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

Charles R Harris charlesr.harris@gmail....
Wed Nov 21 21:35:39 CST 2012


On Wed, Nov 21, 2012 at 7:45 PM, <josef.pktd@gmail.com> wrote:

> On Wed, Nov 21, 2012 at 9:22 PM, Olivier Delalleau <shish@keba.be> wrote:
> > Current behavior looks sensible to me. I personally would prefer no
> warning
> > but I think it makes sense to have one as it can be helpful to detect
> issues
> > faster.
>
> I agree that nan should be the correct answer.
> (I gave up trying to define a default for 0/0 in scipy.stats ttests.)
>
> some funnier cases
>
> >>> np.var([1], ddof=1)
> 0.0
>

This one is a nan in development.


> >>> np.var([1], ddof=5)
> -0
> >>> np.var([1,2], ddof=5)
> -0.16666666666666666
> >>> np.std([1,2], ddof=5)
> nan
>
>
These still do this. Also

In [10]: var([], ddof=1)
Out[10]: -0

Which suggests that the nan is pretty much an accidental byproduct of
division by zero. I think it might make sense to have a definite policy for
these corner cases.

<snip>

Chuck
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