Re: [Numpy-discussion] How to call methods from a class with custom matrices parameters with numpy arrays ?

[David Cournapeau]

Matthieu Brucher wrote:

        You should give ctypes a try, I find it much better than swig most
    of the time for wrapping. You can find some doc here:

        http://www.scipy.org/Cookbook/Ctypes2

        Basically, once you get your dll/so with a function foo(double *a,
    int n), you can call it directly in numpy by passing directly a numpy
    array. The catch is that the layout of the array should be exactly the
    same than your function expects (most of the time a contiguous array),
    but there are numpy functions which enforce that. As long as you
    are not
    calling thousand of function with small arrays, the wrapping is pretty
    efficient in my experience.

        cheers,

        David



Thanks for the fast answer ;)


template<class MatrixType>
struct CostFunction
{
  CostFunction(const MatrixType& points, ...)
  {
  }

  const MatrixType gradient(const Matrixtype& parameters)
  {
    // ...
    return aNewGradient;
  }
};

I was wondering if ctypes would fit better, but in this case, I have to make the wrapper myself, I suppose ? Here is the basic prototype of what I have - it's a cost function I want to optimize with optimizers I proposed on scipy recently - : So I have to create a function that takes a numpy array and that returns and instance of my cost function, this is pretty simple to do, the matrix can be constructed from its two dimensions and a pointer, but for the gradient method, I have trouble seeing how to do wrap it :(

I forgot you are using C++. Using C++ with ctypes makes the wrapping more difficult, I think, specially since you are using templates: you would have to write a C wrapper around your classes, and then using ctypes on the wrapper (there may be better ways, but I don't see an obvious one). Basically, using ctypes is like using code from a dynamically loaded library (ala VST :) ). But writing some C code kills one of the strong point of ctypes (doing all the wrapping in python).

I have attached a simplistic example which show how to wrap some functions.

David

example.tbz2
Description: application/bzip-compressed-tar

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