[R] Ellipsis arguments for plot.formula

[Amit Ganatra]

Thank you for taking the time. 
I get your point about presumptions. 
If you issue the call my.plot( x, y, main = main.str ), it works fine. The problem turns up only when you use a formula as an argument, hence the suspicion that the behavior was unintended. 


--- On Thu, 7/10/08, Bert Gunter <gunter.berton at gene.com> wrote:

> From: Bert Gunter <gunter.berton at gene.com>
> Subject: RE: [R] Ellipsis arguments for plot.formula
> To: amit_d_ganatra at yahoo.com
> Cc: r-help at r-project.org
> Date: Thursday, July 10, 2008, 5:10 PM
> I think it's rather presumptuous of you to ask whether a
> fundamental
> behavior that you don't understand in a mature software
> product that has
> been used by literally thousands of people for around 10
> years (>20 for S)
> is "a bug", don't you? 
> 
> So the answer is, no, this is how R's evaluation
> mechanism works. It is,
> admittedly, a complex issue, but let's see if we can
> trace it through (help
> and corrections from wiser folks would be appreciated if I
> get anything
> wrong...).
> 
> When you issue your call
>  
> > my.plot( y ~ x, tdf, main = main.str )
> 
> at the command line, the "main=main.str" part is
> the "..."  argument (and
> can be recovered within the function e.g. by list(...); see
> ?browser). As
> you understand, R now passes this down to the generic
> plot() call (see
> ?UseMethod for info on S3 generics). This in turn calls
> plot.default() via
> 
> > plot.default(x,y, cex.axis=0.5, main = main.str) .
> 
> Now plot.default has to evaluate the name, main.str, in
> order to execute.
> Where is it to do this evaluation? It is **not** the top
> level (.GlobalEnv),
> which is why you threw the error message. So where does the
> symbol,
> "main.str," get evaluated? As I read the
> ?UseMethod
> documentation(corrections please here if I'm wrong), it
> is in the context of
> the plot call, which is within the my.plot function *** not
> in the parent
> frame as you said*** -- which is the global environment
> (where it *would*
> find it).  It does not find main.str there, and so you get
> an error message.
> As you noted, if you explicitly put main.str into the
> my.plot environment,
> it would find it and the function works as you expect.
> 
> I think the best documentation of all these technicalities
> is V&R's S
> PROGRAMMING, but others may also have their own favorites.
> As I said, it is
> quite tricky.
> 
> As for a "workaround," this too, gets tricky: you
> basically need to
> construct the plot call with the evaluated variables. The
> V&R reference
> shows you one way to do this (see the "Computing on
> the Language" chapter,
> if memory serves). There may well be other slicker tricks
> by now (which
> others may provide, I hope).
> 
> Hope this helps... and please be more circumspect in future
> when asking such
> questions. R is a quite remarkable effort by some really
> smart people who
> have done an enormous amount of work essentially for
> nothing. While bugs
> certainly can and do arise, I think our first and almost
> always correct
> impulse should be to assume that there's something that
> we don't understand
> and phrase our queries accordingly.  We owe the developers
> this courtesy.
> 
> Cheers,
> 
> Bert Gunter
> Genentech, Inc.
> 
> 
> 
>  
> 
> -----Original Message-----
> From: r-help-bounces at r-project.org
> [mailto:r-help-bounces at r-project.org] On
> Behalf Of Amit Ganatra
> Sent: Thursday, July 10, 2008 11:51 AM
> To: r-help at r-project.org
> Subject: [R] Ellipsis arguments for plot.formula
> 
> Hi:
> I have a function as follows:
>  my.plot    <- function( x, y = NULL, ... )
> {
>   plot( x, y, cex.axis=0.5, ...)
> }
> 
> Set up the variables:
> x <- 1:10; y <- x; tdf <- data.frame( x, y );
> main.str <- "test"
> 
> I will exercise the function in two ways:
> 
> > my.plot( y ~ x, tdf, main = "test" )
> This works fine
> 
> > my.plot( y ~ x, tdf, main = main.str )
> Error in eval(expr, envir, enclos) : 
>   ..1 used in an incorrect context, no ... to look in
> 
> Turns out that plot.formula is looking for the name
> "main.str" in its parent
> frame. If I define the name main.str in my.plot the
> function works fine.
> 
> Is this a bug?
> I have a clunky workaround where I get the parent.frame in
> my.plot and
> create the variables in my.plot, but I don't like the
> solution. 
> 
> Thanks
> 
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