petsc-3.7.1 2016-05-15
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# MatGetSubMatrix

Gets a single submatrix on the same number of processors as the original matrix.

### Synopsis

    #include "petscmat.h"
    PetscErrorCode MatGetSubMatrix(Mat mat,IS isrow,IS iscol,MatReuse cll,Mat *newmat)

Collective on Mat Many br

### Input Parameters

| mat | - the original matrix Many br
| isrow | - parallel IS containing the rows this processor should obtain Many br
| 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. Many br
| cll | - either MAT_INITIAL_MATRIX or MAT_REUSE_MATRIX Many br

### Output Parameter

newmat -the new submatrix, of the same type as the old Many br

Many br

### Notes

The submatrix will be able to be multiplied with vectors using the same layout as iscol. Many br

Some matrix types place restrictions on the row and column indices, such Many bras that they be sorted or that they be equal to each other. Many br

The index sets may not have duplicate entries. Many br

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

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

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

### Example usage

Consider the following 8x8 matrix with 34 non-zero values, that is Many brassembled across 3 processors. Let's assume that proc0 owns 3 rows, Many brproc1 owns 3 rows, proc2 owns 2 rows. This division can be shown Many br

### as follows

                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

Many br

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

                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

Many br

### See Also

MatGetSubMatrices()

Level:advanced
Location:src/mat/interface/matrix.c
Index of all Mat routines
Table of Contents for all manual pages
Index of all manual pages

### Examples

src/ksp/ksp/examples/tutorials/ex49.c.html
src/ksp/ksp/examples/tutorials/ex64.c.html