Differences between cell_iterators() and mg_cell_iterators()

[yy.wayne]

I'm a bit confused on the effect of cell_iterators (similar ones are cell_iterators_on_level and dof_handler.begin()) and mg_cell_iterators. In step16, begin_mg() is used when assembling multigrid, while in step56 we use cell_iterators rather than mg_cell_iterators. I think they both loop through all cells, active or not. In documentation the difference is mg_cell_iterators "return an iterator in their level-cell form".  May some expert explain this a little further?

[Timo Heister]

This is not documented well and the doxygen types shown are not very
helpful. In the end it boils down to the following:
*_mg_* functions return level or multigrid iterators that have an
accessor (that is inside the iterator) with the template argument
level_dof_access = true. The only difference, if I remember correctly,
is that get_active_or_mg_dof_indices() will return the result of
get_mg_dof_indices() instead of get_dof_indices(). So, if you use
level iterators, you can use get_active_or_mg_dof_indices() in your
assembly and it will automatically do the right thing. This means you
can use the same assembler for active and mg levels.

See https://www.dealii.org/developer/doxygen/deal.II/classDoFCellAccessor.html#a15ef35e919b1005dc6050f9bca96956d

On Sun, Jan 1, 2023 at 8:45 AM 'yy.wayne' via deal.II User Group
<dea...@googlegroups.com> wrote:
>
> I'm a bit confused on the effect of cell_iterators (similar ones are cell_iterators_on_level and dof_handler. begin()) and mg_cell_iterators. In step16, begin_mg() is used when assembling multigrid, while in step56 we use cell_iterators rather
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> I'm a bit confused on the effect of cell_iterators (similar ones are cell_iterators_on_level and dof_handler.begin()) and mg_cell_iterators. In step16, begin_mg() is used when assembling multigrid, while in step56 we use cell_iterators rather than mg_cell_iterators. I think they both loop through all cells, active or not. In documentation the difference is mg_cell_iterators "return an iterator in their level-cell form". May some expert explain this a little further?
>

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--
Timo Heister
http://www.math.clemson.edu/~heister/

[yy.wayne]

That's great. Thank you.