[Dune] A question about the relationship between iterators and indices in dune-grid

[Oliver Sander]

Hi Helmer!

>
> A) I state the question in 2D (but it obviously applies similarly in 
> 3D). Consider an entity with a given codim thus corresponding to 
> either a vertex, edge or element. Is it so that the associated 
> iterator.begin() always corresponds to (or in other words: points to) 
> the entity with index=0, and moreover, will the operation ++iterator 
> always increase the corresponding entity's index with 1.
No.  Many grids do act that way, but it is not required.
>
> B) If the answer in A) turns at to be negative, i.e. that there is no 
> such obvious relationship between indices and iterators in the 
> dune-grid: how do you relate a given index to the corresponding 
> entity, or in other words, if you know the index of an entity, how do 
> you obtain a pointer to this particular entity?
There is not constant-time way to do so.  The only way is to iterate 
through all entities and stop when
you have found the one with the correct index.  Or, if you have lots of 
memory, you could store an
array of EntityPointers with an entry for each entity.  But both are 
really only theoretical options.

Yours,
Oliver
>
> Thanks in advance,
> Helmer André Friis
>
>
>
>
>
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