#include "petscmat.h" PetscErrorCode MatSetValuesStencil(Mat mat,PetscInt m,const MatStencil idxm[],PetscInt n,const MatStencil idxn[],const PetscScalar v[],InsertMode addv)Not Collective
| mat | - the matrix | |
| m | - number of rows being entered | |
| idxm | - grid coordinates (and component number when dof > 1) for matrix rows being entered | |
| n | - number of columns being entered | |
| idxn | - grid coordinates (and component number when dof > 1) for matrix columns being entered | |
| v | - a logically two-dimensional array of values | |
| addv | - either ADD_VALUES or INSERT_VALUES, where ADD_VALUES adds values to any existing entries, and INSERT_VALUES replaces existing entries with new values |
Calls to MatSetValuesStencil() with the INSERT_VALUES and ADD_VALUES options cannot be mixed without intervening calls to the assembly routines.
The grid coordinates are across the entire grid, not just the local portion
MatSetValuesStencil() uses 0-based row and column numbers in Fortran as well as in C.
For setting/accessing vector values via array coordinates you can use the DMDAVecGetArray() routine
In order to use this routine you must either obtain the matrix with DMCreateMatrix() or call MatSetLocalToGlobalMapping() and MatSetStencil() first.
The columns and rows in the stencil passed in MUST be contained within the ghost region of the given process as set with DMDACreateXXX() or MatSetStencil(). For example, if you create a DMDA with an overlap of one grid level and on a particular process its first local nonghost x logical coordinate is 6 (so its first ghost x logical coordinate is 5) the first i index you can use in your column and row indices in MatSetStencil() is 5.
In Fortran idxm and idxn should be declared as
MatStencil idxm(4,m),idxn(4,n)
and the values inserted using
idxm(MatStencil_i,1) = i
idxm(MatStencil_j,1) = j
idxm(MatStencil_k,1) = k
idxm(MatStencil_c,1) = cetc
For periodic boundary conditions use negative indices for values to the left (below 0; that are to be obtained by wrapping values from right edge). For values to the right of the last entry using that index plus one etc to obtain values that obtained by wrapping the values from the left edge. This does not work for anything but the DM_BOUNDARY_PERIODIC boundary type.
For indices that don't mean anything for your case (like the k index when working in 2d) or the c index when you have a single value per point) you can skip filling those indices.
Inspired by the structured grid interface to the HYPRE package (https://computation.llnl.gov/projects/hypre-scalable-linear-solvers-multigrid-methods)