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# DMPlexPreallocateOperator
Calculate the matrix nonzero pattern based upon the information in the `DM`, the `PetscDS` it contains, and the default `PetscSection`. 
## Synopsis
```
#include "petscdmplex.h"   
PetscErrorCode DMPlexPreallocateOperator(DM dm, PetscInt bs, PetscInt dnz[], PetscInt onz[], PetscInt dnzu[], PetscInt onzu[], Mat A, PetscBool fillMatrix)
```
Collective


## Input Parameters

- ***dm   -*** The `DMPLEX`
- ***bs   -*** The matrix blocksize
- ***dnz  -*** An array to hold the number of nonzeros in the diagonal block
- ***onz  -*** An array to hold the number of nonzeros in the off-diagonal block
- ***dnzu -*** An array to hold the number of nonzeros in the upper triangle of the diagonal block
- ***onzu -*** An array to hold the number of nonzeros in the upper triangle of the off-diagonal block
- ***fillMatrix -*** If `PETSC_TRUE`, fill the matrix with zeros



## Output Parameter

- ***A -*** The preallocated matrix





## See Also
 [](ch_unstructured), `DM`, `DMPLEX`, `DMCreateMatrix()`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/impls/plex/plexpreallocate.c.html#DMPlexPreallocateOperator">src/dm/impls/plex/plexpreallocate.c</A>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/impls/plex/plexpreallocate.c.html#DMPlexPreallocateOperator_2">DMPlexPreallocateOperator_2 in src/dm/impls/plex/plexpreallocate.c</A><BR>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/dm/impls/plex/plexpreallocate.c)


[Index of all DMPlex routines](index.md)  
[Table of Contents for all manual pages](/manualpages/index.md)  
[Index of all manual pages](/manualpages/singleindex.md)