[R] Superassignment (<<-) and indexing
tlumley at u.washington.edu
Sat Sep 3 00:16:17 CEST 2005
On Fri, 2 Sep 2005, Brahm, David wrote:
> In a clean environment under R-2.1.0 on Linux:
>> x <- 1:5
>> x <<- 9
> Error: Object "x" not found
> Isn't that odd? (Note x <<- 9 works just fine.)
Well, yes and no.
It is the result of a bug fix a version or two ago that dealt with the
case where there was a local variable with the same name as a
variable being modified by a complex superassignment.
As the R language definition now explains
x <<- 9
is short for
`*tmp*` <- get(x, envir=parent.env, inherits=TRUE)
`*tmp*` <- 9
x <<- `*tmp*`
and so it doesn't work if x doesn't exist in the parent environment.
x <<- 9
is ok, since it doesn't have to look up x before assigning it, but it is
still a wart that x<<-9 creates a local variable x when executed in the
global environment but not when executed anywhere else.
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