Replace whole row using MatSetValuesStencil with INSERT_VALUES - c++

Question

I am using Petsc Ksp routines.
I construct an operator using MatSetValuesStencil, where in each call of this function I specify one row matrix values of length 5.
There is a case where I sometimes need to completely replace a row from a 5 length stencil to a 3 length one. Will INSERT_VALUES mode leave the two values on non changed positions or it will discard them to zero?

Answer 1

The elements of the matrix that are not specified in the arguments idxm and idxn of the function MatSetValuesStencil(...) are left unchanged, even if INSERT_VALUES is used.
Here is a little code starting from ksp_ex29 to test it :
static char help[] = "Does INSERT_VALUES changes the whole row ? No.\n\n";
#include <petscdm.h>
#include <petscdmda.h>
#include <petscksp.h>
extern PetscErrorCode ComputeMatrix42(DM da,Mat jac);
extern PetscErrorCode ComputeMatrix(DM da,Mat jac);
#undef __FUNCT__
#define __FUNCT__ "main"
int main(int argc,char **argv)
{
DM da;
PetscErrorCode ierr;
Mat matrix;
PetscInitialize(&argc,&argv,(char*)0,help);
ierr = DMDACreate2d(PETSC_COMM_WORLD, DM_BOUNDARY_PERIODIC, DM_BOUNDARY_PERIODIC,DMDA_STENCIL_STAR,-3,-3,PETSC_DECIDE,PETSC_DECIDE,1,1,0,0,&da);CHKERRQ(ierr);
DMCreateMatrix(da,&matrix);
ComputeMatrix(da,matrix);
PetscPrintf(PETSC_COMM_WORLD,"A matrix of negative terms : \n");
MatView(matrix, PETSC_VIEWER_STDOUT_WORLD );
ComputeMatrix42(da,matrix);
PetscPrintf(PETSC_COMM_WORLD,"The diagonal, i-1 and i+1 are set to 42 : \n");
MatView(matrix, PETSC_VIEWER_STDOUT_WORLD );
ierr = DMDestroy(&da);CHKERRQ(ierr);
ierr = MatDestroy(&matrix);CHKERRQ(ierr);
ierr = PetscFinalize();
return 0;
}
#undef __FUNCT__
#define __FUNCT__ "ComputeMatrix"
PetscErrorCode ComputeMatrix(DM da,Mat jac)
{
PetscErrorCode ierr;
PetscInt i,j,mx,my,xm,ym,xs,ys;
PetscScalar v[5];
MatStencil row, col[5];
PetscFunctionBeginUser;
ierr = DMDAGetInfo(da,0,&mx,&my,0,0,0,0,0,0,0,0,0,0);CHKERRQ(ierr);
ierr = DMDAGetCorners(da,&xs,&ys,0,&xm,&ym,0);CHKERRQ(ierr);
for (j=ys; j<ys+ym; j++) {
for (i=xs; i<xs+xm; i++) {
row.i = i; row.j = j;
v[0] = -1; col[0].i = i; col[0].j = j-1;
v[1] = -1; col[1].i = i-1; col[1].j = j;
v[2] = -13; col[2].i = i; col[2].j = j;
v[3] = -1; col[3].i = i+1; col[3].j = j;
v[4] = -1; col[4].i = i; col[4].j = j+1;
ierr = MatSetValuesStencil(jac,1,&row,5,col,v,INSERT_VALUES);CHKERRQ(ierr);
}
}
ierr = MatAssemblyBegin(jac,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
ierr = MatAssemblyEnd(jac,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
PetscFunctionReturn(0);
}
#undef __FUNCT__
#define __FUNCT__ "ComputeMatrix42"
PetscErrorCode ComputeMatrix42(DM da,Mat jac)
{
PetscErrorCode ierr;
PetscInt i,j,mx,my,xm,ym,xs,ys;
PetscScalar v[3];
MatStencil row, col[3];
PetscFunctionBeginUser;
ierr = DMDAGetInfo(da,0,&mx,&my,0,0,0,0,0,0,0,0,0,0);CHKERRQ(ierr);
ierr = DMDAGetCorners(da,&xs,&ys,0,&xm,&ym,0);CHKERRQ(ierr);
for (j=ys; j<ys+ym; j++) {
for (i=xs; i<xs+xm; i++) {
row.i = i; row.j = j;
v[0] = 42; col[0].i = i-1; col[0].j = j;
v[1] = 42; col[1].i = i; col[1].j = j;
v[2] = 42; col[2].i = i+1; col[2].j = j;
ierr = MatSetValuesStencil(jac,1,&row,3,col,v,INSERT_VALUES);CHKERRQ(ierr);
}
}
ierr = MatAssemblyBegin(jac,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
ierr = MatAssemblyEnd(jac,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
PetscFunctionReturn(0);
}
Compile it with the following makefile :
include ${PETSC_DIR}/conf/variables
include ${PETSC_DIR}/conf/rules
main: main.o chkopts
-${CLINKER} -o main main.o ${PETSC_LIB}
${RM} main.o
Output :
A matrix of negative terms :
Mat Object: 1 MPI processes
type: seqaij
row 0: (0, -13) (1, -1) (2, -1) (3, -1) (6, -1)
row 1: (0, -1) (1, -13) (2, -1) (4, -1) (7, -1)
row 2: (0, -1) (1, -1) (2, -13) (5, -1) (8, -1)
row 3: (0, -1) (3, -13) (4, -1) (5, -1) (6, -1)
row 4: (1, -1) (3, -1) (4, -13) (5, -1) (7, -1)
row 5: (2, -1) (3, -1) (4, -1) (5, -13) (8, -1)
row 6: (0, -1) (3, -1) (6, -13) (7, -1) (8, -1)
row 7: (1, -1) (4, -1) (6, -1) (7, -13) (8, -1)
row 8: (2, -1) (5, -1) (6, -1) (7, -1) (8, -13)
The diagonal, i-1 and i+1 are set to 42 :
Mat Object: 1 MPI processes
type: seqaij
row 0: (0, 42) (1, 42) (2, 42) (3, -1) (6, -1)
row 1: (0, 42) (1, 42) (2, 42) (4, -1) (7, -1)
row 2: (0, 42) (1, 42) (2, 42) (5, -1) (8, -1)
row 3: (0, -1) (3, 42) (4, 42) (5, 42) (6, -1)
row 4: (1, -1) (3, 42) (4, 42) (5, 42) (7, -1)
row 5: (2, -1) (3, 42) (4, 42) (5, 42) (8, -1)
row 6: (0, -1) (3, -1) (6, 42) (7, 42) (8, 42)
row 7: (1, -1) (4, -1) (6, 42) (7, 42) (8, 42)
row 8: (2, -1) (5, -1) (6, 42) (7, 42) (8, 42)
I am using PETSC 3.5.2.