[Rd] nlminb: names of parameter vector not passed to objective function

[Thomas Petzoldt]

Dear Prof. Ripley,

thank you very much for the fast response. I am very grateful for all 
the work that the R Core does and so I try to contribute my humble part 
as a tester.

Prof Brian Ripley wrote:
> On Wed, 3 Dec 2008, Thomas Petzoldt wrote:
> 
>> Dear R developers,
>>
>> I tried to use nlminb instead of optim for a current problem (fitting 
>> parameters of a differential equation model). The PORT algorithm 
>> converged much better than any of optim's methods and the identified 
>> parameters are plausible. However, it took me a while before spotting 
>> the reason of a technical problem that nlminb, in contrast to optim, 
>> does not pass names of the start parameters to the objective function.
>>
>> Please find below a minimum reproducible example. There is, of course, 
>> a workaround, but in order to make optim and nlme more compatible I 
>> would ask whether it would be possible to change this idiosyncratic 
>> behavior?
> 
> The 'idiosyncratic behavior' is to expect that a new vector of 
> parameters will magically inherit names from the start vector.  optim() 
> was changed (and documented) because some users asked for this, and if a 
> user who wants it for nlminb provides a tested patch, I am sure it will 
> be considered.

O.K., than I will make my, surely naive, suggestion to change file:

src/library/stats/R/nlminb.R

As far I can oversee it, names are dropped at the beginning in:

  ## Establish the working vectors and check and set options
     n <- length(par <- as.double(start))

so it may be sufficient to add something like:

     names(par) <- names(start)

anywhere before:

    assign(".par", par, envir = rho)

This change was sufficient to make my example working and (at a first 
look) I did not find negative side-effects. At least:

R CMD check stats

passed without problems. I had also a look into port.c and found nothing 
obvious there so I, again naively, assume that there is (hopefully) 
nothing to do at the C level. It would be very kind if someone more 
experienced can validate (or falsify) this.

Thank you very much

Thomas Petzoldt



Index: nlminb.R
===================================================================
--- nlminb.R	(revision 47039)
+++ nlminb.R	(working copy)
@@ -48,6 +48,7 @@
      ## Establish the objective function and its environment
      obj <- quote(objective(.par, ...))
      rho <- new.env(parent = environment())
+    names(par) <- names(start)
      assign(".par", par, envir = rho)

      ## Create values of other arguments if needed