R devel - Feb 2005 - Changing function arguments to NULL

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 back
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 NULL.
But trying to set one of the match.call() arguments to NULL causes it to
be ignored (since the match.call() output is coerced into a list). What
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 and
typing them in explicitly:
e.g. alist(x = 5, y = 2, z = 3).

Re-assigning and then re-evaluating output of the match.call() function
might not be the way to go, but I am not sure, and would appreciate any
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@geodecapital.com



This e-mail, and any attachments hereto, are intended for us...{{dropped}}

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

recursionFUN$x <- list(NULL)

?

-roger

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
back
> 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 NULL.
> But trying to set one of the match.call() arguments to NULL causes it to
> be ignored (since the match.call() output is coerced into a list). What
> 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 and
> typing them in explicitly:
> e.g. alist(x = 5, y = 2, z = 3).
> 
> Re-assigning and then re-evaluating output of the match.call() function
> might not be the way to go, but I am not sure, and would appreciate any
> 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@geodecapital.com
> 
> 
> 
> This e-mail, and any attachments hereto, are intended for us...{{dropped}}
> 
> ______________________________________________
> R-devel@stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

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@jhsph.edu] 
Sent: Monday, February 28, 2005 4:58 PM
To: McGehee, Robert
Cc: r-devel@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)

?

-roger

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
back
> 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
NULL.
> But trying to set one of the match.call() arguments to NULL causes it
to
> be ignored (since the match.call() output is coerced into a list).
What
> 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
and
> typing them in explicitly:
> e.g. alist(x = 5, y = 2, z = 3).
> 
> Re-assigning and then re-evaluating output of the match.call()
function
> might not be the way to go, but I am not sure, and would appreciate
any
> 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@geodecapital.com
> 
> 
> 
> This e-mail, and any attachments hereto, are intended for
us...{{dropped}}
> 
> ______________________________________________
> R-devel@stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>