Re: [Libmesh-users] Create a new element iterator

[Roy S. <roy...@ic...>]

On Wed, 28 Jun 2017, John Peterson wrote:

> On Wed, Jun 28, 2017 at 9:36 AM, Harshad Sahasrabudhe <hsa...@pu...>
> wrote:
>
>> I would like to create a submesh of my mesh inside a particular region.
>> Since the UnstructuredMesh::create_submesh method uses a
>> const_element_iterator on the old mesh to create a new mesh, I would like
>> to create a new single_predicate which identifies elements that lie inside
>> the region of interest. What is the process by which I can create a new
>> predicate and a new element iterator? Will this method work for
>> create_submesh function?
>
> You might not need to define a new type of iterator at all. For example, if
> you could first mark the elements in your region of interest with a
> specific subdomain id, then you could just use the
> existing active_subdomain_elements_begin/end iterators in the call to
> create_submesh().

> The most recent mesh iterator was in added in d7a08b2f, so you should be
> able to use that commit for inspiration... but I would say that it's not
> super straightforward to do.

It's also really only appropriate for iterator predicates which would
be of wide general use.

It would be quite useful to be able to instead create a range of
iterators with any arbitrary one-off user-supplied predicate the user
wants.  Super-useful if the user could assign a lambda function for
their predicate.  We can't do that at the moment (except for an ugly
trick for a special case; see "fake_elem_it" in sparsity_pattern.C)
but it might not be too hard to add.
---
Roy