[R] Cacheing computationally expensive getter methods for S4 objects

[Benilton Carvalho]

If you change 'area' to an environment, you may be able to get  
something close to what you want.

For example:

setClass("Square",
          representation(
                         length='numeric',
                         width='numeric',
                         area='environment'
                         ),
          prototype(
                    length=0,
                    width=0,
                    area=new.env()
                    )
          )

setGeneric("area", function(x) standardGeneric("area"))
setMethod("area", "Square",
           function(x){
             if (length(ls(x at area)) == 0){
               message("Computing")
               assign("area", x at width * x at length, envir=x at area)
             }
             message("Accessing")
             get("area", envir=x at area)
           })

tmp <- new("Square", length=5, width=10)
area(tmp) ## This should show "computing" and "accessing"
area(tmp) ## the 2nd call should show 'accessing' only


b



On Oct 14, 2009, at 3:31 PM, Steve Lianoglou wrote:

> Hi,
>
> I was wondering if there was a way to store the results of a
> computationally expensive "getter" call on an S4 object, so that it is
> only calculated once for each object.
>
> Trivial example: let's say I want to cache the "expensive" area
> calculation of a square object.
>
> setClass("Square",
>   representation(
>     length='numeric',
>     width='numeric',
>     area='numeric'
>   ),
>   prototype(
>     length=0,
>     width=0,
>     area=-1
>   )
> )
>
> setGeneric("area", function(x) standardGeneric("area"))
> setMethod("area", "Square",
> function(x) {
>   if (x at area == -1) {
>     x at area <- x at width * x at height
>   }
>   x at area
> })
>
> Now the first time I call ``area(my.square)`` it computes
> ``my.square at width * my.square at height``, but each subsequent call
> returns ``x at area`` since the area computation has already been calc'd
> and set for this object.
>
> Is this possible? I'm guessing the R pass by value semantics is going
> to make this one difficult ... is there some S4 reference I missed
> that has this type of info from?
>
> Thanks,
> -steve
>
> --
> Steve Lianoglou
> Graduate Student: Computational Systems Biology
>   |  Memorial Sloan-Kettering Cancer Center
>   |  Weill Medical College of Cornell University
> Contact Info: http://cbio.mskcc.org/~lianos/contact
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.