[Numpy-discussion] Problems with Numexpr and discontiguous arrays

Tim Hochberg tim.hochberg at ieee.org
Wed Oct 4 12:13:10 CDT 2006


Sebastian Haase wrote:
> Quick question hopefully somewhat related to this:
> Does numexpr fully support float32 arrays ?
>   
I don't recall. At one point there was a tentative plan to support 
float32 by casting them a block at a time to float64, operating on them 
and them casting them back. That's to limit the number of bytecodes that 
we need to support and keep the switch statement at a manageable size. 
However, it doesn't look like that ever got implemented, so the answer 
is probably no.

-tim



> -Sebastian
>
>
> On Wednesday 04 October 2006 09:32, Tim Hochberg wrote:
>   
>> Ivan Vilata i Balaguer wrote:
>>     
>>> It seemed that discontiguous arrays worked OK in Numexpr since r1977 or
>>> so, but I have come across some alignment or striding problems which can
>>> be seen with the following code::
>>>
>>>     import numpy
>>>     import numexpr
>>>
>>>     array_length = 10
>>>     array_descr = [('c1', numpy.int32), ('c2', numpy.uint16)]
>>>
>>>     array = numpy.empty((array_length,), dtype=array_descr)
>>>     for i in xrange(array_length):
>>>         array['c1'][i] = i
>>>         array['c2'][i] = 0xaaaa
>>>
>>>     print numexpr.evaluate('c1', {'c1': array['c1']})
>>>     print numexpr.evaluate('c1', {'c1': array['c1'].copy()})
>>>
>>> Im my computer, Pentium IV with NumPy 1.0rc1 and Numexpr r2239
>>> (unmodified) this gives the following result::
>>>
>>>     [      0      109226 -1431699456           2      240298 -1431699456
>>>            4      371370           8      633514]
>>>     [0 1 2 3 4 5 6 7 8 9]
>>>
>>> The test works right when ``evaluate()`` is used with 'c2' instead of
>>> 'c1', and also when 'c2' also measures 32 bits and fields are aligned.
>>> Maybe the ``memsteps`` value is not getting used somewhere.  Any ideas
>>> on this?
>>>       
>> I suspect that there are some assumptions that the element separation
>> is an integral multiple of the element size. I certainly didn't have
>> record arrays in mind when I was working on the striding stuff, so it
>> wouldn't surprise me. This should be fixed: preferably to do the right
>> thing and at a minimum to cleanly raise an exception rather than
>> spitting out garbage. I don't know that I'll have time to mess with it
>> soon though.
>>
>> -tim
>>
>>
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