[Rd] Changing function arguments to NULL

McGehee, Robert Robert.McGehee at geodecapital.com
Mon Feb 28 23:13:57 CET 2005

But passing list(NULL) or NULL into a function may have entirely
different results.

For instance,

> paste("Hello world", NULL)
[1] "Hello world "

> paste("Hello world", list(NULL))
[1] "Hello world NULL"

Simply put, I'd like to look at a function's call (i.e. using
match.call()), and replace one of its arguments with NULL and
re-evaluate it. I was hoping for something generic, such that I didn't
have to know all of a function's arguments ahead of time (i.e. I could
just replace the one I wanted without even knowing the names or values
of the other arguments), but this may be easier said than done.

-----Original Message-----
From: Roger D. Peng [mailto:rpeng at jhsph.edu] 
Sent: Monday, February 28, 2005 4:58 PM
To: McGehee, Robert
Cc: r-devel at stat.math.ethz.ch
Subject: Re: [Rd] Changing function arguments to NULL

I'm not sure what you're doing but did you try

recursionFUN$x <- list(NULL)



McGehee, Robert wrote:
> I'm trying to build a recursive set of functions that take a set of
> arguments, change some of the arguments and recursively call the same
> (or different) function.
> For example here's a stupid recursive counting function that prints
> all integers from x to 0 (and ignores arguments y and z)
> cnt <- function(x, y, z) {
> 	stopifnot(is.numeric(x))
> 	print (x)
> 	recursionFUN <- match.call()
> 	recursionFUN$x <- x - 1
> 	if (x <= 0) {
> 		invisible(TRUE)
> 	} else {
> 		eval(recursionFUN)
> 	}
> }
> My problem is that sometimes I want to set one of the arguments to
> But trying to set one of the match.call() arguments to NULL causes it
> be ignored (since the match.call() output is coerced into a list).
> I'd like is that the match.call() output could be converted into an
> alist, so that tagged values with no arguments could be handled and
> passed on to the next function call without being ignored.
> However, I haven't been able to figure out how to construct this alist
> without knowing ahead of time what all of the function arguments are
> typing them in explicitly:
> e.g. alist(x = 5, y = 2, z = 3).
> Re-assigning and then re-evaluating output of the match.call()
> might not be the way to go, but I am not sure, and would appreciate
> comments on the best way to set function arguments to NULL before
> evaluating.
> Thanks,
> Robert
> Robert McGehee
> Geode Capital Management, LLC
> 53 State Street, 5th Floor | Boston, MA | 02109
> Tel: 617/392-8396    Fax:617/476-6389
> mailto:robert.mcgehee at geodecapital.com
> This e-mail, and any attachments hereto, are intended for
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