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

Sebastian Berg sebastian at sipsolutions.net
Thu Nov 22 10:15:54 EST 2012

On Thu, 2012-11-22 at 16:05 +0100, Daπid wrote:
> On Thu, Nov 22, 2012 at 3:54 PM,  <josef.pktd at 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).

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