[R] Whats wrong with NextMethod("[[<-")?

[Prof Brian Ripley]

On Sat, 25 Oct 2003, Henrik Bengtsson wrote:

> Hi. NextMethod("[[<-") do indeed call the "next" "[[<-"() method. The
> thing is that it is also return the update object, which you are *not*
> taking care of. You are just returning the same object again. So here is
> what you want:
> 
> "[[<-.myA"<-function(mvl,name,value) {
>   if(data.class(value) == "myB") {
>     mvl$data[[name]] <- value
>     mvl
>   } else {
>     NextMethod("[[<-")
>   }
> }
> 
> Also, it is better to use data.class(obj) than class(obj)[1]. 

It's not.  First, the effect is the same here, and second data.class is 
really only for back-compatibility when fudging the difference between 
"numeric" and "integer".

It is almost certainly better to use inherits(value, "myB"), though, and
you should probably very rarely test the class (let alone data.class)
directly.

> 
> Cheers
> 
> Henrik Bengtsson
> Lund University
> 
> 
> > -----Original Message-----
> > From: r-help-bounces at stat.math.ethz.ch 
> > [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Eryk Wolski
> > Sent: den 25 oktober 2003 16:27
> > To: r-help at stat.math.ethz.ch
> > Subject: [R] Whats wrong with NextMethod("[[<-")?
> > 
> > 
> > Hi!
> > 
> > Given a list object "myA" with several fields
> > i want to construct the assignment function "[[<-.myA"
> > which behaves differently dependent of the class of the 
> > arguments (value).
> > 
> > If value are a myB object it should append(assign) this 
> > massvector to the data field of myA object if its any other 
> > object the assignement function should work as for any list.
> > 
> > 
> > "[[<-.myA"<-function(mvl,name,value)
> >   {
> >     if(class(value)[1]!="myB")
> >       {
> >         NextMethod("[[<-"); #work as a list
> >       }
> >     else
> >       {
> >         mvl$data[[name]] <- value #work as myA
> >       }
> >     mvl
> >   }
> > 
> > 
> > test<-list(a=1,data=list(1))
> > class(test)<-c("myA","list")
> > 
> > duda<-"hello"
> > class(duda)<-"myB"
> > 
> > test[[2]]<-duda               #works
> > test[["newField"]]<-"bella"   #nothing happens!!!
> > 
> > The assignment test[["newField"]]<-"bella" has no effect. The 
> > list test are not extended by the entry newField. Looks to me 
> > like NextMethod("[[<-") does nothing.
> > 
> > Is it possible to do this what I try to do with S3?
> > If so what i do wrong?
> > If its not possible to do this in this way can anyone suggest
> >  a different solution to what i like to do?
> > 
> > Eryk
> > 
> > ______________________________________________
> > R-help at stat.math.ethz.ch mailing list 
> > https://www.stat.math.ethz.ch/mailma> n/listinfo/r-help
> > 
> >
> 
> ______________________________________________
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> 
> 

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
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