[Numpy-discussion] (newbie) How can I use NumPy to wrap my C++ class with 2-dimensional arrays?

Raymond de Vries reedev@zonnet...
Thu Jul 30 03:52:16 CDT 2009

Hi Matthieu,

Thanks for your quick reply! I did not find this page earlier. I have 
studied the first page and now I realize again that all my efforts so 
far (last few days) have crashed on the lack of calling import_array(). 
Do you have a suggestion for calling import_array() as well? Everywhere 
I see *that* import_array() needs to be called, but not how...

I will take a look at the conversion to a single vector. I hope this can 
be avoided because I cannot simply change the library.


Matthieu Brucher wrote:
> Hi,
> In fact, it's not that complicated. You may know the way how copying a
> vector, and this is all you need
> (http://matt.eifelle.com/2008/01/04/transforming-a-c-vector-into-a-numpy-array/).
> You will have to copy your data, it is the safest way to ensure that
> the data is always valid.
> For the std::vector<>[], I suggest you convert it to a single vector,
> as the data inside this "array" is not contiguous and it can thus be
> cumbersome to create a Numpy array from that.
> Once the structure is prepared, you have to allocate the dimensions,
> but this may be available online.
> In case you know that the C++ data won't go away before the Python
> array, you can always wrap the container
> (http://matt.eifelle.com/2008/11/04/exposing-an-array-interface-with-swig-for-a-cc-structure/)
> with SWIG.
> Matthieu
> 2009/7/30 Raymond de Vries <reedev@zonnet.nl>:
>> Hi everyone,
>> (I sent this message yesterday as well but somehow it didn't come
>> through...)
>> I would like to ask your advice how I can use NumPy to wrap my existing
>> C++ library with 2-dimensional arrays. I am wrapping the library with
>> swig and I have the typemap function declaration. But now I am
>> struggling a lot to find an implementation for getting access to the
>> 2-dimensional arrays in my C++ classes. I have wrapped most of the
>> library but the multi-dimensional arrays are problematic. From what I
>> have read, I can use NumPy for this..
>> There is so much information about NumPy, Numeric etc that it is not
>> clear anymore which example I can use best. It's clear that NumPy is
>> probably the way to go.
>> This is a small example of the class that I would like to wrap:
>> struct MyClass {
>>  float array1[2][2];
>>  std::vector<float> array2[2];
>> };
>> I've also read that I need to call import_array() but where should I put
>> this?
>> Is there a small example with NumPy c-api to wrap/convert my
>> 2-dimensional arrays to python?
>> Thanks a lot already
>> Raymond
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