I believe that a recommended S4 convention is simply to write methods for
slot extraction with the same name as the slot. Thus, for an object,x of
class 'myclass'
slotname(x)
would give [EMAIL PROTECTED] This is a more natural interface than either @ or $
(and I am not sure that methods for $ can be defined due to S3/S4 class
consistency details).
Obviously, it is trivial, but a bit tedious, to manually write such methods
for any class you care to. However, I wrote a small generator function that
automatically generates the slot extraction methods for any class if you
wish to do things this way. I just run this right after I've define a new
class,'myclass', to generate slot access methods by: slotget('myclass')
The code is below. I would appreciate feedback on any errors or suggestions
for improvement.
Cheers,
-- Bert Gunter
Genentech Non-Clinical Statistics
South San Francisco, CA
slotGet<-function(x,where=topenv())
#automatically generates accessor and replacement methods for class x
{
if(!isClass(x,where=where))stop(paste('Class',x,'not found'))
slots<-slotNames(getClass(x,where=where))
for(slt in slots){
## slot accessor
fun<-function(object)NULL
if(!isGeneric(slt,where=where)){
if(exists(slt,where=where,mode="function"))
fun<-get(slt,pos=where,mode="function")
else body(fun)<-substitute(standardGeneric(slt))
setGeneric(slt,fun,where=where)
}
if(!existsMethod(slt,x,where=where)){
body(fun)<-substitute([EMAIL PROTECTED])
setMethod(slt,x,fun,where=where)
}
}
invisible()
}
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of Uwe Ligges
> Sent: Friday, November 18, 2005 8:46 AM
> To: Ulrike Grömping
> Cc: r-help@stat.math.ethz.ch
> Subject: Re: [R] Method for $
>
> Ulrike Grömping wrote:
>
> > Dear R experts,
> >
> > I have defined a class "myclass" and would like the slots
> to be extractable
> > not only by "@" but also by "$". I now try to write a
> method for "$" that
> > simply executes the request [EMAIL PROTECTED], whenever someone calls
> > object$slotname for any object of class "myclass".
> > I don't manage to find out how I can provide this function
> with "slotname",
> > so that one and the same function works for any arbitrary
> slotname a user
> > might choose.
> >
> > I have tried
> >
> > setMethod("$", signature(x="myclass"), function(x,slotname){
> > [EMAIL PROTECTED]
> > }
> > )
>
>
> Ulrike,
>
> what about (untested!):
> slot(x, slotname)
>
> Best wishes from Dortmund and Lena,
> Uwe Ligges
>
>
> > This produced the error message:
> > In method for function "$": expanding the signature to
> > include omitted arguments in definition: name = "missing"
> > Error in rematchDefinition(definition, fdef, mnames,
> fnames, signature) :
> > methods can add arguments to the generic only if
> '...' is an argument
> > to the generic
> >
> > My searches for a solution of this problem have not been
> successful. Can
> > someone help?
> >
> > Thanks and regards,
> > Ulrike Grömping, Berlin
> >
> > ______________________________________________
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> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide!
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