R-alpha: "..." arg.matching [was 'Problems with dimnames and names']

[Martin Maechler]

>>>>> "Martyn" == Martyn Plummer <plummer@iarc.fr> writes:

    Martyn> I have rounded up three buglets in R-0.50-a4. Two of them I can
    Martyn> fix and a patch is supplied below. I hope this is useful for the
    Martyn> current source (if these haven't been fixed already :)

Both of these bugs are already fixed in 0.60

    Martyn> 1) cov
    Martyn> cov() fails when it's argument is a matrix with one column and with
    Martyn> column names defined. It tries to apply dimnames to the output but
    Martyn> the output is a vector of length 1.

    >> x <- matrix(rnorm(100),ncol=1,dimnames=list(NULL,"foo"))
    >> var(x)
    Martyn> Error: dimnames applied to non-array

    Martyn> 2) Subscripting one-dimensional arrays.
    Martyn> These are treated as vectors when subscripting
    Martyn> so subscripting by dimnames fails.

    >> x <- c(0,0,0,1,1,1,1,3)
    >> table(x)
    Martyn> 0 1 3
    Martyn> 3 4 1
    >> table(x)["3"]
    Martyn> Error: subscript out of bounds

-- Now to the ``real'' one : --

    Martyn> 3) I have no idea what's going on here.

    >> c(r=1,s=2)
    Martyn> s
    Martyn> 2
    >> c(u=100,b=4)
    Martyn> b
    Martyn> 4

    Martyn> This seems to happen only for "r" and "u". Other single-letter
    Martyn> names are OK. This doesn't happen on my home PC which is still
    Martyn> running R-0.50-a1.

Well, we (i.e., Peter and Martyn) have found *why* it happens.

However, this is a semantic change from older versions of R and from S
which I consider serious [a bug ?!?] an we should discuss.

The general accepted semantic for S is:

  <> <>  If a function has a '...' argument,                    <> <>
  <> <>  all the other argument names must be matched EXACTLY,  <> <>    (S)
  <> <>  not only up to the unique string beginning.            <> <>

This behaviour (S) definitely IS a pain sometimes,
because it forces you not abbreviate argument names for all these
functions.

For the ``function writer'', this leads to 
  1) you really should only use "..." arguments if you must.
  2) if you have "..." argument(s),
	use relatively short argument names for the other arguments
  3) if there are named arguments in "..." (which is the usual case),
	make sure 
		[if you can, which is the case mostly, but not for 'c(.)']
	that all the argument names are valid.


Unfortunately, "3)" is not followed too often in S-plus,
which leads to the ``famous'' situation, that

	glm(ni ~ x1 * x2, familiy = binomial)

will fit a normal instead of a logistic regression,
with no warning, just because of a 1 letter typo ..

	(misspelling of 'familiy' makes the whole argument be ignored,
	  and family = gaussian , the default, be taken).

--------

So, we now see why Ross(?) may have wanted
to change the  argument matching behavior in the "..." case.
[[Rule: If  S  does things wierdly, R should do better ...]]

I still think however that this must be carefully tought over,
and I am not yet convinced if we shouldn't follow the S way after all.
Where we could do better (in our `own' functions) is to folllow my rule 
"3)" above as strictly as possible.


Martin Maechler <maechler@stat.math.ethz.ch>		 <><
Seminar fuer Statistik, SOL G1
ETH (Federal Inst. Technology)	8092 Zurich	 SWITZERLAND
phone: x-41-1-632-3408		fax: ...-1086
http://www.stat.math.ethz.ch/~maechler/

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