RE: [R] defining a template for functions via do.call and substitute.

[Thomas Lumley]

On Wed, 29 Sep 2004, Berton Gunter wrote:
John:
Andy and Dimitris have already fully answered your query. However, as you
seem to mainly be interested in simple string substitution, I wanted to
point out that there is a perhaps more transparent approach using gsub()
that does just this.
Here's a slightly more complicated function form to illustrate the idea:
f<-function(a,b){z<-a+b; b+z*sin(b)}

## Get the body of the function and convert it to a character vector:
 bf<-deparse(body(f))

orig<-c('a','b')  ## original symbols
changed<-c('x','y') ## the character strings you wish to substitute for
them
The problem with this is that it will substitute xx for aa, and xy for ab, 
and so on.  You need at least to tokenize the code, even if you don't 
necessarily need to parse it.

        -thomas

## perform the string substitution
 for(i in seq(along=changed))bf<-gsub(orig[i],changed[i],bf)

## re-parse the body and change the formals
 body(f)<-parse(text=bf)
 names(formals(f))<-changed

f
function (x, y)
{
   z <- x + y
   y + z * sin(y)
}
substitute's real power lies in its ability to substitute whole expressions
bound to a symbol in an environment, and this might be overkill here.
Cheers,
-- Bert Gunter
Genentech Non-Clinical Statistics
South San Francisco, CA
"The business of the statistician is to catalyze the scientific learning
process."  - George E. P. Box

-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of
[EMAIL PROTECTED]
Sent: Wednesday, September 29, 2004 4:17 AM
To: [EMAIL PROTECTED]
Subject: [R] defining a template for functions via do.call
and substitute.
Hi,
Given a function
  fun <- function(a, b) a + b
how do I generate the function 'function(x, y) x + y'?
Working from the help files and Bill Venables' R-news article
(June 2002),
I have tried various permutations with substitute without success.
e.g.
  do.call("substitute", list(fun, list(a = as.name("x"), b =
as.name("y"))))
Regards,
John.
John Gavin <john.gavin at ubs.com>,
Quantitative Risk Models and Statistics,
UBS Investment Bank, 6th floor,
100 Liverpool St., London EC2M 2RH, UK.
Phone +44 (0) 207 567 4289
Fax   +44 (0) 207 568 5352
Visit our website at http://www.ubs.com
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Thomas Lumley                       Assoc. Professor, Biostatistics
[EMAIL PROTECTED]       University of Washington, Seattle
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