Re: [R] Hashes as S4 Classes, or: How to separate environments

From: Martin Morgan <mtmorgan_at_fhcrc.org>
Date: Sat, 10 May 2008 05:46:55 -0700

Hi Hans --

Hans W Borchers <hwborchers_at_gmail.com> writes:

> For learning purposes mainly I attempted to implement hashes/maps/dictionaries
> (Python lingua) as S4 classes, see the coding below. I came across some rough S4
> edges, but in the end it worked (for one dictionary).
>
> When testing ones sees that the dictionaries D1 and D2 share their environments
> D1_at_hash and D2_at_hash, though I thought a new and empty environment would be
> generated each time 'new("Dict")' is called.
>
> QUESTION: How can I separate the environments D1_at_hash and D2_at_hash ?

The prototype is created once, at class creation time. So all objects derived from the prototype share the same environment (other types like 'list' would have the illusion of being created anew, because of copy-on-change semantics). A solution is to have an initialize method that recreates the hash each time.

setMethod("initialize", signature=signature(.Object="Dict"),

    function(.Object,

             hash=new.env(hash=TRUE, parent=emptyenv()),
             ...) {
        callNextMethod(.Object, hash=hash, ...)
    })

Another solution is to have a constructor that feeds Dict a new environment

Dict <- function() {

    new("Dict", hash=new.env(hash=TRUE, parent=emptyenv()) }

The latter is perhaps preferable, both from an aesthetic / user point of view ('Dict()' better than 'new("Dict")') and from the nuances-of-S4 point of view (e.g., the default 'initialize' method is documented, though not in so many words, as a copy constructor, taking slots in it's first argument as defaults; 'initialize' above does not satisfy that contract).

I know others have implemented Dict objects; it would be fun to hear from them.

Martin

> Reading the articles mentioned in "Tipps and Tricks" didn't help me really.
> Of course, I will welcome other corrections and improvements as well.
> Working in R 2.7.0 under Windows.
>
> Hans Werner
>
>
> #-- Class and method definition for dictionaries -------------------------------
>
> setClass("Dict",
> representation (hash = "environment"),
> prototype (hash = new.env(hash=T, parent = emptyenv()))
> )
>
> setMethod("show", signature(object="Dict"),
> definition = function(object) ls(object_at_hash)
> )
>
> setGeneric("hclear", function(object) standardGeneric("hclear"))
> setMethod("hclear", signature(object="Dict"),
> function(object) rm(list=ls(object_at_hash), envir=object_at_hash)
> )
>
> setGeneric("hput", function(object, key, value) standardGeneric("hput"))
> setMethod("hput", signature(object="Dict", key="character", value="ANY"),
> function(object, key, value) assign(key, value, envir=object_at_hash)
> )
>
> setGeneric("hget", function(object, key, ...) standardGeneric("hget"))
> setMethod("hget", signature(object="Dict", key="character"),
> function(object, key) {
> if (exists(key, envir=object_at_hash, inherits = FALSE)) {
> get(key, envir=object_at_hash)
> } else {
> return(NULL)
> }
> }
> )
>
> # ---- Some tests ----
> D1 <- new("Dict")
> hput(D1, "a", 1) # Same as: D1_at_hash$a <- 1
> hput(D1, "b", 2)
> hget(D1, "a")
> hget(D1, "b")
> show(D1)
>
> D2 <- new("Dict")
> hput(D2, "c", 3)
> hput(D2, "d", 4)
> hget(D2, "a") # Wrong: was defined only for D1
> hget(D2, "b")
> show(D2)
>
> hclear(D2) # Wrong: clears D1 too
> show(D1)
> #---------------------
>
> ______________________________________________
> R-help_at_r-project.org mailing list
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> PLEASE do read the posting guide 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 M2 B169
Phone: (206) 667-2793

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Received on Sat 10 May 2008 - 12:51:54 GMT

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