[Numpy-discussion] weird problem with subtracting ndarrays

Moroney, Catherine M (398D) Catherine.M.Moroney at jpl.nasa.gov
Wed Jun 12 15:25:33 EDT 2013


I've got two arrays of the same shape that I read in from a file, and I'm trying to
difference them.  Very simple stuff, but I'm getting weird answers.

Here is the code:

>>> counts1 = hfile1.read_grid_field("CFbA", "TerrainReferencedRCCMFraction_Num")
>>> counts2 = hfile2.read_grid_field("CFbA", "TerrainReferencedRCCMFraction_Num")
>>> counts1.max(), counts2.max()
(13, 13)
>>> counts1.min(), counts2.min()
(0, 0)
>>> numpy.all(counts1 == counts2)
>>> diff = counts1 - counts2
>>> diff.max()
4294967295      !! WHAT IS HAPPENING HERE ??
>>> sum = counts1 + counts2
>>> sum.max()

As you can see, the range of values in both arrays is 0 to 13, and the sum
behaves normally, but the difference gives this weird number.

When I create dummy arrays, the subtraction works fine.  So there must be some funny value
lurking in either the counts1 or counts2 array, but the numpy.isnan() test returns False.

Any ideas for how I debug this?  


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