[Rd] apply with zero-row matrix

[Martin Maechler]

>>>>> David Hugh-Jones 
>>>>>     on Mon, 30 Jul 2018 10:12:24 +0100 writes:

    > Hi Martin, Fair enough for R functions in general. But the
    > behaviour of apply violates the expectation that apply(m,
    > 1, fun) calls fun n times when m has n rows.  That seems
    > pretty basic.

Well, that expectation is obviously wrong ;-)  see below

    > Also, I understand from your argument why it makes sense
    > to call apply and return a special result (presumably
    > NULL) for an empty argument; but why should apply call fun?

    > Cheers David

The reason is seen e.g. in

    > apply(matrix(,0,3), 2, quantile)
	 [,1] [,2] [,3]
    0%     NA   NA   NA
    25%    NA   NA   NA
    50%    NA   NA   NA
    75%    NA   NA   NA
    100%   NA   NA   NA
    > 

and that is documented (+/-) in the first paragraph of the
'Value:' section of help(apply) :

 > Value:
 > 
 >      If each call to ‘FUN’ returns a vector of length ‘n’, then ‘apply’
 >      returns an array of dimension ‘c(n, dim(X)[MARGIN])’ if ‘n > 1’.
 >      If ‘n’ equals ‘1’, ‘apply’ returns a vector if ‘MARGIN’ has length
 >      1 and an array of dimension ‘dim(X)[MARGIN]’ otherwise.  If ‘n’ is
 >      ‘0’, the result has length 0 but not necessarily the ‘correct’
 >      dimension.


To determine 'n', the function *is* called once even when
length(X) ==  0

It may indeed be would helpful to add this explicitly to the
help page  ( <R>/src/library/base/man/apply.Rd ).
Can you propose a wording (in *.Rd if possible) ?

With regards,
Martin