:orphan:
# MatCreateSubMatrix
Gets a single submatrix on the same number of processors as the original matrix. 
## Synopsis
```
#include "petscmat.h" 
PetscErrorCode MatCreateSubMatrix(Mat mat, IS isrow, IS iscol, MatReuse cll, Mat *newmat)
```
Collective


## Input Parameters

- ***mat -*** the original matrix
- ***isrow -*** parallel `IS` containing the rows this processor should obtain
- ***iscol -*** parallel `IS` containing all columns you wish to keep. Each process should list the columns that will be in IT's "diagonal part" in the new matrix.
- ***cll -*** either `MAT_INITIAL_MATRIX` or `MAT_REUSE_MATRIX`



## Output Parameter

- ***newmat -*** the new submatrix, of the same type as the original matrix





## Notes
The submatrix will be able to be multiplied with vectors using the same layout as `iscol`.

Some matrix types place restrictions on the row and column indices, such
as that they be sorted or that they be equal to each other. For `MATBAIJ` and `MATSBAIJ` matrices the indices must include all rows/columns of a block;
for example, if the block size is 3 one cannot select the 0 and 2 rows without selecting the 1 row.

The index sets may not have duplicate entries.

The first time this is called you should use a cll of `MAT_INITIAL_MATRIX`,
the `MatCreateSubMatrix()` routine will create the newmat for you. Any additional calls
to this routine with a mat of the same nonzero structure and with a call of `MAT_REUSE_MATRIX`
will reuse the matrix generated the first time.  You should call `MatDestroy()` on `newmat` when
you are finished using it.

The communicator of the newly obtained matrix is ALWAYS the same as the communicator of
the input matrix.

If `iscol` is `NULL` then all columns are obtained (not supported in Fortran).


## Example usage
Consider the following 8x8 matrix with 34 non-zero values, that is
assembled across 3 processors. Let's assume that proc0 owns 3 rows,
proc1 owns 3 rows, proc2 owns 2 rows. This division can be shown
as follows
```none
            1  2  0  |  0  3  0  |  0  4
    Proc0   0  5  6  |  7  0  0  |  8  0
            9  0 10  | 11  0  0  | 12  0
    -------------------------------------
           13  0 14  | 15 16 17  |  0  0
    Proc1   0 18  0  | 19 20 21  |  0  0
            0  0  0  | 22 23  0  | 24  0
    -------------------------------------
    Proc2  25 26 27  |  0  0 28  | 29  0
           30  0  0  | 31 32 33  |  0 34
```


Suppose `isrow` = [0 1 | 4 | 6 7] and `iscol` = [1 2 | 3 4 5 | 6].  The resulting submatrix is

```none
            2  0  |  0  3  0  |  0
    Proc0   5  6  |  7  0  0  |  8
    -------------------------------
    Proc1  18  0  | 19 20 21  |  0
    -------------------------------
    Proc2  26 27  |  0  0 28  | 29
            0  0  | 31 32 33  |  0
```



## See Also
 [](ch_matrices), `Mat`, `MatCreateSubMatrices()`, `MatCreateSubMatricesMPI()`, `MatCreateSubMatrixVirtual()`, `MatSubMatrixVirtualUpdate()`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/interface/matrix.c.html#MatCreateSubMatrix">src/mat/interface/matrix.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex49.c.html">src/ksp/ksp/tutorials/ex49.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex59.c.html">src/ksp/ksp/tutorials/ex59.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex64.c.html">src/ksp/ksp/tutorials/ex64.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/snes/tutorials/ex73f90t.F90.html">src/snes/tutorials/ex73f90t.F90</A><BR>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/aij/mpi/mpiaij.c.html#MatCreateSubMatrix_MPIAIJ">MatCreateSubMatrix_MPIAIJ in src/mat/impls/aij/mpi/mpiaij.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/aij/seq/aij.c.html#MatCreateSubMatrix_SeqAIJ">MatCreateSubMatrix_SeqAIJ in src/mat/impls/aij/seq/aij.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/baij/mpi/mpibaij.c.html#MatCreateSubMatrix_MPIBAIJ">MatCreateSubMatrix_MPIBAIJ in src/mat/impls/baij/mpi/mpibaij.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/baij/seq/baij2.c.html#MatCreateSubMatrix_SeqBAIJ">MatCreateSubMatrix_SeqBAIJ in src/mat/impls/baij/seq/baij2.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/blockmat/seq/blockmat.c.html#MatCreateSubMatrix_BlockMat">MatCreateSubMatrix_BlockMat in src/mat/impls/blockmat/seq/blockmat.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/dense/mpi/mpidense.c.html#MatCreateSubMatrix_MPIDense">MatCreateSubMatrix_MPIDense in src/mat/impls/dense/mpi/mpidense.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/dense/seq/dense.c.html#MatCreateSubMatrix_SeqDense">MatCreateSubMatrix_SeqDense in src/mat/impls/dense/seq/dense.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/is/matis.c.html#MatCreateSubMatrix_IS">MatCreateSubMatrix_IS in src/mat/impls/is/matis.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/kaij/kaij.c.html#MatCreateSubMatrix_KAIJ">MatCreateSubMatrix_KAIJ in src/mat/impls/kaij/kaij.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/maij/maij.c.html#MatCreateSubMatrix_MAIJ">MatCreateSubMatrix_MAIJ in src/mat/impls/maij/maij.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/nest/matnest.c.html#MatCreateSubMatrix_Nest">MatCreateSubMatrix_Nest in src/mat/impls/nest/matnest.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/sbaij/mpi/mpisbaij.c.html#MatCreateSubMatrix_MPISBAIJ">MatCreateSubMatrix_MPISBAIJ in src/mat/impls/sbaij/mpi/mpisbaij.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/sbaij/seq/sbaij2.c.html#MatCreateSubMatrix_SeqSBAIJ">MatCreateSubMatrix_SeqSBAIJ in src/mat/impls/sbaij/seq/sbaij2.c</A><BR>


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