dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Dec 01, 2009 at 07:16:36PM +0000, Garth N. Wells wrote:
>
>
> Anders Logg wrote:
> > On Tue, Dec 01, 2009 at 07:06:43PM +0000, Garth N. Wells wrote:
> >>
> >> Anders Logg wrote:
> >>> On Tue, Dec 01, 2009 at 09:59:18AM -0800, Johan Hake wrote:
> >>>> On Tuesday 01 December 2009 00:45:50 Anders Logg wrote:
> >>>>> Would it help to add a new class on the C++ side that is used only for
> >>>>> passing array data back and forth between C++ and Python? We have had
> >>>>> this before (SimpleArray) and it would be fairly easy to extend the
> >>>>> C++ with extra functions in the interface that use SimpleArray instead
> >>>>> of std::vector.
> >>>>>
> >>>>> Then perhaps we can have one single typemap that hits SimpleArray
> >>>>> everywhere and converts it to a NumPy array.
> >>>> Yes, something in that direction is what I had in mind. In addition we could
> >>>> also add a foo.array() function to get a NumPy view from this class. This
> >>>> would be nice when we do not want to have all the communication through
> >>>> typemaps, but actually using the SimpleArray in Python as return argument from
> >>>> some function that wants to resize the array.
> >>>>
> >>>> We would also need some stuff to handle memory management.
> >>>>
> >>>> I see two fundamental ways such a class could be used:
> >>>>   1) A replacement for the previous use of double/uint/int*, now std::vector
> >>>>   2) A replacement for communication using std::vector where resize
> >>>>      flexibility is needed.
> >>>>
> >>>> I think 1, speaks for it self. 2 is where we need to resize any passed vector.
> >>>> This goes for GenericMatrix.getrow, foo.intersection,
> >>>> GenericFunction.comput_vertex_values.
> >>>>
> >>>>> And the work would be to add the extra stuff on the C++ side. The
> >>>>> advantage would be less complex wrapper code and that Garth and I
> >>>>> are capable of handling the complexities on the C++ side.
> >>>> Yes this must be a goal. I agree that the present SWIG situation has grown out
> >>>> of hands.
> >>>>
> >>>>> But what I don't understand is why it would be easier to write a
> >>>>> typemap for SimpleArray than for std::vector. Both of them use
> >>>>> contiguous memory.
> >>>> Yes, but in std::vector it is now way, I suppose, to prevent a vector to
> >>>> delete its data when it goes out of scope. This is necessary in a typical in
> >>>> typemap.
> >>> ok, let's create a very flexible array class that is targeted at
> >>> simple communication between C++ and Python/NumPy. We had a class
> >>> before named SimpleArray. We might call it NumPyArray or PythonArray.
> >>>
> >> Can we just call it Array? It will be visible in the C++ interface (e.g.
> >> in eval) so it would be good to have a nice name.
> >
> > I thought we should not name it Array so as to not encourage use of
> > it, or do you suggest using it instead of std::vector.
> >
>
> Yes.
>
> > I was thinking of having it in addition to std::vector.
> >
>
> We would have multiple versions of eval functions if we support
> std::vector and Array. It wouldn't be clear to a user which to use.
> Also, we could avoid some copying of data if we use Array since an Array
> could be initialised with a pointer to some data, whereas a vector can't.

It makes sense, but I'll need some more convincing. We have an Array
class before. The reason I added it then was that we wanted a
nice-looking interface with minimal visibility of other libraries.
We also had a List class for a linked list etc.

But having an Array class does make some sense considering we have
Vector and Matrix classes (that happen to be implemented using for
example PETSc).

> >>> I have created a blueprint:
> >>>
> >>> https://blueprints.launchpad.net/dolfin/+spec/array-typemaps
> >>>
> >> I'll add something. I was thinking already about this. With a smart
> >> pointer to the underlying data we should be to devise an elegant memory
> >> model and be able to tell an Array object when it does and doesn't own
> >> the data upon construction, and be able to change during execution.
> >
> > Sounds good.
> >
> >>> We can fill out the details together.
> >>>
> >>>> I will fix the interface of getrow this evening. I was about to do it
> >>>> yesterday, but instead I got grumpy :) But a good night sleep makes wonders!
> >>> Good! :-)
> >>>
> >>> Will you make a fast/temporary fix so that we can get ready for a
> >>> release of 0.9.5 and then we can move the PythonArray implementation
> >>> to a future release?
> >>>
> >> The fast fix would be revert back to the
> >>
> >>     eval(double*, std::vector<double>&)
> >>
> >> interface. No point wasting time on typemaps for std::vector if we're
> >> not going to use them.
> >
> > Good point, but if it's possible to fix with moderate work I suggest
> > we (Johan...) fix it before the release.
> >
> > This might be the last release in a long time with major interface
> > changes to the Expression/Function classes and then it would be good
> > to have it all in place at once.
> >
>
> You mean that you want a new Array class in place? I don't think that is
> feasible in a short time.
>
> Garth

No, not if we go for that option. Then it doesn't really matter about
the real* vs std::vector thing.

But the Python interface will remain unchanged which is good. C++
users are a bit more hardcore and can live with the changes.

--
Anders

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