[Numpy-discussion] Weird problem with subtracting ndarrays
Moroney, Catherine M (398D)
Catherine.M.Moroney at jpl.nasa.gov
Wed Jun 12 15:38:58 EDT 2013
Thanks for the tip. I thought it must be something simple like that. When I convert
the arrays to numpy.int32 things behave normally.
Another question though: a numpy.all() on the signed-int arrays shows that they
are equal. So why would the subtraction of the unsigned arrays wrap around like that?
Catherine
On Jun 12, 2013, at 12:25 PM, <numpy-discussion-request at scipy.org>
wrote:
> Message: 9
> Date: Wed, 12 Jun 2013 15:30:22 -0400
> From: Warren Weckesser <warren.weckesser at gmail.com>
> Subject: Re: [Numpy-discussion] weird problem with subtracting
> ndarrays
> To: Discussion of Numerical Python <numpy-discussion at scipy.org>
> Message-ID:
> <CAGzF1udhTAmxPWwHsd9xQ386CBz2iXymmYC4kJJ3AOTRPZQzyg at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> On Wed, Jun 12, 2013 at 3:25 PM, Moroney, Catherine M (398D) <
> Catherine.M.Moroney at jpl.nasa.gov> wrote:
>
>> Hello,
>>
>> 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)
>> False
>>>>> diff = counts1 - counts2
>>>>> diff.max()
>> 4294967295 !! WHAT IS HAPPENING HERE ??
>>>>> sum = counts1 + counts2
>>>>> sum.max()
>> 26
>>
>> 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?
>>
>> Catherine
>>
>>
> Check the dtype of the arrays. They are probably unsigned integers, and
> the subtraction leads to wrap-around in some cases.
>
> For example:
>
> In [1]: x = np.array([0, 1, 2], dtype=np.uint32)
>
> In [2]: y = np.array([1, 1, 1], dtype=np.uint32)
>
> In [3]: x - y
> Out[3]: array([4294967295, 0, 1], dtype=uint32)
>
>
> Warren
>
>
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