[Numpy-discussion] record arrays with char*?

[Chris Barker]

On Mon, Feb 10, 2014 at 10:43 PM, Christopher Jordan-Squire <cjordan1 at uw.edu
> wrote:

> I'm trying to wrap some C code using cython. The C code can take
> inputs in two modes: dense inputs and sparse inputs. For dense inputs
> the array indexing is naive. I have wrappers for that. In the sparse
> case the matrix entries are typically indexed via names. So, for
> example, the library documentation includes this as input you could
> give:
>
> struct
> {
>  char* ind;
>  double val, wght;
> } data[] = { {"camera", 15, 2}, {"necklace", 100, 20}, {"vase", 90, 20},
>                  {"pictures", 60, 30}, {"tv", 40, 40}, {"video", 15, 30}};
>
> At the C level, data is passed to the function by directly giving its
> address. (i.e. the C function takes as an argument (unsigned long)
> data, casting the data pointer to an int)
>

wow -- that's prone to error! but I"m still not sure which pointer you're
talking about -- a pointer to this struct?

I'd like to create something similar using record arrays, such as
>
> np.array([("camera", 15, 2), ("necklace", 100, 20), ... ],
> dtype='object,f8,f8').
>



> Unfortunately this fails because
> (1) In cython I need to determine the address of the first element and
> I can't take the address of a an input whose type I don't know (the
> exact type will vary on the application, so more or fewer fields may
> be in the C struct)
>

still a bit confused, but if this is types as an array in Cython, you
should be abel to do somethign like:

&the_array[i]

to get the address of the ith element.

(2) I don't think a python object type is what I want--I need a char*
> representation of the string. (Unfortunately I can't test this because
> I haven't solved (1) -- how do you pass a record array around in
> cython and/or take its address?)
>

well, and object type will give you a pointer to a pyobject. If you know
for sure that that pyobject is a string object (probably want a bytes
object -- you son't want unicode here), then you should be abel to get the
address of the underlying char array. But that would require passing
something different off to the C code that the address of that element.

You could use an unsigned long for that first field, as you are assuming
that in the C code anyway  but I don't hink there is a way in numpy to set
that to a pointer to a char allocated elsewhere -- where would it
be allocated?

So I would give up on expecting to store the struct directly in numpy
array, and rather, put something reasonable (maybe what you have above) in
the numpy array, and build the C struct you need from that rather than
passing a pointer in directly.

-Chris



-- 

Christopher Barker, Ph.D.
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