"Gabor Grothendieck" <ggrothendi...@gmail.com> wrote in message
news:aanlktilszaicycu3lz2f5d_bxq1g8m8f7jsjsbj2l...@mail.gmail.com...
> On Tue, Jul 20, 2010 at 9:58 AM, <nas...@uottawa.ca> wrote:
>> For nls, the fixing (or masking) of parameters is not, to my knowledge,
>> possible.
>>
>> This is something I've been trying to get in such routines for over 2
>> decades. Masks are
>> available, but not yet well documented, in Rcgmin and Rvmmin packages.
>> However, these use
>> an optim() style approach, which is quite different from nls(). If
>> there's
>> sufficient
>> interest and some collaboration, I'd be willing to have a go at providing
>> such
>> functionality, but it would take quite a bit of work to provide the full
>> capability of nls().
>>
>
> You can optimize over b while fixing m like this:
>
> m <- 1
> nls(demand ~ m + b * Time, BOD, start = c(b = 1))
>
Thanks Gabor,
I'd recognised that approach in my original post starting this thread (19
July) where I'd used the example:
#--------------------------------
# fix parameter and use explicit start values works fine
nls(density ~ SSgompertz(log(conc), Asym, b2, b3=0.8), data = DNase.1,
start=list(Asym=3, b2=2))
#---------------------------
I'm fitting many different models so I need a more generic approach.
I'd appreciate any comments on the suggestion in my 20 July post? (repeated
here with some typo's corrected)
-----------------
nls <- function(formula, data=parent.frame(), start, ...){
if (missing(start)) start <- getInitial(formula, data)
stats:::nls(formula, data, start=start[names(start) %in%
all.vars(formula)], ...)
}
----------------------
I see it breaks nls for non-selfStart functions; e.g. omitting the explicit
start from your example
nls(demand ~ m + b * Time, BOD)
works fine with vanilla nls (albeit with a warning)
# No starting values specified for some parameters.
# Intializing 'b' to '1.'.
# Consider specifying 'start' or using a selfStart model
but my wrapper breaks it altogether.
# Error in getInitial.default(func, data, mCall = as.list(match.call(func,
:
# no 'getInitial' method found for "function" objects
I think that's OK in my application - I either use selfStart functions or
explicit start lists - but I don't really want to remove existing
functionality.
I guess I could (!?!) dive into nls and implement the same kind of approach
internally, but that's very deep water for me :-{
Best regards,
Keith J
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