[R] assigment operator question

[Duncan Murdoch]

On 12-04-07 4:51 PM, Henrik Bengtsson wrote:
> On Sat, Apr 7, 2012 at 1:30 PM, Mark Heckmann<mark.heckmann at gmx.de>  wrote:
>> Hello,
>>
>> using the<<- assignment operator I do not understand why the following does not work.
>>
>> l<<- list()
>> l
>> list()
>> l$arg1<<- "test"
>> error in l$arg1<<- "test" : Objekt 'l' not found
>>
>> ?"<<-" says:  "The operators<<- and ->>  cause a search to made through the environment for an existing definition of the variable being assigned. If such a variable is found (and its binding is not locked) then its value is redefined, otherwise assignment takes place in the global environment. "
>>
>> Still I do noch understand why the above does not work. The object l is in the global environment. Can someone explain it to me?
>
> Yes, the object 'l' is in the global environment, but 'l$arg1' is not,
> cf. exists("l$arg1") and exists("l").  Instead, this works:

I don't think that is the problem:  <<- normally works with complex 
assignments.  I think the problems are

  - Mark was working at top level in the console
  - The documentation isn't clear about what happens in that case.
  - R is inconsistent between simple and complex assignments with <<-.

The <<- operator is designed to be used in a function, and it makes an 
assignment in the environment of the function, not in the local 
evaluation frame.  If it can't find a variable there, it assigns into 
the global environment.  So if the l$arg1 <<- "test" was a line in a 
function, everything would have worked as expected.

When you use it at the top level, it tries looking in the parent of the 
global environment, its parent, and so on, back to the empty environment 
at the root of all the namespaces.  If you happened to use a name like 
"mean" which exists, it will try to assign to that object (and fail, 
because that binding is locked).

If you pick a name that doesn't exist, then R is inconsistent.  If 
you're doing a simple assignment like

l <<- 5

it will be done in the global environment, creating a new variable if 
necessary.  If you're doing a complex assignment like

l$arg1 <- "test"

it will just fail, there's no global environment fallback.

I don't think this inconsistency is really worth fixing, since <<- is 
designed for use in functions.  There really isn't any use for it at the 
top level.

But maybe all of this should be documented.

Duncan Murdoch

>
> l<<- list();
> l$args1<- "test";
>
> but much much much much much much much much much much much much much
> much much much much much much much much much much much much much much
> much much much much much much much much much much much better, do NOT
> use "<<-" (or assign()/get()) unless you fully understand what you're
> doing and have a very good reason for doing it, and that reason should
> be able pass the embarrassment-test on public R mailing lists.  Got
> the point?  Don't use it - there is another way to do what you want to
> achieve - you just have to ask/find out how.
>
> /H
>
>>
>> Thanks
>> --Mark
>> ––––––––––––––––––––––––––––––––––––
>> Mark Heckmann
>> Blog: www.markheckmann.de
>> R-Blog: http://ryouready.wordpress.com
>>
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>
> ______________________________________________
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.