Frank,
Sorry for the confusion. MatCopy() and MatAXPY() only work for identically sized matrices. Even if the two matrices are the same size but one, say A, has a subset of nonzeros than the other, say B, what you want to do is non-trivial. This is because to do a copy you must MatAssembly the matrix first and this squeezes out the zero locations that you would like to fill in B. A simple solution is in the code that computes the subset of entries, call MatSetValues() twice, once for A and once for B. Not entirely satisfying but it will work and be reasonably efficient. Barry > On Sep 26, 2024, at 1:50 PM, Frank Bramkamp <bramk...@nsc.liu.se> wrote: > > Dear PETSc team, > > > I would like to create two matrices. The first matrix is based on a compact > stencil (COMPACT), and the second one has an extended stencil (WIDE) > So the non-zero elements of the COMPACT matrix is contained in the WIDE > matrix as well. But the WIDE matrix shall contain some additional elements. > I try to create the COMPACT matrix first, then copy it to the WIDE matrix and > then add some more terms to the WIDE matrix. > We want to create a matrix that is based on a lower order approximation of > our scheme which serves as basis matrix for preconditioning, > where we want to have a more accurate jacobian which has a larger stencil, > that is based on the lower approximation plus some additional non-zero > elements. > We want to avoid that we have to call the same routines twice to setup the > more accurate matrix, but copy the elements from the lower order (compact) > matrix. > > > I try to use MatCopy to copy the COMPACT matrix into the WIDE matrix. Somehow > I cannot figure out to do this, > since there is always a problem wirh non conforming sizes > > [0]PETSC ERROR: Nonconforming object sizes > [0]PETSC ERROR: Mat A,Mat B: global dim (4,8) (4,8) > > > Here is a basic example that I try. > Adding additional elements to the WIDE matrix is disabled right now, since > MatCopy already gives a problem. > I thought that in MatCopy(COMPACT, WIDE, DIFFERENT_NONZERO_PATTERN, ierr) > the option DIFFERENT_NONZERO_PATTERN should enable to copy different matrix > sizes into each other. > Or is that not possible to do ?! > > Here the COMPACT matrix is a 4x4 matrix and the WIDE matrix 8x8, for a basic > test. > > Thanks, Frank Bramkamp > > > > > program petsc_matrix_example > #include <petsc/finclude/petsc.h> > use petsc > implicit none > > PetscErrorCode :: ierr > Mat :: COMPACT, WIDE > PetscInt :: m, n, MM, NN, d_nz > PetscInt :: i, j > PetscScalar :: additional_value > > ! Initialize PETSc > call PetscInitialize(PETSC_NULL_CHARACTER, ierr) > > ! Set dimensions for COMPACT matrix > m = 4 ! Local rows > n = 4 ! Local columns > MM = 4 ! Global rows > NN = 4 ! Global columns > d_nz = 4 ! Estimated non-zeros per row (diagonal) o_nz = 0 ! Estimated > non-zeros per row (off-diagonal) > > ! Create COMPACT matrix > call MatCreateSeqAIJ(PETSC_COMM_SELF, MM, NN, PETSC_DECIDE, > PETSC_NULL_INTEGER, COMPACT, ierr) > > ! Set some values in COMPACT matrix (example) do i = 0, M-1 > do i = 0, NN-1 > !do j = max(0, i-1), min(NN-1, i+1) > do j = 0,NN-1 > call MatSetValue(COMPACT, i, j, 1.0_PETSC_REAL_KIND, INSERT_VALUES, ierr) > end do > end do > > ! Assemble COMPACT matrix > call MatAssemblyBegin(COMPACT, MAT_FINAL_ASSEMBLY, ierr) > call MatAssemblyEnd(COMPACT, MAT_FINAL_ASSEMBLY, ierr) > > ! Set dimensions for WIDE matrix > n = 8 ! Increase local columns > NN = 8 ! Increase global columns > m = 8 > MM = 8 > d_nz = 8 > > ! Create WIDE matrix > call MatCreateSeqAIJ(PETSC_COMM_SELF, MM, NN, PETSC_DECIDE, > PETSC_NULL_INTEGER, WIDE, ierr) > > ! Assemble WIDE matrix: MUST WE ASSMBLE THE MATRIX BEFORE MatCOPY ?! > call MatAssemblyBegin(WIDE, MAT_FINAL_ASSEMBLY, ierr) > call MatAssemblyEnd(WIDE, MAT_FINAL_ASSEMBLY, ierr) > > ! Copy elements from COMPACT to WIDE > call MatCopy(COMPACT, WIDE, DIFFERENT_NONZERO_PATTERN, ierr) > > > > !call MatAssemblyBegin(WIDE, MAT_FINAL_ASSEMBLY, ierr) > > ! Add additional elements to WIDE matrix > !!$ additional_value = 10.0 > !!$ > !!$ ! original > !!$ !do i = 0, MM-1 > !!$ ! do j = NN/2, NN-1 > !!$ ! call MatSetValue(WIDE, i, j, additional_value, INSERT_VALUES, ierr) > !!$ ! end do > !!$ !end do > !!$ > !!$ do i = 0, NN-1 > !!$ do j = max(0, i-1), min(NN-1, i+1) > !!$ call MatSetValue(WIDE, i, j, 10.0_PETSC_REAL_KIND, ADD_VALUES, ierr) > !!$ end do > !!$ end do > > > ! Assemble WIDE matrix again > call MatAssemblyBegin(WIDE, MAT_FINAL_ASSEMBLY, ierr) > call MatAssemblyEnd(WIDE, MAT_FINAL_ASSEMBLY, ierr) > > ! View matrices (optional) > call PetscPrintf(PETSC_COMM_WORLD, "COMPACT matrix:\n", ierr) > call MatView(COMPACT, PETSC_VIEWER_STDOUT_WORLD, ierr) > call PetscPrintf(PETSC_COMM_WORLD, "WIDE matrix:\n", ierr) > call MatView(WIDE, PETSC_VIEWER_STDOUT_WORLD, ierr) > > ! Clean up > call MatDestroy(COMPACT, ierr) > call MatDestroy(WIDE, ierr) > > ! Finalize PETSc > call PetscFinalize(ierr) > > end program petsc_matrix_example > >
