[Rd] c(<Matrix>, <Matrix>) / help(dotsMethods) etc

[John Chambers]

(Brief reply, I'm traveling but as per below, this is on my radar right now so wanted to comment.)

Two points regarding "dotsMethods".

1.  To clarify the limitation.  It's not that all the arguments have to be of the same class, but there must be one class that they belong to or subclass.  (So, as in the example in the documentation, the method could be defined for a class union or other virtual class that all the actual arguments inherit from.)

2.  The current documentation is a mess.  In common with lots of other very old documentation.  I'm in the process of rewriting a large chunk of the documentation including that for dotsMethods.  Sometime in the next few weeks, I hope to have it coherent enough to commit.

So far, I'm not trying to change any significant aspects of the code, including for "..." methods, which seem to do roughly what was intended.

John


On Sep 10, 2016, at 8:27 AM, Martin Maechler <maechler at stat.math.ethz.ch> wrote:

> I have been asked  (by Roger; thank you for the good question,
>       	    	    and I hope it's fine to answer to the public) :
> 
>> with Pi a sparse matrix and x,y, and ones
>> compatible n-vectors — when I do:
> 
>>> c(t(x) %*% Pi %*% ones, t(ones) %*% Pi %*% y )
>> [[1]] 1 x 1 Matrix of class "dgeMatrix"
>> [,1] [1,]
>> 0.1338527
>> [[2]] 1 x 1 Matrix of class "dgeMatrix"
>     [,1] [1,]
>> 0.7810341
> 
>> I get a list whereas if Pi is an ordinary matrix I get a
>> vector.  Is this intentional?
> 
> Well, no.  But it has been "unavoidable" in the sense that it had not
> been possible to provide S4 methods for '...' in the "remote"
> past, when  Matrix was created.
> 
> Later ... also quite a few years ago, John Chambers had added
> that possibility, with still some limitation (all '...' must be
> of the same class), and also plans to remove some of the
> limitations, see   ?dotsMethods  in R.
> 
> I honestly have forgotten the history of my trying to provide 'c'
> methods for our "Matrix" objects after the  'dotsMethods'
> possibility had emerged,  but I know I tried and had not seen a
> way to succeed "satisfactorily",
> but maybe I now think I maybe should try again.
> I currently think this needs changes to R before it can be done
> satisfactorily, and this is the main reason why this is a public
> answer to R-devel at ..., but I'm happy if I'am wrong.
> 
> The real challenge here is that I think that if it  should "work",
> it should work so in all cases, e.g., also for
> 
>    c(NA, 3:2, Matrix(2:1), matrix(10:11))
> 
> and that's not so easy, e.g., the following class and method
> definitions do *not* achieve the desired result:
> 
> ## "mMatrix" is already hidden in Matrix pkg:
> setClassUnion("mMatrix", members = c("matrix", "Matrix"))
> setClassUnion("numMatrixLike", members =
>                c("logical", "integer","numeric", "mMatrix"))
> 
> c.Matrix <- function(...) unlist(lapply(list(...), as.vector))
> ## NB: Must use   signature  '(x, ..., recursive = FALSE)' :
> setMethod("c", "Matrix", function(x, ..., recursive) c.Matrix(x,
> ...))
> ## The above is not sufficient for
> ##    c(NA, 3:2, <Matrix>, <matrix>) :
> setMethod("c", "numMatrixLike", function(x, ..., recursive)
>   c.Matrix(x, ...))
> 
> ## but the above does not really help:
> 
>> c(Diagonal(3), NA, Matrix(10:11))   ## works fine,
> [1]  1  0  0  0  1  0  0  0  1 NA 10 11
> 
>> c(NA, Diagonal(3)) ## R's lowlevel c() already decided to use list():
> [[1]]
> [1] NA
> 
> [[2]]
>     [,1] [,2] [,3]
>     [1,]    1    .    .
>     [2,]    .    1    .
>     [3,]    .    .    1
> 
>> 
> ----------------------------------------------
> 
> BTW, I (and the package users) suffer from exactly the same
> problem with the "MPFR" (multi precision numbers) provided by my
> package Rmpfr:
> 
>> require(Rmpfr)
>> c(mpfr(3,100), 1/mpfr(7, 80)) ## works fine
> 2 'mpfr' numbers of precision  80 .. 100  bits
> [1]                            3 0.14285714285714285714285708
> 
>> c(pi, 1/mpfr(7, 80)) ## "fails" even worse than in 'Matrix' case
> [[1]]
> [1] 3.141593
> 
> [[2]]
> 'mpfr1' 0.14285714285714285714285708
> 
>> 
> 
> 
> Yes, it would be very nice  if  c(.)  could be used to
> concatenate quite arbitrary  R objects into one long atomic
> vector, but I don't see how to achieve this easily.
> 
> The fact, that  c()  just builds a list of its arguments if it
> ("thinks" it) cannot dispatch to a method, is a good strategy,
> but I'd hope it should be possible to have c() try to do better
> (and hence work for this case, and
> without a noticable performance penalty.
> 
> Suggestions are very welcome.
> Martin
> 
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