {
    std::cout << "elems[" << k << "] = " << elems[k] << std::endl;
  }
```
and get
```
elems[0] = 5764607523034234881
elems[1] = 5764607523034234882
elems[2] = 5764607523034234883
// ...
```
That came as a bit of a surprise to me since I had expected low consecutive
numbers, but I guess there's no guarantee for that.

Now, I would like to store additional data per element, and I initially
planned to just create a `std::vector<double>`. However, to associate the
data `data[42]` with a particular `elem`, I'll have to do something else. I
suppose I could use a `std::map<EntityHandle, double>` instead.

How to you typically organize this? Is there, for example, a helper method
that converts an `elem` ID into something that I can use as an index into
`data`?

Cheers,
Nico
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