[petsc-users] Symmetric matrix: Setting entries below diagonal

[Barry Smith]

> On Apr 7, 2017, at 6:40 AM, Florian Lindner <mailinglists at xgm.de> wrote:
> 
> Hello,
> 
> two questions about symmetric (MATSBAIJ) matrices.
> 
> + Entries set with MatSetValue below the main diagonal are ignored. Is that by design?

   Yes

> I rather expected setting A_ij to
> have the same effect as setting A_ji.

   You need to check the relationship between i and j and swap them if needed before the call.

> 
> + Has MatSetOption to MAT_SYMMETRIC and MAT_SYMMETRIC_ETERNAL any gain on MATSBAIJ matrices?

  No

> 
> Thanks,
> Florian
> 
> Test programm:
> 
> 
> #include "petscmat.h"
> #include "petscviewer.h"
> 
> int main(int argc, char **argv)
> {
>  PetscInitialize(&argc, &argv, "", NULL);
>  PetscErrorCode ierr = 0;
> 
>  Mat A;
>  ierr = MatCreate(PETSC_COMM_WORLD, &A); CHKERRQ(ierr);
>  MatSetType(A, MATSBAIJ); CHKERRQ(ierr);
>  ierr = MatSetSizes(A, 4, 4, PETSC_DECIDE, PETSC_DECIDE); CHKERRQ(ierr);
>  ierr = MatSetUp(A); CHKERRQ(ierr);
>  ierr = MatSetOption(A, MAT_SYMMETRIC, PETSC_TRUE); CHKERRQ(ierr);
>  ierr = MatSetOption(A, MAT_SYMMETRY_ETERNAL, PETSC_TRUE); CHKERRQ(ierr);
> 
>  // Stored
>  ierr = MatSetValue(A, 1, 2, 21, INSERT_VALUES); CHKERRQ(ierr);
>  ierr = MatSetValue(A, 1, 1, 11, INSERT_VALUES); CHKERRQ(ierr);
> 
>  // Ignored
>  ierr = MatSetValue(A, 2, 1, 22, INSERT_VALUES); CHKERRQ(ierr);
>  ierr = MatSetValue(A, 3, 2, 32, INSERT_VALUES); CHKERRQ(ierr);
>  ierr = MatSetValue(A, 3, 1, 31, INSERT_VALUES); CHKERRQ(ierr);
> 
>  ierr = MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
>  ierr = MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
> 
>  PetscViewer viewer;
>  ierr = PetscViewerCreate(PETSC_COMM_WORLD, &viewer); CHKERRQ(ierr);
>  ierr = PetscViewerSetType(viewer, PETSCVIEWERASCII); CHKERRQ(ierr);
>  ierr = PetscViewerPushFormat(viewer, PETSC_VIEWER_ASCII_DENSE); CHKERRQ(ierr);
>  ierr = MatView(A, viewer); CHKERRQ(ierr);
>  ierr = PetscViewerPopFormat(viewer); CHKERRQ(ierr);
>  ierr = PetscViewerDestroy(&viewer); CHKERRQ(ierr);
> 
>  PetscFinalize();
>  return 0;
> }