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

[Henrik Bengtsson]

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]. 

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
> 
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