R help - Sep 2005 - Superassignment (<<-) and indexing

In a clean environment under R-2.1.0 on Linux:
> x <- 1:5
> x[3] <<- 9
Error: Object "x" not found

Isn't that odd?  (Note x <<- 9 works just fine.)

Why am I doing this?  Because I'm stepping through code that
normally lives inside a function, where "<<-" is appropriate.

-- David Brahm (brahm at alum.mit.edu)

Permit a mild protest on the word "appropriate" in this context.  The 
global assignment operator "<<-" provides, for my tastes,
excessive
opportunities for problems.  If I define "x" someplace else and then 
call your function, it may change my "x" in ways that generate 
considerable wailing and gnashing of teeth.  Unless I assign the 
function output to "x", then the action of your function will change
my
"x" in ways I did not anticipate, possibly generating many problems
for
me later -- with extreme difficulties in finding the source of the 
problem.  Moreover, if your library expects to later find in "x" what 
your function stored there, there could be other problems, because I 
might redefine "x" before you use it.  The library might work fine
when
you use it but not for someone else -- and tracing the problem can be 
difficult.

	  I understand that "<<-" may allow your function f1 to call f2
and
have f2 change "x" in f1.  However, if your f2 gets called some other 
way or if the name of "x" is misspelled or changed in either f1 or f2,
we could be back to the situation I just described.

	  spencer graves

Brahm, David wrote:
> In a clean environment under R-2.1.0 on Linux:
> 
>>x <- 1:5
>>x[3] <<- 9
> 
> Error: Object "x" not found
> 
> Isn't that odd?  (Note x <<- 9 works just fine.)
> 
> Why am I doing this?  Because I'm stepping through code that
> normally lives inside a function, where "<<-" is
appropriate.
> 
> -- David Brahm (brahm at alum.mit.edu)
> 
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-- 
Spencer Graves, PhD
Senior Development Engineer
PDF Solutions, Inc.
333 West San Carlos Street Suite 700
San Jose, CA 95110, USA

spencer.graves at pdf.com
www.pdf.com <http://www.pdf.com>
Tel:  408-938-4420
Fax: 408-280-7915

On Fri, 2 Sep 2005, Brahm, David wrote:
> In a clean environment under R-2.1.0 on Linux:
>> x <- 1:5
>> x[3] <<- 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[3] <<- 9
is short for
   `*tmp*` <- get(x, envir=parent.env, inherits=TRUE)
   `*tmp*`[3] <- 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.

 	-thomas