Dear David--
I think I am straight on the dopiness of my original question.
In trying to make sure I understand your example, I ran this function
below. It seems to assign the value of V from the calling environment to X
as a default, ignored if X is given a value positionally, used if not. It
does not assign a name to X so far as I can tell, unless V has a name, in
which case it assigns to X the name of V (not the name "V") to X, even if
X is given a different numerical value positionally. Does it seem to you
that I am understanding or misunderstanding your point about the = in a
function definition assigning names?
V<-2
Names(V)<- Bob
foo <- function(X = V) {
cat("name of x:", names(X), "\n")
print("V:")
print(V)
print("X:")
print(X)
}
foo(4)
foo()
On Sun, Dec 29, 2013 at 8:05 PM, David Winsemius <dwinsem...@comcast.net>wrote:
>
> On Dec 29, 2013, at 3:57 PM, andrewH wrote:
>
> Dear David--
>>
>> Thanks so much for your helpful reply!
>>
>> David Winsemius wrote:
>>
>>> The LHS X becomes a name, the RHS X will be looked up in the calling
>>>>
>>> environment and fails if no value is positionally matched and then no X
>> is
>> found (at the time of the function definition.
>>
>> Does X really have to exist when the function is defined?
>>
>
> No
>
>
>
> I thought it was
>> enough if it existed in the environment of the calling function, or
>> somewhere up the environment chain of the calling function. If this is not
>> true, then that means it matters a lot whether you write a function inside
>> another function or just call it in that function. Suppose a function
>> with
>> a reflexive assignment X=X
>>
>
> Arrrgh. The is no "reflexive assignment". You are making up a concept.
>
>
> is defined in the global environment but called
>> inside another function, and X has a different value in those two places.
>> Will it look first in the global environment and only then in the calling
>> environment? And is this different from the behavior without the reflexive
>> assignment?
>>
>> I should not bother you with those questions. I should just run it both
>> ways
>> and see what happens.calling function and will it look first in the
>>
>> If you use`X <- value` in the argument list, then what is returned is
>>>> only
>>>>
>>> the value and the name `X` may be lost. Or in the case of data.frame
>> morphed
>> into a strange name:
>>
>> [example omitted]
>> I am not sure that I am understanding you correctly here. Are you saying
>> that assignment using the "=" retains the name (and other attributes?
>> which
>> ones?) of the RHS, while "<-" does not?
>>
>
> Using "=" assigns a name. Using "<-" retruns a value and whether the value
> gets a name depends on the particular function.
>
> > foo <- function(X <- V) { print(X)}
> Error: unexpected assignment in "foo <- function(X <-"
> > foo <- function(X = V) { print(X)}
> > foo(4)
> [1] 4
> > foo <- function(X = V) { print(V)}
> > foo(4)
> Error in print(V) : object 'V' not found
>
>
> --
>
> David Winsemius, MD
> Alameda, CA, USA
>
>
--
J. Andrew Hoerner
Director, Sustainable Economics Program
Redefining Progress
(510) 507-4820
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