petsc-3.13.6 2020-09-29

MatGetRow

Gets a row of a matrix. You MUST call MatRestoreRow() for each row that you get to ensure that your Section 1.5 Writing Application Codes with PETSc 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.

Fortran Notes

The calling sequence from Fortran 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/mat/tutorials/ex12.c.html
src/mat/tutorials/ex16.c.html
src/ksp/ksp/tutorials/ex59.c.html

Implementations