[Numpy-discussion] the mean, var, std of empty arrays

josef.pktd@gmai... josef.pktd@gmai...
Thu Nov 22 09:31:28 CST 2012


On Thu, Nov 22, 2012 at 10:15 AM, Sebastian Berg
<sebastian@sipsolutions.net> wrote:
> On Thu, 2012-11-22 at 16:05 +0100, Daπid wrote:
>> On Thu, Nov 22, 2012 at 3:54 PM,  <josef.pktd@gmail.com> wrote:
>> > Why don't operations on empty arrays not return empty arrays?
>>
>> Because functions like mean or std are expected to return a scalar.
>> Functions that are piecewiese can (and should) return an empty array,
>> but not the mean.
>
> I agree, this makes sense, note that:
>
> In [2]: a = np.empty((5,0))
>
> In [3]: a.std(0)
> Out[3]: array([], dtype=float64)
>
> In [4]: a.std(1)
> /usr/bin/ipython:1: RuntimeWarning: invalid value encountered in divide
>   #!/usr/bin/env python
> Out[4]: array([ nan,  nan,  nan,  nan,  nan])
>
> However you are reducing, and with reducing you expect exactly 1 scalar
> result (along that dimension).

Ok, I see.
we cannot have an empty 1-D array shape (5,)

Josef

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