[R] Pass-through of unevaluated arguments

[Peter Dalgaard BSA]

fharrell at virginia.edu writes:

> Here's the situation.  xYplot needs to fiddle with
> arguments formula and data (which I default to 
> sys.frame(sys.parent()) which I believe will pass through
> to xyplot OK).  There are several arguments passed
> through to xyplot which were the "..." arguments to xYplot.
> I want to defer evaluation of these to inside xyplot where
> it will evaluate the arguments with respect to the
> "data" argument.  A good example is the groups= variable:
> 
> xYplot(formula, data=d, groups=g)
> 
> where g might be in d or in the frame that invoked
> xYplot.  Using for example do.call("xyplot", list(...))
> from within xYplot will result in "g not defined".
> I can do something like
>  
>  if(!missing(groups)) groups <- eval(substitute(groups),data)

I think 

eval(substitute(xyplot(...)), data, environment(formula))

will get you there if xYplot is function(formula,data,...).
environment(formula) is somewhat preferable to parent.frame() in
general, although they're usually the same thing. 

The tricky bit is when you want to mix external arguments with
something computed inside xYplot. We just had this aired on R-core in
a similar context, and I *think* you get by with substituting in local
variables (notice that R's substitute() is not quite identical to
S's).
  
-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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