[R] seeking help with with()

[Simon Fear]

Thank you so much for that fix (to my understanding).

I would be willing to add such an example to the help 
page for future releases - though I'm sure others would 
do it better - there are currently no examples where
 INDICES is a name.

In fact in my real application it is more or less essential
that INDICES is a name or at least deparse(substituted 
as a subscript; in a slight elaboration of my previous "fix"

fnz <- function(dframe, by.vars=treat)
  for (pop in 1:2) {
    dframe.pop <- subset(dframe, ITT==pop)
    attach(dframe.pop)
    print(by(dframe.pop, by.vars, summary))
    detach(dframe.pop)
  }

the second call (when pop=2) to by() will crash because by.vars 
is not re-evaluated afresh - it retains its value 
from the first loop.

So, my "fix" was wrong and I am happy to stand corrected.


> -----Original Message-----
> From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk]
> Sent: 27 August 2003 14:08
> To: Simon Fear
> Cc: R-help at stat.math.ethz.ch
> Subject: Re: [R] seeking help with with()
> 
> 
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> ________________________________________________________________
> 
> On Wed, 27 Aug 2003, Simon Fear wrote:
> 
> > I tried to define a function like:
> > 
> > fnx <- function(x, by.vars=Month)
> >   print(by(x, by.vars, summary))
> > 
> > But this doesn't work (does not find x$Month; unlike other 
> functions,
> > such as
> > subset(), the INDICES argument to "by" does not look for 
> variables in
> > dataset
> > x. Is fully documented, but I forget every time). So I tried using
> > "with":
> > 
> > fnxx <- function(x, by.vars=Month)
> >   print(with(x, by(x, by.vars, summary)))
> > 
> > Still fails to find object x$Month. 
> 
> That's not the actual error message, is it?
> 
> > I DO have a working solution (below) - this post is just to ask: Can
> > anyone
> > explain what happened to the with()?
> 
> Nothing!
> 
> by.vars is a variable passed to fnxx, so despite lazy 
> evaluation, it is
> going to be evaluated in the environment calling fnxx().  If 
> that fails
> to
> find it, it looks for the default value, and evaluates that in the
> environment of the body of fnxx.  It didn't really get as far as with.
> 
> (I often forget where default args are evaluated, but I 
> believe that is 
> correct in R as well as in S.)
> 
> I think you intended Months to be a name and not a variable.  With
> 
> X <- data.frame(z=rnorm(20), Month=factor(rep(1:2, each=10)))
> 
> fnx <- function(x, by.vars="Month")
>    print(by(x, x[by.vars], summary))
> 
> will work, as will
> 
> fnx <- function(x, by.vars=Month)
>    print(by(x, x[deparse(substitute(by.vars))], summary))
> 
> 
> -- 
> 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
>
 

Simon Fear
Senior Statistician
Syne qua non Ltd
Tel: +44 (0) 1379 644449
Fax: +44 (0) 1379 644445
email: Simon.Fear at synequanon.com
web: http://www.synequanon.com
 
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