[petsc-users] Testing MatGetSubMatrices

[Bikash Kanungo]

Hi,

I was trying to perform a simple test on MatGetSubMatrices. I have an
MPIDENSE matrix X which has been populated and assembled and another matrix
XTemp of the same type and size. Since X is MPIDENSE in order to extract a
sub-matrix from it, the IS must be sorted. This can be done easily through
ISSort. Once the sub-matrix is obtained, I'm trying to set the values in
XTemp by using the sorted IS as the global indices and the sub-matrix as
the values to be set. Once XTemp is set and assembled, I evaluate XTemp - X
and see if the norm of the resultant matrix is zero or not. The norm turned
out to be as high as 1.0e6.

I'm providing a simplified version of the code that performs the above
test. Kindly let me know if I have done any mistake is designing the test.


      //
      // Given an initialized and assembled MPIDENSE Mat X and
      // an uninitialized MPIDENSE Mat XTemp of the same size as that of X
      //
      IS  * ISRowIds, * ISColIds;
      PetscMalloc(1*sizeof(IS), &ISRowIds);
      PetscMalloc(1*sizeof(IS), &ISColIds);

      //
      // globalRowIds and globalColIds are STL vectors containing
      // the global node ids for which the sub-matrix from X need
      // to be extracted
      //
      const PetscInt * rowIdsPtr = globalRowIds.data();
      const PetscInt * colIdsPtr = globalColIds.data();

      ISCreateGeneral(PETSC_COMM_WORLD, globalRowIds.size(), rowIdsPtr,
PETSC_COPY_VALUES, &ISRowIds[0]);
      ISCreateGeneral(PETSC_COMM_WORLD, globalColIds.size(), colIdsPtr,
PETSC_COPY_VALUES, &ISColIds[0]);
      ISSort(ISRowIds[0]);
      ISSort(ISColIds[0]);

      Mat * subMat;
      MatGetSubMatrices(X, 1, ISRowIds, ISColIds, MAT_INITIAL_MATRIX,
&subMat);

      PetscScalar * subMatValues;
      MatDenseGetArray(*subMat, &subMatValues);

      const PetscInt * rowIndicesSorted, * colIndicesSorted;
      ISGetIndices(ISRowIds[0], &rowIndicesSorted);
      ISGetIndices(ISColIds[0], &colIndicesSorted);

      MatSetValues(XTemp,
                   globalRowIds.size(),
                   rowIndicesSorted,
                   globalColIds.size(),
                   colIndicesSorted,
                   subMatValues,
                   INSERT_VALUES);

      //
      // commencing assembly
      //
      MatAssemblyBegin(XTemp,
                         MAT_FINAL_ASSEMBLY);

      //
      // concluding assembly
      //
      MatAssemblyEnd(XTemp,
               MAT_FINAL_ASSEMBLY);

      MatAXPY(XTemp, -1.0, X, DIFFERENT_NONZERO_PATTERN);
      double diffNorm;
      MatNorm(XTemp, NORM_1, &diffNorm);



Thanks,
Bikash
-- 
Bikash S. Kanungo
PhD Student
Computational Materials Physics Group
Mechanical Engineering
University of Michigan
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