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# MatISSetPreallocation
Preallocates memory for a `MATIS` parallel matrix. 
## Synopsis
```
#include "petscmat.h" 
PetscErrorCode MatISSetPreallocation(Mat B, PetscInt d_nz, const PetscInt d_nnz[], PetscInt o_nz, const PetscInt o_nnz[])
```
Collective


## Input Parameters

- ***B -*** the matrix
- ***d_nz  -*** number of nonzeros per row in DIAGONAL portion of local submatrix
(same value is used for all local rows)
- ***d_nnz -*** array containing the number of nonzeros in the various rows of the
DIAGONAL portion of the local submatrix (possibly different for each row)
or `NULL`, if `d_nz` is used to specify the nonzero structure.
The size of this array is equal to the number of local rows, i.e `m`.
For matrices that will be factored, you must leave room for (and set)
the diagonal entry even if it is zero.
- ***o_nz  -*** number of nonzeros per row in the OFF-DIAGONAL portion of local
submatrix (same value is used for all local rows).
- ***o_nnz -*** array containing the number of nonzeros in the various rows of the
OFF-DIAGONAL portion of the local submatrix (possibly different for
each row) or `NULL`, if `o_nz` is used to specify the nonzero
structure. The size of this array is equal to the number
of local rows, i.e `m`.


If the *_nnz parameter is given then the *_nz parameter is ignored




## Note
This function has the same interface as the `MATMPIAIJ` preallocation routine in order to simplify the transition
from the asssembled format to the unassembled one. It overestimates the preallocation of `MATIS` local
matrices; for exact preallocation, the user should set the preallocation directly on local matrix objects.


## See Also
 [](ch_matrices), `Mat`, `MatCreate()`, `MatCreateIS()`, `MatMPIAIJSetPreallocation()`, `MatISGetLocalMat()`, `MATIS`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/is/matis.c.html#MatISSetPreallocation">src/mat/impls/is/matis.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/tutorials/ex3.c.html">src/mat/tutorials/ex3.c</A><BR>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/is/matis.c.html#MatISSetPreallocation_IS">MatISSetPreallocation_IS in src/mat/impls/is/matis.c</A><BR>


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