dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

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.

>>> 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


> --
> Anders