MatGetRow#

Gets a row of a matrix. You MUST call MatRestoreRow() for each row that you get to ensure that your application does not bleed memory.

Synopsis#

#include "petscmat.h" 
PetscErrorCode MatGetRow(Mat mat, PetscInt row, PetscInt *ncols, const PetscInt *cols[], const PetscScalar *vals[])

Not Collective

Input Parameters#

mat - the matrix
row - the row to get

Output Parameters#

ncols - if not NULL, the number of nonzeros in the row
cols - if not NULL, the column numbers
vals - if not NULL, the values

Notes#

This routine is provided for people who need to have direct access to the structure of a matrix. We hope that we provide enough high-level matrix routines that few users will need it.

MatGetRow() always returns 0-based column indices, regardless of whether the internal representation is 0-based (default) or 1-based.

For better efficiency, set cols and/or vals to NULL if you do not wish to extract these quantities.

The user can only examine the values extracted with MatGetRow(); the values cannot be altered. To change the matrix entries, one must use MatSetValues().

You can only have one call to MatGetRow() outstanding for a particular matrix at a time, per processor. MatGetRow() can only obtain rows associated with the given processor, it cannot get rows from the other processors; for that we suggest using MatCreateSubMatrices(), then MatGetRow() on the submatrix. The row index passed to MatGetRow() is in the global number of rows.

Use MatGetRowIJ() and MatRestoreRowIJ() to access all the local indices of the sparse matrix.

Use MatSeqAIJGetArray() and similar functions to access the numerical values for certain matrix types directly.

Fortran Note#

The calling sequence is

   MatGetRow(matrix,row,ncols,cols,values,ierr)
         Mat     matrix (input)
         integer row    (input)
         integer ncols  (output)
         integer cols(maxcols) (output)
         double precision (or double complex) values(maxcols) output

where maxcols >= maximum nonzeros in any row of the matrix.

Caution#

Do not try to change the contents of the output arrays (cols and vals). In some cases, this may corrupt the matrix.

Level#

advanced

Location#

src/mat/interface/matrix.c

Examples#

src/ksp/ksp/tutorials/ex19.c
src/ksp/ksp/tutorials/ex59.c
src/mat/tutorials/ex12.c
src/mat/tutorials/ex16.c

Implementations#

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Index of all Mat routines
Table of Contents for all manual pages
Index of all manual pages