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 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
######################################################################
Attention:\ This e-mail message is privileged and confid...{{dropped:9}}
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 > > ###################################################################### > Attention:\ This e-mail message is privileged and confid...{{dropped:9}} > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code.
Note the difference between
test <- function(a) {
exists("a", mode = "symbol")
}
test()
and
test2 <- function(a) {
exists("a", mode = "numeric") #say
}
test2()
and then note that the default mode argument to exists is "any".
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 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
>
> ######################################################################
> Attention:\ This e-mail message is privileged and confid...{{dropped:9}}
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
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 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?Oops, I didn't explain why this is the way it should be. Say your "whatever" above makes use of a, but you didn't pass an a in. Then you'd like an error, or you'd like "missing(a)" to evaluate to TRUE, or something along those lines. But if a was completely undefined and nonexistent, R would just go looking for a global, and make use of that. So it has to be marked as missing. Duncan Murdoch