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

[giles.heywood@uk.abnamro.com]

I have found this a useful correspondence.  My own wish is to define a new
S4 class which differs from
class 'array' only in its default handling of the 'drop' argument i.e.
drop=FALSE - not an unusual wish.

My solution is the following:

setClass("noDropArray",representation("array"),prototype=array(NA,2:4))

setMethod('[','noDropArray',function(x,i,j,drop,...){
    cl<-as.list(sys.call())
    if( !("drop"%in%names(as.list(sys.call()))) )
        cl <-  c(cl,drop=FALSE)
    cl[[2]]<-x at .Data
    eval(as.call(cl))
    })

new("noDropArray")[1,1,1]

This behaves as I would expect.  However I have an uncomfortable feeling
that this is not the most
elegant solution.

Any comments?

- Giles


> 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.

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