[petsc-users] determining dof at runtime

[Juha Jäykkä]

Hi list!

I am trying to create a 3D DA such that I can use DMDAVecGetArray() and access 
it using array indexing. I know how to get several dof in the DA and how using 
a struct to access the members works, when the dof is determined at compile-
time (so accesses like array[1][2][3].member1[2] are computed correctly.

However, I would like to determine dof at run-time. Creating a struct with 
pointers (even of the type member[]) of course gives me a struct whose size 
depends on the *pointer* size, not my dof. So I was wondering if I could work 
around this somehow, but do not seem to be able to.

My DA is created by

DMDACreate3d(PETSC_COMM_WORLD,
             DMDA_BOUNDARY_GHOSTED, DMDA_BOUNDARY_GHOSTED, 
             DMDA_BOUNDARY_GHOSTED, DMDA_STENCIL_BOX,
             2, 2, 2,
             PETSC_DECIDE, PETSC_DECIDE, PETSC_DECIDE,
             ndof, NGHOSTS,
             PETSC_NULL, PETSC_NULL, PETSC_NULL, 
             &da);

so my Vec does have the correct dof, but I cannot seem to fool C to address 
this as array[z][y][x][n] or something of the like, where the last index is 
the index to ndof. (Usually I would index that as array[z][y][x].member[n], 
but now I do not know how to make that happen since I cannot define a struct 
whose storage size is not known to the compiler.)

Any ideas? Or am I trying to accomplish something which is not possible? (If 
so, why would that be? The vector is of the correct size.)

Cheers,
-Juha

-- 
		 -----------------------------------------------
		| Juha Jäykkä, juhaj at iki.fi			|
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