[Numpy-discussion] Problems with Numexpr and discontiguous arrays

Sebastian Haase haase at msg.ucsf.edu
Wed Oct 4 12:23:25 CDT 2006


On Wednesday 04 October 2006 10:13, Tim Hochberg wrote:
> 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

Does that mean its considered "impratical" to ever add native float32 
support ?   Is the switch-statement you mention written by hand or is that 
automatically generated ?
-Sebastian



>
> > -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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