[R] detecting if a variable has changed

[William Dunlap]

I don't know what you mean by "without having to use any special
interfaces", but "reference classes" will do what I think you want.  E.g.,
the following makes a class called 'SortedNumeric' that only sorts the
vector when you want to get its value, not when you append values.  It
stores the sorted vector so it does not get resorted each time you ask for
it.

SortedNumeric <- setRefClass("sortedNumeric",
            fields = list(
                fData = "numeric",
                fIsUnsorted = "logical"),
            methods = list(
                initialize = function(Data = numeric(), isUnsorted = TRUE) {
                    fData <<- Data
                    stopifnot(is.logical(isUnsorted),
                              length(isUnsorted)==1,
                              !is.na(isUnsorted))
                    fIsUnsorted <<- isUnsorted
                },
                getData = function() {
                    if (isUnsorted) {
                        fData <<- sort(fData)
                        fIsUnsorted <<- FALSE
                    }
                    fData
                },
                appendData = function(newEntries) {
                    fData <<- c(fData, newEntries)
                    fIsUnsorted <<- TRUE
                }
            ))

Use it as:

> x <- SortedNumeric$new()
> x$appendData(c(4,2,5))
> x$appendData(c(1,8,9))
> x
Reference class object of class "sortedNumeric"
Field "fData":
[1] 4 2 5 1 8 9
Field "fIsUnsorted":
[1] TRUE
> x$getData()
[1] 1 2 4 5 8 9
> x
Reference class object of class "sortedNumeric"
Field "fData":
[1] 1 2 4 5 8 9
Field "fIsUnsorted":
[1] FALSE


Outside of base R, I think the R6 package gives another approach to this.


Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Sun, Jun 5, 2016 at 6:53 AM, Neal H. Walfield <neal at walfield.org> wrote:

> Hi,
>
> I have a huge list.  Normally it is sorted, but I want to be able to
> add elements to it without having to use any special interfaces and
> then sort it on demand.  My idea is to use something like weak
> references combined with attributes.  Consider:
>
>   # Initialization.
>   l = as.list(1:10)
>   # Note that it is sorted.
>   attr(l, 'sorted') = weakref(l)
>
>   # Modify the list.
>   l = append(l, 1:3)
>
>   # Check if the list is still sorted.  (I use identical here, but it
>   # probably too heavy weight: I just need to compare the addresses.)
>   if (! identical(l, attr(l, 'sorted'))) {
>     l = sort(unlist(l))
>     attr(l, 'sorted') = weakref(l)
>   }
>   # Do operation that requires sorted list.
>   ...
>
> This is obviously a toy example.  I'm not actually sorting integers
> and I may use a matrix instead of a list.
>
> I've read:
>
>   http://www.hep.by/gnu/r-patched/r-exts/R-exts_122.html
>   http://homepage.stat.uiowa.edu/~luke/R/references/weakfinex.html
>
> As far as I can tell, weakrefs are only available via the C API.  Is
> there a way to do what I want in R without resorting to C code?  Is
> what I want to do better achieved using something other than weakrefs?
>
> Thanks!
>
> :) Neal
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.
>

	[[alternative HTML version deleted]]