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

[Duncan Murdoch]

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