[Trilinos-Users] Epetra_Map

[Heroux, Mike]

Both of your points are well taken.  The relationship between block and
non-block entities in Epetra has turned out to be cumbersome.  Tpetra does
this differently (as a result of the lessons learned from Epetra).  In
Tpetra, "block-ness" is a decoration to a map object, so the
specialization relationship between Epetra_Map and Epetra_BlockMap is no
longer present.  The issues you are running into should be gone with
Tpetra.

Mike

On 12/9/13 10:21 AM, "Littlewood, David John" <djlittl at sandia.gov> wrote:

>Mike,
>
>Wanted to chime in here because I've had the same question as Luca.
>Everything you said makes sense, but it's still not clear why the
>constructor for Epetra_CrsMatrix takes an Epetra_Map.  It seems like it
>should take an Epetra_BlockMap.
>
>The fact that it takes an Epetra_Map has caused some minor grief for me,
>since I generally use Epetra_BlockMap to create vectors which I'd then
>like to multiply with an Epetra_FECrsMatrix.  This doesn't work without
>some data juggling, because I created the matrix using an Epetra_Map (as
>required by the constructor) and thus it is incompatible with my vectors
>(created with Epetra_BlockMap, element size = 3).
>
>Dave
>
>
>On 12/6/13 11:25 PM, "Heroux, Mike" <MHeroux at csbsju.edu> wrote:
>
>>Luca,
>>
>>As you pointed out, Epetra_Map "isa" Epetra_BlockMap, with a block size
>>of 1.  If you are only using a block size of 1, then you should just use
>>Epetra_Map.  Doing this should make your code much simpler.  So when you
>>are working with Epetra_CrsGraph, just use an Epetra_Map.
>>
>>In general Epetra_Map (and Epetra_BlockMap, and similar classes in Tpetra
>>and soon Kokkos) encodes a description of the dimensions and distribution
>>of data objects.  By encoding this information into a separate class, we
>>can create many data objects using the same map and know that they are
>>compatible by construction.  We can also use maps to create
>>redistributions of data objects, and a few other things.
>>
>>In straightforward use cases, the map may seem like unnecessary overhead,
>>but even in these cases, the maps are being used by underlying
>>functionality to improve correct and efficient behavior.
>>
>>Epetra_CrsGraph uses Epetra_BlockMap because it serves as the graph class
>>for both Epetra_Map and Epetra_BlockMap based object.  For example,
>>Epetra_CrsMatrix and Epetra_VbrMatrix both use Epetra_CrsGraph.
>>
>>I hope this helps.
>>
>>Mike
>>
>>On Dec 7, 2013, at 1:20 AM, "Bertagna, Luca"
>><lbertag at emory.edu<mailto:lbertag at emory.edu>> wrote:
>>
>>
>>Hello,
>>
>>I've been using the epetra package for a while, but I still don't
>>understand what's the point of Epetra_Map. I understand it's a map where
>>the element size is equal to 1, but I don't get why Epetra_CrsMatrix uses
>>it when Epetra_CrsGraph does not. Calling the method FillComplete in
>>CrsMatrix is always a pain, cause I need to cast the BlockMap of the
>>graph into a Map. I guess my question is, if the underlying graph does
>>not use Epetra_Map but uses Epetra_BlockMap instead, why can't
>>Epetra_CrsMatrix just use Epetra_BlockMap as well?
>>
>>Thanks
>>
>>
>>Luca Bertagna
>>Math&CS, Emory University, Atlanta
>>www.mathcs.emory.edu/~lbertag<http://www.mathcs.emory.edu/~lbertag>
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