[Numpy-discussion] Problem with absolute value
Matthew Brett
matthew.brett at gmail.com
Wed Jul 3 13:38:04 EDT 2019
Hi,
On Wed, Jul 3, 2019 at 9:08 AM Charles R Harris
<charlesr.harris at gmail.com> wrote:
>
>
>
> On Wed, Jul 3, 2019 at 9:08 AM Hameer Abbasi <einstein.edison at gmail.com> wrote:
>>
>> Hi,
>>
>> It turns out you're running into a bit-error. In general, the two's complement of -2 ** (n-1) with the bit-length being limited to n bits is itself... No way around that. And integers don't set hardware exceptions so checking for errors like these is hard as well.
>>
>> TL;DR: It's an error with how the integer is stored in memory and how you're running out of space.
>>
>> Regards,
>> Hameer Abbasi
>
>
> More like the eight bit twos complement of -128 is -128, bytes cannot represent 128. Matlab used to (still does?) solve this problem by returning 127 instead :) Basically, the data needs more precision. Returning an unsigned type would lead to it's own problems with unexpected promotions when the result was used. We could, I suppose, raise a warning, although that might be considered noisy. If you use `absolute` instead, you can specify the dtype.
I still think this is a major wart in numpy's abs.
I'd really like to add a new function, `uabs` which would return an
unsigned int for integer inputs. I'm happy to do a pull request if
that also seems sensible to y'all,
Cheers,
Matthew
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