dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

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.

I have created a blueprint:

https://blueprints.launchpad.net/dolfin/+spec/array-typemaps

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?

--
Anders

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