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# MatAssemblyBegin

Begins assembling the matrix. This routine should be called after completing all calls to `MatSetValues()`.

## Synopsis

    #include "petscmat.h"
    PetscErrorCode MatAssemblyBegin(Mat mat, MatAssemblyType type)

Collective

## Input Parameters

- mat - the matrix

- type - type of assembly, either `MAT_FLUSH_ASSEMBLY` or `MAT_FINAL_ASSEMBLY`

## Notes

`MatSetValues()` generally caches the values that belong to other MPI processes. The matrix is ready to use only after `MatAssemblyBegin()` and `MatAssemblyEnd()` have been called.

Use `MAT_FLUSH_ASSEMBLY` when switching between `ADD_VALUES` and `INSERT_VALUES` in `MatSetValues()`; use `MAT_FINAL_ASSEMBLY` for the final assembly before using the matrix.

ALL processes that share a matrix MUST call `MatAssemblyBegin()` and `MatAssemblyEnd()` the SAME NUMBER of times, and each time with the same flag of `MAT_FLUSH_ASSEMBLY` or `MAT_FINAL_ASSEMBLY` for all processes. Thus you CANNOT locally change from `ADD_VALUES` to `INSERT_VALUES`, that is a global collective operation requiring all processes that share the matrix.

Space for preallocated nonzeros that is not filled by a call to `MatSetValues()` or a related routine are compressed out by assembly. If you intend to use that extra space on a subsequent assembly, be sure to insert explicit zeros before `MAT_FINAL_ASSEMBLY` so the space is not compressed out.

## See Also

Matrices, `Mat`, `MatAssemblyEnd()`, `MatSetValues()`, `MatAssembled()`

## Level

beginner

## Location

src/mat/interface/matrix.c

## Examples

src/dm/impls/stag/tutorials/ex1.c
src/dm/impls/stag/tutorials/ex3.c
src/ksp/ksp/tutorials/ex12.c
src/dm/impls/stag/tutorials/ex4.c
src/ksp/ksp/tutorials/ex15.c
src/dm/impls/stag/tutorials/ex8.c
src/ksp/ksp/tutorials/ex13.c
src/dm/impls/stag/tutorials/ex2.c
src/ksp/ksp/tutorials/ex11.c
src/ksp/ksp/tutorials/ex1.c

## Implementations

MatAssemblyBegin_MPIAdj() in src/mat/impls/adj/mpi/mpiadj.c
MatAssemblyBegin_MPIAIJ() in src/mat/impls/aij/mpi/mpiaij.c
MatAssemblyBegin_MPIBAIJ() in src/mat/impls/baij/mpi/mpibaij.c
MatAssemblyBegin_MPIDense() in src/mat/impls/dense/mpi/mpidense.c
MatAssemblyBegin_Elemental() in src/mat/impls/elemental/matelem.cxx
MatAssemblyBegin_HYPRE() in src/mat/impls/hypre/mhypre.c
MatAssemblyBegin_IS() in src/mat/impls/is/matis.c
MatAssemblyBegin_Nest() in src/mat/impls/nest/matnest.c
MatAssemblyBegin_Preallocator() in src/mat/impls/preallocator/matpreallocator.c
MatAssemblyBegin_MPISBAIJ() in src/mat/impls/sbaij/mpi/mpisbaij.c
MatAssemblyBegin_ScaLAPACK() in src/mat/impls/scalapack/matscalapack.c
MatAssemblyBegin_MPISELL() in src/mat/impls/sell/mpi/mpisell.c

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