[R] Using subset() in a user-defined function

[Tobin, Jared]

Thanks for the quick response, Duncan.

The given code doesn't seem to work, and possibly due to this reason I
found in the online help for missing() (if I understand it correctly):

"Currently missing() can only be used in the immediate body of the
function that defines the argument, not in the body of a nested function
or a local call. This may change in the future."

So as I understand it, missing() cannot refer to the arguments of
function1 if it is used in an argument of subset()?  It seems to remain
a promising function for this situation regardless, but I'm not sure how
I could implement it into the subset() arguments offhand.

--

jared tobin, student research assistant
dept. of fisheries and oceans
tobinjr at dfo-mpo.gc.ca

-----Original Message-----
From: Duncan Murdoch [mailto:murdoch at stats.uwo.ca] 
Sent: Thursday, June 14, 2007 11:28 AM
To: Tobin, Jared
Cc: r-help at stat.math.ethz.ch
Subject: Re: [R] Using subset() in a user-defined function

On 6/14/2007 9:38 AM, Tobin, Jared wrote:
> Hello,
> 
> I'm having a problem with using subset() inside a function I'm
writing.
> Ignoring everything else in the function, the problem can be 
> illustrated by (where master.frame is the data frame I'm using):
> 
> 
> function1 <- function(arg1="", arg2="", arg3=""){
> 
> 	temp.frame <- subset(master.frame, a == arg1 & b == arg2 & c ==
> arg3)
> 
> }
> 
> 
> This works fine if the user specifies all arguments, but if any one or

> more of the arguments isn't specified, say arg1 for example, the 
> subset is empty because subset() goes looking for values of a == "" in

> master.frame, and there are none.  I want it to work such that if an 
> argument is not specified, it is not included in what subset() goes 
> looking for.  So if I were to input:
> 
> function1(arg2=5, arg3=6)
> 
> then in function1, the subset command will look like
> 
> 	temp.frame <- subset(master.frame, b == 5 & c == 6)
> 
> 
> Any suggestions would be much appreciated.

Code it like this:

subset(master.frame, (missing(arg1) | a == arg1) &
                      (missing(arg2) | b == arg2) &
                      (missing(arg3) | c == arg3))

I haven't tried this, and I forget what happens in subset() if you pass
it a subset of the wrong length, so it might fail if all args are
missing, but otherwise I think it should work.  It does depend on
defaults for the args existing and not causing errors in the equality
tests (it's not using shortcut evaluation).

Duncan Murdoch