[Trilinos-Users] Question on use of Epetra_BlockMap with Epetra_Vector

[Robert Crockett]

Hello,

The context:
I would like to be able to instantiate an Epetra_Vector from data in 
existing memory, but only using a subset of that data. If anyone can 
provide answers to the following questions, I would be most thankful.

The questions:
(1) Is this possible and, if so, is my below proposal a good way
     to do it?
(2) Is there existing example code that does this?

The details:
We have data in an existing (parallel) data structure, which holds it in 
a contiguous swath of memory, eg.

int m_dataSize;  // size of data
double* m_data;  // pointer to data

I would like to pass this memory into Epetra_Vector for performing 
solves elsewhere in Trilinos. The catch is that only parts of m_data are 
valid degrees of freedom for the solves. These parts are contiguous, so 
I believe using an Epetra_BlockMap fits the bill. Here is my proposed 
pseudo-code for how I would do this, which is the basis for my questions 
above:

<code>
Epetra_Comm* m_epCommPtr;  // initialized elsewhere
Epetra_BlockMap* m_mapPtr;
Epetra_Vector* m_vecPtr;
vector<int> myGlobalElements;
vector<int> elementSizeList;
// iterate only over valid DoF
DataIterator it(m_data, m_dataSize, VALID_CELLS);
/// add all valid cells to global element list for this proc,
//+ and add a one for element size (will optimize for contiguous
//+ blocks -- ie. element size > 1 -- later)
for (it.begin(); it.ok(); ++it) {
   myGlobalElements.push_back(it.globalIndex());
   elementSizeList.push_back(1);
}

// compute all element counts
int numMyElements = myGlobalElements.size();
int numGlobalElements;
m_comm.allReduceSum(&numMyElements, &numGlobalElements);
int indexBase = 0;
m_mapPtr = new Epetra_BlockMap(numGlobalElements, numMyElements,
                                myGlobalElements, elementSizeList,
                                indexBase, *m_epCommPtr);
m_vecPtr = new Epetra_Vector(View, *m_mapPtr, m_dataPtr);
</code>

That's it! Thanks,
Robert

-- 
"The mind is not a vessel to be filled but a fire to be kindled."
-Plutarch
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Robert Crockett, Ph.D.            Research Scientist, Tech-X Corporation
rkcrockett at txcorp.com             5621 Arapahoe Avenue, Suite A
303/996-2037                      Boulder, CO 80303
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