Allow Re-Listing an unlist()ed Object

Description

relist() is an S3 generic function with a few methods in order to allow easy inversion of unlist(obj) when that is used with an object obj of (S3) class "relistable".

Usage

relist(flesh, skeleton)
## Default S3 method:
relist(flesh, skeleton = attr(flesh, "skeleton"))
## S3 method for class 'factor'
relist(flesh, skeleton = attr(flesh, "skeleton"))
## S3 method for class 'list'
relist(flesh, skeleton = attr(flesh, "skeleton"))
## S3 method for class 'matrix'
relist(flesh, skeleton = attr(flesh, "skeleton"))

as.relistable(x)
is.relistable(x)

## S3 method for class 'relistable'
unlist(x, recursive = TRUE, use.names = TRUE)

Arguments

Details

Some functions need many parameters, which are most easily represented in complex structures, e.g., nested lists. Unfortunately, many mathematical functions in R, including optim and nlm can only operate on functions whose domain is a vector. R has unlist() to convert nested list objects into a vector representation. relist(), its methods and the functionality mentioned here provide the inverse operation to convert vectors back to the convenient structural representation. This allows structured functions (such as optim()) 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 cannot operate on functions that take lists as input; it only likes numeric vectors. The solution is conversion. Given a function mvdnorm(x, mean, vcov, log = FALSE) which computes the required probability density, then

        ipar <- list(mean = c(0, 1), vcov = c bind(c(1, 1), c(1, 0)))
        initial.param <- as.relistable(ipar)

        ll <- function(param.vector)
        {
           param <- relist(param.vector, skeleton = ipar)
           -sum(mvdnorm(x, mean = param$mean, vcov = param$vcov,
                        log = TRUE))
        }

        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(flesh, skeleton)

will put the content of flesh on the skeleton. You don't need to specify skeleton explicitly if the skeleton is stored as an attribute inside flesh. In particular, if flesh was created from some object obj with unlist(as.relistable(obj)) then the skeleton attribute is automatically set. (Note that this does not apply to the example here, as optim is creating a new vector to pass to ll and not its par argument.)

As long as skeleton has the right shape, it should be an inverse of unlist. These equalities hold:

   relist(unlist(x), x) == x
   unlist(relist(y, skeleton)) == y

   x <- as.relistable(x)
   relist(unlist(x)) == x

Note however that the relisted object might not be identical to the skeleton because of implicit coercions performed during the unlisting step. All elements of the relisted objects have the same type as the unlisted object. NULL values are coerced to empty vectors of that type.

Value

an object of (S3) class "relistable" (and "list").

Author(s)

R Core, based on a code proposal by Andrew Clausen.

See Also

unlist

Examples

 ipar <- list(mean = c(0, 1), vcov = cbind(c(1, 1), c(1, 0)))
 initial.param <- as.relistable(ipar)
 ul <- unlist(initial.param)
 relist(ul)
 stopifnot(identical(relist(ul), initial.param))