[petsc-users] IJacobian "PetscReal a"

[Matthew Knepley]

On Thu, Aug 29, 2013 at 4:01 PM, Jin, Shuangshuang <
Shuangshuang.Jin at pnnl.gov> wrote:

>  Hello, when I look at IJacobian function in the example: *
> http://www.mcs.anl.gov/petsc/petsc-current/src/ts/examples/tutorials/ex19.c
> *<http://www.mcs.anl.gov/petsc/petsc-current/src/ts/examples/tutorials/ex19.c>
>
> static PetscErrorCode IJacobian(TS ts,PetscReal t,Vec X,Vec Xdot,PetscReal
> a,Mat *A,Mat *B,MatStructure *flag,void *ctx)
> {
>   PetscErrorCode ierr;
>   PetscInt       rowcol[] = {0,1};
>   PetscScalar    *x,J[2][2];
>
>   PetscFunctionBeginUser;
>   ierr    = VecGetArray(X,&x);CHKERRQ(ierr);
>   J[0][0] = a;    J[0][1] = -1.;
>   J[1][0] = 1.;   J[1][1] = -1. + x[1]*x[1];
>   ierr    =
> MatSetValues(*B,2,rowcol,2,rowcol,&J[0][0],INSERT_VALUES);CHKERRQ(ierr);
>   ierr    = VecRestoreArray(X,&x);CHKERRQ(ierr);
>
>   ierr = MatAssemblyBegin(*A,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
>   ierr = MatAssemblyEnd(*A,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
>   if (*A != *B) {
>     ierr = MatAssemblyBegin(*B,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
>     ierr = MatAssemblyEnd(*B,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
>   }
>   *flag = SAME_NONZERO_PATTERN;
>   PetscFunctionReturn(0);
> }
>
> There’s a PetscReal a. Is it a constant? And if the value of a known
> outside of the IJacobian function, saying the main() part.
>

Its the shift for J_udot. See 6.1.1 in the manual.

   Matt


> If it is, then as shown in this example, J[0][0] = a; J[0][1] = -1.;
> J[1][0] = 1.; are constant through all the iterations. How can I use MatRetrieveValues(*A)
> and MatStoreValues(*A) to reuse them?
>
> I’m asking this question because I have a large Jacobian matrix, but half
> of the matrix contains constant values. However, I don’t know what’s
> “PetscReal a” when I try to use it to compute my constant elements of
> Jacobian matrix which doesn’t depend on the value of x at all.
>
> Thanks,
> Shuangshuang
>
>
>
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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