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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