Several years ago I asked a similar question and this is what Barry told me:
>>>>>>>>>>
Randy,
Take a look at DMDAGetRay() in src/dm/impls/da/dasub.c (now called
DMDACreateRay()) this takes a row or column from a 2d DADM. You can use the
same kind of approach to get a slice from a 3d DMDA.
Barry
<<<<<<<<<<
Maybe this would help you?
Randy
> On Jan 23, 2021, at 4:45 PM, Sajid Ali <sajidsyed2...@u.northwestern.edu>
> wrote:
>
> Hi PETSc-developers,
>
> For an application, I'd like to extract a 2D slice from a 3D DMDA vector,
> perform a `MatMult` on it (a discretized rotation on the 2D vector) and place
> the resulting vector back into the 3D vector. The approach I'd taken was to
> use `DMDACreatePatchIS` to create an IS that selects the slice (say this
> slice is [:,:,n_z] with no loss of generality). This IS is then used to
> extract the 2D slice via the `VecGetSubVector` utility. The issue that arises
> from this scheme is that the extracted 2D vector does not represent a
> flattened 2D vector, it is instead ordered as a DMDA vector (aka each rank
> having the portion of vector it owns in a column major ordering). How do I
> convert this DMDA ordered 2D subvector to a vector that represents a
> flattened 2D array? Since I can't predict how the extracted sub-vector
>
> While PETSc provides `DMDAGlobalToNatural` routines, those don't apply to
> extracted sub-vectors and extracting the 2D slice from a natural vector
> (filled by a scatter from the global vector) does not work either. On the
> other hand, converting the output of DMDACreatePatchIS to a natural IS via
> `AOPetscToApplicationIS` before extracting a subvector extracts the wrong
> indices from the 3D vector.
>
> Could someone tell me how I can achieve the desired goal (converting a 2D
> vector ordered as per a 3D DMDA grid onto a natural flattened index) ? Thanks
> in advance for the help!
>
> PS : Should it help, I'm attaching a slightly modified version of
> `src/dm/tests/ex53.c` to demonstrate the issue. The array I wish to obtain
> (the [:,:,2]) slice is : `[196., 194., 192., 190., 198., 197., 196., 195.,
> 200., 200., 200.,200., 202., 203., 204., 205.]` (a row-major flattening of
> the 2D slice or alternatively a column-major flattening). What I obtain from
> the program (by creating the PatchIS and extracting the subvector) instead is
> : `[0 (A)] 196,198,194,197 [1 (B)] 200,202,200,203 [2 (C)] 192,196,190,195 [3
> (D)] 200,204,200,205` (where each square bracket represents the rank and
> portion of the 2D vector being flattened via a column-major formatting with
> the layout shown below)
> |---------|
> | A | C |
> | B | D |
> |---------|
>
>
> Thank You,
> Sajid Ali (he/him) | PhD Candidate
> Applied Physics
> Northwestern University
> s-sajid-ali.github.io <http://s-sajid-ali.github.io/><ex53.c>
