Re: [Rd] [R] learning R

From: Wacek Kusnierczyk <Waclaw.Marcin.Kusnierczyk_at_idi.ntnu.no>
Date: Wed, 25 Feb 2009 10:32:03 +0100

a quick follow-up:

    e = new.env()
    e$a = 1
    names(e)
    # NULL
    names(e) = 'a'
    # error in names(e) = "foo" : names() applied to a non-vector

this is surprising. names(e) 'works', there is no complaint, but when names<- is used, the error is about the use of names, not names<-.

btw. ?names says:

"Description:

     Functions to get or set the names of an object.

Usage:

     names(x)
     names(x) <- value

Arguments:

       x: an R object.
"

and there is no clarification in the rest of the page that x cannot be an environment, or that it has to be a vector. furthermore:

    p = pairlist(a=1)
    names(p)
    # "a"
    names(p) = 'b'
    # fine
    is.vector(p)
    # FALSE which is incoherent with the above error message, in that p is *not* a vector.

vQ

Wacek Kusnierczyk wrote:
>
> the following:
>
> names(a[2]) = 'foo'
>
> has (partially) a functional flavour, in that you assign to the names of
> a *copy* of a part of a, while
>
> names(a)[2] = 'foo'
>
> does not have the flavour, in that you assign to the names of a; it
> seems, according to the man page you quote, to be equivalent to:
>
> a = 'names<-'(a, '[<-.'(names(a), 2, 'foo'))
>
> which proceeds as follows:
>
> tmp1 = names(a)
> # get a copy of the names of a, no effect on a
>
> tmp2 = '[<-'(tmp1, 2, 'foo')
> # get a copy of tmp1 with the second element replaced with 'foo'
> # no effect on either a or tmp1
>

> tmp3 = 'names<-'(a, tmp2)
> # get a copy of a with its names replaced with tmp2
> # no effect on either a, tmp1, or tmp2
>
> a = tmp3
> # backassign the result to a
>



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