[Trilinos-Users] all global indices in a map

[Mike Heroux]

Bora,

I have done this kind of thing quite a bit using the following approach:

1) Create an Epetra_Map, map2, that has the same NumMyElements() value on
each processor as your original map, map1.  This new map will have the same
number of elements but will have a contiguous list of global IDs from 0 to
NumGlobalElements()-1.

	Epetra_Map map2(-1, map1.NumMyElements(), map1.IndexBase(),
map1.Comm());

2) Create another Epetra_Map, map3, that has NumMyElements on PE 0 set to
map2.NumGlobalElements(), and NumMyElements set to zero on all other
processors.  This map will have a contiguous list of global IDs with the
same range as map2, but all IDs will be assigned to PE 0.

	int ne = (map2.Comm().MyPID()==0) ? Map2.NumGlobalElements() : 0;
	Epetra_Map map3(-1, ne, map2.IndexBase(), map2.Comm()); 

3) Create an Epetra_IntVector that contains the values of your original map:

	Epetra_IntVector v1(View, map2, map1.MyGlobalElements());

4) Create an Epetra_IntVector using map3.  This will be used to collect the
global IDs to PE 0:

	Epetra_IntVector v2(map3);

5) Create importer to get global ids stored in v1 into v2:
	Epetra_Import importer(map3, map2);

6) Do the import

	v2.Import(v1, importer, Insert);

7) Construct the new map containing all original global ids on PE 0:

	Epetra_Map map4(-1, v2.MyLength(), v2.Values(), map2.IndexBase(),
map2.Comm());

I hope this helps.

Mike

> -----Original Message-----
> From: trilinos-users-bounces at software.sandia.gov 
> [mailto:trilinos-users-bounces at software.sandia.gov] On Behalf 
> Of Bora Ucar
> Sent: Thursday, March 16, 2006 7:19 PM
> To: trilinos-users at software.sandia.gov
> Subject: [Trilinos-Users] all global indices in a map
> 
> Hi,
> 
> I need to gather global indices distributed with an 
> Epetra_Map to a single processor.
> How can I do that? Is there an easy way, or do I have to call 
> communication routines?
> Note: the global indices are not consecutive and are not 
> linearly distributed.
> 
> My scenario is the following. I have a distributed Epetra_CrsMatrix.
> Each processor extracts a set of rows and the corresponding 
> columns to form a submatrix from its local matrix. Now, I 
> need to gather all those extracted submatrices.
> 
> Best wishes,
> 
> Bora
> 
> 
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