[petsc-users] Suggestions for code with dof >> 1 ?

[Jed Brown]

Christophe Ortiz <christophe.ortiz at ciemat.es> writes:
> I am facing some problems with this when filling in the Jacobian for a
> problem of 2 diffusion equations (dof=2). See ex25.c.
>
> If I'm right, then for node i, there should be 3 blocks of 2x2:
>
> X  0  |  Y  0  |  X  0
> 0  X  |  0  Y  |  0  X
>
> Is it correct ?
>
> First I tried what you said, to pass a one dimensional array:
>    row = i;
>    cols[0] = i-1; cols[1] = i; cols[2] = i+1;
>
>    j = 0;
>    vallin[j] = X; vallin[j++] = 0; 

The expression j++ in vallin[j++] is post-increment, so the two
statements above both write to vallin[0] and vallin[11] is undefined
(whatever garbage was in the array before).

> Then, I tried a 2-dimensional array. I saw in some examples of ts/tutorials
> that it is also possible to pass a multi-dimensional arrays. After all, in
> memory the rows are stored contiguously, so I guess it is equivalent to a
> one-dimensional array.
>
>       j = 0; k = 0;
>       valdiff[j][k] = X; valdiff[j][k++] = 0;

Same problem here.

>       valdiff[j][k++] = Y; valdiff[j][k++] = 0;
>       valdiff[j][k++] = X; valdiff[j][k++] = 0;
>
>       j= 1; k = 0;
>       valdiff[j][k] = 0; valdiff[j][k++] = X;
>       valdiff[j][k++] = 0; valdiff[j][k++] = Y;
>       valdiff[j][k++] = 0; valdiff[j][k++] = X;
>
>   ierr =
> MatSetValuesBlocked(*Jpre,1,&row,3,cols,&valdiff[0][0],INSERT_VALUES);
>
> I get the same error message.
> What did I do wrong ?
>
> In ex25.c they use a 3-dimensional array:
>
>  const PetscScalar vals[2][3][2] = {{{dxxL,0},{a *hx+dxx0,0},{dxxR,0}},
>
>  {{0,dxxL},{0,a*hx+dxx0},{0,dxxR}}};
>
> and &vals[0][0][0] is passed in MatSetValuesBlocked (....,....,
> &vals[0][0][0],...), and it works, though I don't understand how.

That grouping in the 3D array is the same ordering, but (perhaps) makes
it easier to see which expression refers to which entry of the matrix.
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