Does that mean, then, that the help page's "x$name" just accidentally coincides
with the arguments x and name that the "$"-function requires in S4?
Regards, Ulrike
---------- Original Message -----------
From: Prof Brian Ripley <[EMAIL PROTECTED]>
To: Ulrike Grömping <[EMAIL PROTECTED]>
Cc: Duncan Temple Lang <[EMAIL PROTECTED]>, r-help@stat.math.ethz.ch
Sent: Tue, 22 Nov 2005 08:39:32 +0000 (GMT)
Subject: Re: [R] Method for $
> On Fri, 18 Nov 2005, Ulrike Grömping wrote:
>
> > From: Duncan Temple Lang <[EMAIL PROTECTED]>
>
> [...]
> >> For the record, the problem you are experiencing is that
> >> you are not using the same argument names in your
> >> method as the function "$" uses. Hence there is a mismatch.
> >> Use
> >> setMethod("$", signature(x="myclass"), function(x, name){
> >> slot(x, name)
> >> } )
> >>
> >> i.e. rather than slotname as the second argument, use name
> >> as that is what $ uses.
> >
> > Thanks very much for clarifying that. I just wasn't aware of the
> > argument names of $. It's obvious from the help file, once you know what
> > to look for, but ...
>
> `For the record', this is rather misleading. $ is a primitive function in
> R, and does _not_ have argument names (it uses only positional matching
> like all primitive functions). To check this, try
>
> xx <- list(a=1, b=2)
> `$`(y=xx, zz="b")
>
> which is perfectly valid R and works just like xx$b. Hence there is no
> reason for the help page to specify argument names (and it does in fact
> explain that they are not used).
>
> However, the S4 model needs argument names, and so when setting S4 methods
> for primitive functions one needs to use the argument names which it
> _assumes_. Sometimes the only way to find those out is to read the code.
> They are in src/library/methods/R/BasicFunsList.R which contains
>
> "$" = function(x, name)
> {
> name <- as.character(substitute(name))
> standardGeneric("$")
> }
>
> S3 methods for primitives do not need to match argument names, fortunately
> as Duncan TL has just added an S3 method with the (mis-matching) arguments
>
> > args(`$.DLLInfo`)
> function (x, i, ...)
>
> Since there are no argument names for the generic, the QC tools do not
> check such cases.
>
> --
> Brian D. Ripley, [EMAIL PROTECTED]
> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> University of Oxford, Tel: +44 1865 272861 (self)
> 1 South Parks Road, +44 1865 272866 (PA)
> Oxford OX1 3TG, UK Fax: +44 1865 272595
------- End of Original Message -------
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