[petsc-users] Getting vectors of field variables of a DMPlex

[Andrea Irwin via petsc-users]

Hi,

I'm new to PETSc, and I'm trying to learn how to use the DMPlex object. I've 
learned that you can assign labeled "fields" to a PETScSection/DMPlex, but I'm 
a bit lost as to how to access the data corresponding to each field.

The documentation suggests that data from different fields is stored in the 
same big vector, and you can access it with PetscSectionGetFieldOffset(). I've 
tried this in the snippet below (using petsc4py)

# get points from the layout
pstart, pend = section.getChart()

# set vector values according to field
arr = vec.getArray()

for point in range(pstart, pend):
    print(point)
    offset_u = section.getFieldOffset(point, 0)
    offset_v = section.getFieldOffset(point, 1)
    offset_w = section.getFieldOffset(point, 2)

    print(offset_u, offset_v, offset_w)

    arr[offset_u] = 1

    for d in range(num_comp[1]):
        arr[offset_v + d] = 2

    for d in range(num_comp[2]):
        arr[offset_w + d] = 3

Unfortunately, this either this doesn't work or I'm not using it right. The 
offsets are often equal when they should be different for different variables, 
and they get larger than the number of points in the vector. Despite being 
recommended directly in the documentation, there aren't any examples of it 
being used in context either.

A previous entry in the mailing list suggests using DMCreateSubDM() to instead 
split the DM (and the vectors, in the process) up by field. There's also 
DMCreateFieldDecomposition(), which appears to do something similar.

Is there a "canonical" way to access different fields? Does anyone have some 
simple examples of different field variables being declared, allocated, set, 
and accessed? I'm still new enough that I'm not even sure what questions to ask 
at this point, but any help would be appreciated.

Thank you,

Andrea Irwin