R devel - Dec 2006 - question about trailing arguments in an S4 method

I'm trying to add arguments to the AIC method
for some classes -- things like
  weights=TRUE to calculate AIC weights
  corr=TRUE, nobs to calculate AICc
  delta=TRUE  to put a delta-AIC column in the output.

 The problem is that AIC is defined as
AIC(object, ..., k=2)  where k is the constant associated
with the penalty term and ... is a list of objects
that will have their AICs calculated and listed.
Thus I'm not allowed (I think) to extend the definition to

AIC(object, ..., nobs, corr=FALSE, delta=FALSE, weights=FALSE, k=2)

  An ugly solution to this is to go through the ... list and
pull out the values of any of the other arguments I want, and
then set them to NULL, leaving the list composed only of
additional objects to calculate the AIC for.  But it seems
really ugly ...

   Any ideas for a better way to do this would be
appreciated.  (Please be gentle if possible, I'm a bit out
of my depth with S4 ...)

  cheers
    Ben Bolker


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I don't see how this can work, in S3 or S4.  Callers of AIC are entitied 
to expect it to behave as described on the help page, and so to be able to 
pass objects called (e.g.) 'delta' and get back exactly the value 
mentioned.

For what you seem to want to do, I think you need your own generic.


On Sat, 16 Dec 2006, Ben Bolker wrote:
>
>  I'm trying to add arguments to the AIC method
> for some classes -- things like
>  weights=TRUE to calculate AIC weights
>  corr=TRUE, nobs to calculate AICc
>  delta=TRUE  to put a delta-AIC column in the output.
>
> The problem is that AIC is defined as
> AIC(object, ..., k=2)  where k is the constant associated
> with the penalty term and ... is a list of objects
> that will have their AICs calculated and listed.
> Thus I'm not allowed (I think) to extend the definition to
>
> AIC(object, ..., nobs, corr=FALSE, delta=FALSE, weights=FALSE, k=2)
>
>  An ugly solution to this is to go through the ... list and
> pull out the values of any of the other arguments I want, and
> then set them to NULL, leaving the list composed only of
> additional objects to calculate the AIC for.  But it seems
> really ugly ...
>
>   Any ideas for a better way to do this would be
> appreciated.  (Please be gentle if possible, I'm a bit out
> of my depth with S4 ...)
>
>  cheers
>    Ben Bolker
>
>
>
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

Ben Bolker <bolker at zoo.ufl.edu> writes:
>   I'm trying to add arguments to the AIC method
> for some classes -- things like
>   weights=TRUE to calculate AIC weights
>   corr=TRUE, nobs to calculate AICc
>   delta=TRUE  to put a delta-AIC column in the output.
>
>  The problem is that AIC is defined as
> AIC(object, ..., k=2)  where k is the constant associated
> with the penalty term and ... is a list of objects
> that will have their AICs calculated and listed.
> Thus I'm not allowed (I think) to extend the definition to
>
> AIC(object, ..., nobs, corr=FALSE, delta=FALSE, weights=FALSE, k=2)
I think you can add args before the '...'.  However, you will need to
provide values for them in all calls or else they will steal a fit
from the '...' args.

I don't understand really what you are trying to achieve, so this may
not be helpful.  Nevertheless, here is a dummy example that you may
find useful.


setClass("Foo",
         representation=representation(
           fit="lm"))

setMethod("AIC", "Foo",
          function(object, weights=TRUE, delta=TRUE, ..., k=2) {
              ans <- AIC(object=object at fit, k=k, ...)
              args1 <- list(weights=weights, delta=delta, k=k)
              cat("args\n")
              print(args1)
              cat("answer:\n")
              print(ans)
          })

example("AIC")
foo <- new("Foo", fit=lm1)
AIC(foo)
## AIC(foo, lm1)   # this doesn't work, lm1 gets picked up as the weights
arg
AIC(foo, weights=FALSE, delta=TRUE, lm1, lm1, lm1)



+ seth