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<div class="moz-cite-prefix">Hi.<br>
I was also looking at the large amount of work and thought gone
into this - <br>
trying to figure out the use case of random access the whole time.<br>
<br>
Is it then planed that one could write something like<br>
(*it+gridView.step(0,3)) to go three elements in the x-direction
(direction 0)? <br>
I can easily see the use case for something like this to implement
FD type schemes on structured grids<br>
and have occasionally thought about options for doing this.<br>
Of course this wouldn't have required full random access iterators
but just some +operator....<br>
<br>
Is there some other usage of this feature you have in mind?<br>
<br>
Greetings<br>
Andreas<br>
<br>
On 01/10/14 07:58, Oliver Sander wrote:<br>
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<blockquote cite="mid:542BA60F.2050600@igpm.rwth-aachen.de"
type="cite">
<pre wrap="">Hi Jö,
thanks for the work. As usual, it seems very well thought-out.
However, mainly to satisfy my curiosity, here a two questions:
- was this discussed in Berlin? I can't find it in the meeting minutes,
- what is the use-case for random access iterators?
Again, this is not to say that I object to the feature.
Cheers,
Oliver
Am 30.09.2014 um 23:17 schrieb Jö Fahlke:
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<pre wrap="">Hi!
The random-access entity iterators are now ready for general review.
<a class="moz-txt-link-freetext" href="https://cgit.dune-project.org/repositories/dune-grid/log/?h=feature/random-access-entity-iterators">https://cgit.dune-project.org/repositories/dune-grid/log/?h=feature/random-access-entity-iterators</a>
@Carsten: are you willing to include this in 2.4, provided noone finds any
serious issues?
A grid declares that certain entity iterators are random-access by doing
typedef std::random_access_iterator_tag iterator_category;
in the iterator implementation class. The facade class Dune::EntityIterator
notices this and defines the additional methods needed for random access
iterators in terms of the member functions advance(n) and distanceTo(otherIt)
of the iterator implementation class. If your grids entity iterators do
typedef std::forward_iterator_tag iterator_category;
or omit the member type completely, the EntityIterator will be a forward
interator, as before.
To do this, Dune::EntityIterator no longer implements the iterator methods
itself, but derives from Dune::EntityIteratorBase, which has a template
parameter category to select which type of iterator to implement. Previously
the ancestry looked like
EntityPointer
EntityIterator
now it looks like (for forward iterators)
EntityPointer
EntityIteratorBase<..., std::forward_iterator_tag>
EntityIterator
or (for random access iterators)
EntityPointer
EntityIteratorBase<..., std::forward_iterator_tag>
EntityIteratorBase<..., std::random_access_iterator_tag>
EntityIterator
The template argument list of EntityIterator is completely the same as before.
There is one function that has been omitted from random access iterators for
the time being, namely operator[]. There are three possibilities for the
return type of operator[]: Either return the entity by value -- but that needs
copyable entities, which are not ready yet. Or return a reference to the
entity -- not possible, since we have nothing that stores that entity. Or
return a proxy object -- but I'm too lazy to implement that, since we will
have copyable entities soon. Note that instead of the (currently) invalid
expression "it[n]" you can write "*(it+n)": The + creates a temporary
iterator that holds the entity.
I also omitted implementing bidirectional iterators, since I could not come up
with a use case for iterating backwards through the grid. If a grid declares
an iterator implementation as bidirectional, the resulting EntityIterator will
nevertheless just be a forward iterator. It should be easy to add
bidirectional iterators, should the need arise.
Regards,
Jö.
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