# [R] Cacheing computationally expensive getter methods for S4 objects

Thu Oct 15 00:08:21 CEST 2009

```Just a note: this technique is called memoisation.

On Wed, Oct 14, 2009 at 3:42 PM, Benilton Carvalho <bcarvalh at jhsph.edu> wrote:
> Thank you very much, Martin. :)
>
> b
>
> On Oct 14, 2009, at 5:23 PM, Martin Morgan wrote:
>
>> Steve Lianoglou wrote:
>>>
>>> Very clever, that looks to do the trick!
>>
>> I think though that all Square's then share the same environment
>> (created in the prototype) and hence area.
>>
>>> area(new("Square", length=50, width=100))
>>
>> Accessing
>> [1] 50
>>
>>
>> Which is quite efficient at doing the calculation, but maybe not what is
>> expected...
>>
>> The solution is to create the area environment in an 'initialize' method.
>>
>> A different approach, but along similar lines, might make 'area' a
>> function that exploits lexical scope. Here's an area 'factory'
>>
>> areaf <- function() {
>>   area <- NULL
>>   function(x) {
>>       if (is.null(area)) {
>>           message("expensive")
>>           area <<- x at length * x at width
>>       }
>>       area
>>   }
>> }
>>
>> that we use in the initialize method
>>
>> setMethod(initialize, "Rect",
>>         function(.Object, ..., length=.Object at length,
>>                  width=.Object at width)
>> {
>>   callNextMethod(.Object, area=areaf(), length=length,
>>                  width=width, ...)
>> })
>>
>> setMethod(area, "Rect", function(x) x at area(x))
>>
>> The signature of initialize is such that one could
>>
>> setReplaceMethod("length", c("Rect", "numeric"), function(x, value) {
>>   initialize(x, length=value)
>> })
>>
>>
>> so
>>
>>> a <- new("Rect", length=10, width=5)
>>> area(a)
>>
>> expensive
>> [1] 50
>>>
>>> area(a) # cheap
>>
>> [1] 50
>>>
>>> b <- a
>>> area(b)
>>
>> [1] 50
>>>
>>> length(b) <- 20
>>> area(a)
>>
>> [1] 50
>>>
>>> area(b)
>>
>> expensive
>> [1] 100
>>
>> Martin
>>
>>>
>>> Thanks,
>>>
>>> -steve
>>>
>>> On Oct 14, 2009, at 2:57 PM, Benilton Carvalho wrote:
>>>
>>>> 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
>>>>> http://www.R-project.org/posting-guide.html
>>>>> and provide commented, minimal, self-contained, reproducible code.
>>>>
>>>
>>> --
>>> 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
>>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>> --
>> Martin Morgan
>> Computational Biology / Fred Hutchinson Cancer Research Center
>> 1100 Fairview Ave. N.
>> PO Box 19024 Seattle, WA 98109
>>
>> Location: Arnold Building M1 B861
>> Phone: (206) 667-2793
>
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