# 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 to remove

• rows - the global row indices

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

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

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

## 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 does not change the nonzero structure of the matrix, it merely zeros those entries in the matrix.

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.

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

intermediate

## Location#

src/mat/interface/matrix.c

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