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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