Dear all,
The following code is inspired by the help file for the relist()
function (see?relist), which explicitly details how you can use a
relistable object in conjunction with optim to pass and reconstruct
complex parameter structures/groupings. The idea is that the optim()
function can only work with vectors, but in many cases you would like
to use a complex structure inside the objective function- relist is
one way to do that. The problem is that optim appears to be stripping
the attributes and therefore the example doesn't seem to run, giving
the error at the bottom.
> rb.banana <- function(params) {
+ #Params is initially a vector
+ cat("Params initially has the attributes:\n")
+ print(names(attributes(params)))
+ #Relisting it turns it into a list...
+ params <- relist(params)
+ cat("---------\n")
+ #..which can then be called in the standard list manner
+ return( (1-params$x)^2 + 100*(params$y - params$x^2)^2)
+ }
>
> ipar <- as.relistable(list(x=5,y=0))
> initial.params <- unlist(ipar)
>
> #Test to see if rb.banana works properly in the "normal" case
> rb.banana(initial.params)
Params initially has the attributes:
[1] "names" "skeleton"
---------
[1] 62516
>
> #OK, that's good. How about with optim though?
> optim(initial.params,rb.banana)
Params initially has the attributes:
[1] "names"
Error in relist(params) :
The flesh argument does not contain a skeleton attribute.
Either ensure you unlist a relistable object, or specify the skeleton
separately.
>
What's going on here? Has this functionality been removed and the
documentation in relist() not updated? Or has the feature been broken?
Or have I misinterpreted something here (it wouldn't be the first
time!!)
Am running R version 2.7.1 (2008-06-23) under windows.
Cheers,
Mark
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