[Numpy-discussion] the mean, var, std of empty arrays
Charles R Harris
charlesr.harris at gmail.com
Wed Nov 21 22:35:39 EST 2012
On Wed, Nov 21, 2012 at 7:45 PM, <josef.pktd at gmail.com> wrote:
> On Wed, Nov 21, 2012 at 9:22 PM, Olivier Delalleau <shish at 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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