[Numpy-discussion] abs for max negative integers - desired behavior?

Matthew Brett matthew.brett@gmail....
Sat Oct 15 14:00:45 CDT 2011


Hi,

On Wed, Oct 12, 2011 at 8:31 AM, David Cournapeau <cournape@gmail.com> wrote:
> On 10/12/11, "V. Armando Solé" <sole@esrf.fr> wrote:
>> On 12/10/2011 10:46, David Cournapeau wrote:
>>> On Wed, Oct 12, 2011 at 9:18 AM, "V. Armando Solé" wrote:
>>>>   From a pure user perspective, I would not expect the abs function to
>>>> return a negative number. Returning +127 plus a warning the first time
>>>> that happens seems to me a good compromise.
>>> I guess the question is what's the common context to use small
>>> integers in the first place. If it is to save memory, then upcasting
>>> may not be the best solution. I may be wrong, but if you decide to use
>>> those types in the first place, you need to know about overflows. Abs
>>> is just one of them (dividing by -1 is another, although this one
>>> actually raises an exception).
>>>
>>> Detecting it may be costly, but this would need benchmarking.
>>>
>>> That being said, without context, I don't find 127 a better solution than
>>> -128.
>>
>> Well that choice is just based on getting the closest positive number to
>> the true value (128). The context can be anything, for instance you
>> could be using a look up table based on the result of an integer
>> operation ...
>>
>> In terms of cost, it would imply to evaluate the cost of something like:
>>
>> a = abs(x);
>>   if (a < 0) {a -= MIN_INT;}
>> return a;
>
> Yes, this is costly: it adds a branch to a trivial operation. I did
> some preliminary benchmarks (would need confirmation when I have more
> than one minute to spend on this):
>
>  int8, 2**16 long array. Before check: 16 us. After check: 92 us. 5-6
> times slower
>  int8, 2**24 long array. Before check: 20ms. After check: 30ms. 30 % slower.
>
> There is also the issue of signaling the error in the ufunc machinery.
> I forgot whether this is possible at that level.

I suppose that returning the equivalent uint type would be of zero cost though?

I don't think the problem should be relegated to 'people should know
about this' because this a problem for any signed integer type, and it
can lead to nasty errors which people are unlikely to test for.

See you,

Matthew


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