[R] Existence of formal arguments.

[Duncan Murdoch]

On 05/06/2008 8:23 PM, Rolf Turner wrote:
> I just discovered what seems to me to be a slight funny in respect
> of formal argument names.  If I define a function
> 
> 	foo <- function(a,b){ ... whatever ...}
> 
> then ``inside'' foo() the exists() function will return TRUE
> from ``exists("a") whether an object named ``a'' exists or not.
> But get("a") will yield an error ``object "a" not found''
> in these circumstances.

I don't see that.  I see this:

 > foo <- function(a, b) { get("a") }
 > get("a")
Error in get("a") : variable "a" was not found
 > foo()

I'm guessing that after you got a, you tried to do something with it, 
but I can't guess what:  I get a different error in this code.

 > foo <- function(a, b) { x <- get("a"); x }
 > foo()
Error in foo() : argument "x" is missing, with no default


The thing is, formals are always defined in the local scope when you 
call a function.  But if you didn't assign a value or a default to them, 
they are defined as a special missing value.  As the second example 
shows, you can move that missing around, but not do anything with it.

Duncan Murdoch




> 
> I presume there is a reason for specifying that an object named
> by a formal argument always exists --- but it is mysterious by my
> standards.  Can anyone explain the reason for this behaviour?
> 
> This is just idle curiosity --- or my hunger for knowledge, whichever
> way you want to look at it :-) --- it doesn't really matter.
> 
> 	cheers,
> 
> 		Rolf Turner
> 
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