Re: [Rd] relist, an inverse operator to unlist

From: Martin Maechler <maechler_at_stat.math.ethz.ch>
Date: Mon, 14 May 2007 09:53:31 +0200

Nice ideas, Gabor and Andrew.

While I agree with Andrew that such a utility makes for nicer and considerably better maintainable code in examples like his, and I do like to provide "inverse operator functions" in R whenever sensible,
OTOH, we have strived to keep R's "base" package as lean and clean as possible, so I think this had to go to "utils".

One further small proposal: I'd use class name "relistable" since that's what the object of this class are and hence as.relistable().

What do other R-develers think?
Martin

>>>>> "GaGr" == Gabor Grothendieck <ggrothendieck_at_gmail.com> >>>>> on Mon, 14 May 2007 02:54:22 -0400 writes:

    GaGr> unlist would not attach a skeleton to every vector it
    GaGr> returns, only the relist method of unlist would.
    GaGr> That way just that method needs to be added and no
    GaGr> changes to unlist itself are needed.

    GaGr> Before applying unlist to an object you would coerce
    GaGr> the object to class "relist" to force the relist     GaGr> method of unlist to be invoked.

    GaGr> Here is an outline of the code:

    GaGr> as.relist <- function(x) {
    GaGr>  if (!inherits(x, "relist")) class(x) <- c("relist", class(x))
    GaGr>  x
    GaGr> }

    GaGr> unlist.relist <- function(x, ...) {
    GaGr>  y <- x
    GaGr>  cl <- class(y)
    GaGr>  class(y) <- cl[- grep("relist", cl)]
    GaGr>  z <- unlist(y)
    GaGr>  attr(z, "relist") <- y

    GaGr> as.relist(z)
    GaGr> }
    GaGr> relist <- function(x, skeleton = attr(x, "relist")) {
    GaGr>  # simpler version of relist so test can be executed
    GaGr>  skeleton
    GaGr> }

    GaGr> # test
    GaGr> x <- list(a = 1:2, b = 3)
    GaGr> class(as.relist(x))

    GaGr> unlist(as.relist(x))
    GaGr> relist(unlist(as.relist(x)))

    GaGr> On 5/14/07, Andrew Clausen <clausen_at_econ.upenn.edu> wrote:
>> Hi GaGr,
>>
>> Thanks for the interesting suggestion. I must confess I got lost -- is
>> it something like this?
>> * unlist() could attach skeleton to every vector it returns.
>> * relist() could then use the skeleton attached to the vector to reconstruct
>> the object. The interface might be
>>
>> relist <- function(flesh, skeleton=attributes(flesh)$skeleton)
>>
>> For example:
>>
>> par <- list(mean=c(0, 0), vcov(rbind(c(1, 1), c(1, 1))))
>> vector.for.optim <- unlist(par)
>> print(attributes(vector.optim)$skeleton) # the skeleton is stored!
>> converted.back.again <- relist(par)
>>
>> Some concerns:
>> * the metadata might get lost in some applications -- although it seems
>> to work fine with optim(). But, if we provide both interfaces (where
>> skeleton=flesh$skeleton is the default), then there should be no problem.
>> * would there be any bad side-effects of changing the existing unlist
>> interface? I suppose an option like "save.skeleton" could be added to unlist.
>> I expect there would be some objections to enabling this as default behaviour,
>> as it would significantly increase the storage requirements of the output.
>>
>> Cheers,
>> Andrew
>>
>> On Sun, May 13, 2007 at 07:02:37PM -0400, GaGr Grothendieck wrote:
>> > I suggest you define a "relist" class and then define an unlist
>> > method for it which stores the skeleton as an attribute. Then
>> > one would not have to specify skeleton in the relist command
>> > so
>> >
>> > relist(unlist(relist(x))) === x
>> >
>> > 1. relist(x) is the same as x except it gets an additional class "relist".
>> > 2. unlist(relist(x)) invokes the relist method of unlist on relist(x)
>> > returning another relist object
>> > 3. relist(unlist(relist(x))) then recreates relist(x)
>> >
>> >
>> > On 5/13/07, Andrew Clausen <clausen_at_econ.upenn.edu> wrote:
>> > >Hi all,
>> > >
>> > >I wrote a function called relist, which is an inverse to the existing
>> > >unlist function:
>> > >
>> > > http://www.econ.upenn.edu/~clausen/computing/relist.R
>> > >
>> > >Some functions need many parameters, which are most easily represented in
>> > >complex structures. Unfortunately, many mathematical functions in R,
>> > >including optim, nlm, and grad can only operate on functions whose domain
>> > >is
>> > >a vector. R has a function to convert complex objects into a vector
>> > >representation. This file provides an inverse operation called "unlist" to
>> > >convert vectors back to the convenient structural representation.
>> > >Together,
>> > >these functions allow structured functions to have simple mathematical
>> > >interfaces.
>> > >
>> > >For example, a likelihood function for a multivariate normal model needs a
>> > >variance-covariance matrix and a mean vector. It would be most convenient
>> > >to
>> > >represent it as a list containing a vector and a matrix. A typical
>> > >parameter
>> > >might look like
>> > >
>> > > list(mean=c(0, 1), vcov=cbind(c(1, 1), c(1, 0)))
>> > >
>> > >However, optim can't operate on functions that take lists as input; it
>> > >only likes vectors. The solution is conversion:
>> > >
>> > > initial.param <- list(mean=c(0, 1), vcov=cbind(c(1, 1), c(1, 0)))
>> > >
>> > > ll <- function(param.vector)
>> > > {
>> > > param <- relist(initial.param, param.vector)
>> > > -sum(dnorm(x, mean=param$mean, vcov=param$vcov, log=TRUE))
>> > > # note: dnorm doesn't do vcov... but I hope you get the
>> > > point
>> > > }
>> > >
>> > > optim(unlist(initial.param), ll)
>> > >
>> > >"relist" takes two parameters: skeleton and flesh. Skeleton is a sample
>> > >object that has the right "shape" but the wrong content. "flesh" is a
>> > >vector
>> > >with the right content but the wrong shape. Invoking
>> > >
>> > > relist(skeleton, flesh)
>> > >
>> > >will put the content of flesh on the skeleton.
>> > >
>> > >As long as "skeleton" has the right shape, it should be a precise inverse
>> > >of unlist. These equalities hold:
>> > >
>> > > relist(skeleton, unlist(x)) == x
>> > > unlist(relist(skeleton, y)) == y
>> > >
>> > >Is there any easy way to do this without my new relist function? Is there
>> > >any
>> > >interest in including this in R's base package? (Or anywhere else?) Any
>> > >comments on the implementation?
>> > >
>> > >Cheers,
>> > >Andrew



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