On Mon, Jul 15, 2013 at 8:34 AM, Sebastian Berg <[email protected]>wrote:
> On Mon, 2013-07-15 at 07:52 -0600, Charles R Harris wrote: > > > > > > On Sun, Jul 14, 2013 at 3:35 PM, Charles R Harris > > <[email protected]> wrote: > > > > <snip> > > > > > For nansum, I would expect 0 even in the case of all > > nans. The point > > of these functions is to simply ignore nans, correct? > > So I would aim > > for this behaviour: nanfunc(x) behaves the same as > > func(x[~isnan(x)]) > > > > > > Agreed, although that changes current behavior. What about the > > other cases? > > > > > > > > Looks like there isn't much interest in the topic, so I'll just go > > ahead with the following choices: > > > > Non-NaN case > > > > 1) Empty array -> ValueError > > > > The current behavior with stats is an accident, i.e., the nan arises > > from 0/0. I like to think that in this case the result is any number, > > rather than not a number, so *the* value is simply not defined. So in > > this case raise a ValueError for empty array. > > > To be honest, I don't mind the current behaviour much sum([]) = 0, > len([]) = 0, so it is in a way well defined. At least I am not sure if I > would prefer always an error. I am a bit worried that just changing it > might break code out there, such as plotting code where it makes > perfectly sense to plot a NaN (i.e. nothing), but if that is the case it > would probably be visible fast. > I'm talking about mean, var, and std as statistics, sum isn't part of that. If there is agreement that nansum of empty arrays/columns should be zero I will do that. Note the sums of empty arrays may or may not be empty. In [1]: ones((0, 3)).sum(axis=0) Out[1]: array([ 0., 0., 0.]) In [2]: ones((3, 0)).sum(axis=0) Out[2]: array([], dtype=float64) Which, sort of, makes sense. > > > 2) ddof >= n -> ValueError > > > > If the number of elements, n, is not zero and ddof >= n, raise a > > ValueError for the ddof value. > > > Makes sense to me, especially for ddof > n. Just returning nan in all > cases for backward compatibility would be fine with me too. > > > Nan case > > > > 1) Empty array -> Value Error > > 2) Empty slice -> NaN > > 3) For slice ddof >= n -> Nan > > > Personally I would somewhat prefer if 1) and 2) would at least default > to the same thing. But I don't use the nanfuncs anyway. I was wondering > about adding the option for the user to pick what the fill is (and i.e. > if it is None (maybe default) -> ValueError). We could also allow this > for normal reductions without an identity, but I am not sure if it is > useful there. > > Chuck
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