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# MatZeroRows

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

## Synopsis

    #include "petscmat.h"
    PetscErrorCode MatZeroRows(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 zero

- rows - the global row indices

- diag - value put in the diagonal of the zeroed 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 entries

## Notes

This routine, along with `MatZeroRowsColumns()`, 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`.

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.

May be followed by using a `PC` of type `PCREDISTRIBUTE` to solve the reduced problem (`PCDISTRIBUTE` completely eliminates the zeroed rows and their corresponding columns) from the matrix.

Unlike `MatZeroRowsColumns()` for the `MATAIJ` and `MATBAIJ` matrix formats this removes the old nonzero structure, from the eliminated rows of the matrix but does not release memory. Because of this removal matrix-vector products with the adjusted matrix will be a bit faster. For the dense formats this does not alter the nonzero structure.

If the option `MatSetOption`(mat,`MAT_KEEP_NONZERO_PATTERN`,`PETSC_TRUE`) the nonzero structure of the matrix is not changed the values are merely zeroed.

The user can set a value in the diagonal entry (or for the `MATAIJ` format formats can optionally remove the main diagonal entry from the nonzero structure as well, by passing 0.0 as the final argument).

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.

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).

You can call `MatSetOption`(mat,`MAT_NO_OFF_PROC_ZERO_ROWS`,`PETSC_TRUE`) if each process indicates only rows it owns that are to be zeroed. This saves a global synchronization in the implementation.

## See Also

Matrices, `Mat`, `MatZeroRowsIS()`, `MatZeroRowsColumns()`, `MatZeroRowsLocalIS()`, `MatZeroRowsStencil()`, `MatZeroEntries()`, `MatZeroRowsLocal()`, `MatSetOption()`, `MatZeroRowsColumnsLocal()`, `MatZeroRowsColumnsLocalIS()`, `MatZeroRowsColumnsIS()`, `MatZeroRowsColumnsStencil()`, `PCREDISTRIBUTE`, `MAT_KEEP_NONZERO_PATTERN`

## Level

intermediate

## Location

src/mat/interface/matrix.c

## Examples

src/ksp/ksp/tutorials/ex51.c
src/ksp/ksp/tutorials/ex72.c
src/ksp/ksp/tutorials/ex49.c
src/ksp/ksp/tutorials/ex52.c
src/ksp/ksp/tutorials/ex3.c
src/tao/tutorials/ex3.c

## Implementations

MatZeroRows_MPIAIJ() in src/mat/impls/aij/mpi/mpiaij.c
MatZeroRows_SeqAIJ() in src/mat/impls/aij/seq/aij.c
MatZeroRows_MPIBAIJ() in src/mat/impls/baij/mpi/mpibaij.c
MatZeroRows_SeqBAIJ() in src/mat/impls/baij/seq/baij.c
MatZeroRows_MPIDense() in src/mat/impls/dense/mpi/mpidense.c
MatZeroRows_SeqDense() in src/mat/impls/dense/seq/dense.c
MatZeroRows_CUDA() in src/mat/impls/hypre/cuda/hypre1.cu
MatZeroRows_HIP() in src/mat/impls/hypre/hip/hypre2.hip.c
MatZeroRows_Kokkos() in src/mat/impls/hypre/kokkos/hypre3.kokkos.cxx
MatZeroRows_HYPRE() in src/mat/impls/hypre/mhypre.c
MatZeroRows_IS() in src/mat/impls/is/matis.c
MatZeroRows_Shell() in src/mat/impls/shell/shell.c

Index of all Mat routines
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Index of all manual pages