[R] How to define S4 methods for '['

[Berton Gunter]

Thanks to Robert and Gabor for their replies, but neither was what I was
looking for, undoubtedly because of the poor phrasing of my question (+ a
typo -- however, even if correctly typed it doesn't work). I finally
realized that the "elegant" approach I sought can easily be done without
setGeneric:

setMethod('[','foo',function(x,i,j,drop,...){
    cl<-as.list(sys.call())
    cl[[2]]<-x at dat
    eval(as.call(cl))
    })

Indeed, this is a general template for this sort of thing. Should have
thought of this before, as V&R's S PROGRAMMING has numerous such examples...
Sigh...

-- Bert

-----Original Message-----
From: Gabor Grothendieck [mailto:ggrothendieck at gmail.com] 
Sent: Monday, June 20, 2005 1:31 PM
To: Berton Gunter
Cc: r-help at stat.math.ethz.ch
Subject: Re: [R] How to define S4 methods for '['

On 6/20/05, Berton Gunter <gunter.berton at gene.com> wrote:
> Folks:
> 
> This is a question about the S4 formal class system.
> 
> Suppose I have a class, 'foo', defined by:
> 
> setClass('foo',representation(dat='matrix', id='character') )
> 
> I wish to define a '[' method for foo that will extract from the 'dat'
slot.
> I would have thought that the following would work, but it doesn't:
> 
> setMethod("[","foo",function(x,i, j, .,drop=TRUE)callGeneric(x at dat,i,
> j,drop=drop) )
> 
> The only way I have succeeded in defining this method is using brute force
> eval(parse(. :
> 
> {eval(parse(text=paste('.dat(x)[',
>        ifelse(missing(i),',','i,'),
>        ifelse(missing(j),']','j]'))))
>        }
> 
> This works. However, I am not able under any circumstances to pass the
drop
> argument -- it is ignored.
> 
> I would appreciate any pointers about how to do this properly. If  this is
> explicitly in the Green Book (I do not have it with me at the moment),
that
> will suffice.
> 

Download the source to the 'its' package where an S4 [ method is defined.