# DMCreateMatrix#

Gets an empty matrix for a DM that is most commonly used to store the Jacobian of a discrete PDE operator.

## Synopsis#

#include "petscdm.h"
#include "petscdmlabel.h"
#include "petscds.h"
PetscErrorCode DMCreateMatrix(DM dm, Mat *mat)


Collective on dm

## Input Parameter#

• dm - the DM object

## Output Parameter#

• mat - the empty Jacobian

## Options Database Keys#

• -dm_preallocate_only - Only preallocate the matrix for DMCreateMatrix() and DMCreateMassMatrix(), but do not fill it with zeros

## Notes#

This properly preallocates the number of nonzeros in the sparse matrix so you do not need to do it yourself.

By default it also sets the nonzero structure and puts in the zero entries. To prevent setting the nonzero pattern call DMSetMatrixPreallocateOnly()

For DMDA, when you call MatView() on this matrix it is displayed using the global natural ordering, NOT in the ordering used internally by PETSc.

For DMDA, in general it is easiest to use MatSetValuesStencil() or MatSetValuesLocal() to put values into the matrix because MatSetValues() requires the indices for the global numbering for the DMDA which is complicated to compute

DMDestroy(), DMView(), DMCreateGlobalVector(), DMCreateInterpolation(), DMSetMatType(), DMCreateMassMatrix()`

beginner

## Location#

src/dm/interface/dm.c

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