# MatZeroRowsColumns#

Zeros all entries (except possibly the main diagonal) of a set of rows and columns of a matrix.

## Synopsis#

#include "petscmat.h"
PetscErrorCode MatZeroRowsColumns(Mat mat, PetscInt numRows, const PetscInt rows[], PetscScalar diag, Vec x, Vec b)


Collective

## Input Parameters#

• mat - the matrix

• numRows - the number of rows/columns to zero

• rows - the global row indices

• diag - value put in the diagonal of the eliminated rows

• x - optional vector of the solution for zeroed rows (other entries in vector are not used), these must be set before this call

• b - optional vector of the right-hand side, that will be adjusted by provided solution entries

## Notes#

This routine, along with MatZeroRows(), is typically used to eliminate known Dirichlet boundary conditions from a linear system.

For each zeroed row, the value of the corresponding b is set to diag times the value of the corresponding x. The other entries of b will be adjusted by the known values of x times the corresponding matrix entries in the columns that are being eliminated

If the resulting linear system is to be solved with KSP then one can (but does not have to) call KSPSetInitialGuessNonzero() to allow the Krylov method to take advantage of the known solution on the zeroed rows.

For the parallel case, all processes that share the matrix (i.e., those in the communicator used for matrix creation) MUST call this routine, regardless of whether any rows being zeroed are owned by them.

Unlike MatZeroRows(), this ignores the MAT_KEEP_NONZERO_PATTERN option value set with MatSetOption(), it merely zeros those entries in the matrix, but never removes them from the nonzero pattern. The nonzero pattern of the matrix can still change if a nonzero needs to be inserted on a diagonal entry that was previously missing.

Each processor can indicate any rows in the entire matrix to be zeroed (i.e. each process does NOT have to list only rows local to itself).

The option MAT_NO_OFF_PROC_ZERO_ROWS does not apply to this routine.

Matrices, Mat, MatZeroRowsIS(), MatZeroRows(), MatZeroRowsLocalIS(), MatZeroRowsStencil(), MatZeroEntries(), MatZeroRowsLocal(), MatSetOption(), MatZeroRowsColumnsLocal(), MatZeroRowsColumnsLocalIS(), MatZeroRowsColumnsIS(), MatZeroRowsColumnsStencil()

intermediate

## Location#

src/mat/interface/matrix.c