[Octave-bug-tracker] [bug #48690] basic statistics functions should return NaN on empty input

[Nicholas Jankowski]

Follow-up Comment #18, bug #48690 (project octave):

hold off on the patch for cov. All others should be fine. 

I missed its usage with multiple inputs, and that collides with previous
accepted matlab incompatibility for the function. in short, if with numbers
matlab would produce a matrix output, and octave's does not, should the
'matlab compatibility fix' for empty and nan inputs follow the matlab or
octave convention?

ex, normal inputs:

cov([1 2 3 4],[1 2 3 4])
Octave:  ans = 1.6667
Matlab: ans = [1.6667,1.6667; 1.6667,1.6667] 


ex, matlab outputs for this case:

cov ([]) = NaN
cov (NaN) = NaN 

cov([],[]) = NaN(2,2)
cov(NaN(0,2)) = NaN(2,2)
cov(NaN(2,0)) = [] 


also, there are a couple tests/errors in the current implementation that are
problematic:

scalar 'y' treated only as an 'opt' for nargin==2 if it's ascalar, out of
bounds error if it's not 0 or 1. Matlab doesn't error, but output is a
matrix:


%!error cov (1, 3)

matlab output
>> cov(1,3)
ans =
     0     0
     0     0 


last, there's a special codepath at line 97  (scalar 'x' outputs 0) that
shortcuts everything else, including code that should error out:


>> cov (5, [1 2])
ans = 0

matlab output:

>> cov (5,[1 2])
Error using cov (line 107)
The number of elements in x and y must match.


previous mailing list discussions on the accepted incompatibility:
http://octave.1599824.n4.nabble.com/covariance-matrix-td1607376.html
http://octave.1599824.n4.nabble.com/cov-x-y-matrix-or-number-td4465087.html


started a discussion on maintainers. no discussion but linking so nothing gets
lost:
http://octave.1599824.n4.nabble.com/compatibility-of-cov-past-discussions-and-how-to-interpret-for-current-bugfix-td4681490.html

I've also cc'd a couple people involved in the past cov compatibility
discussion.


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