[Rd] surprising behaviour of names<-

[Simon Urbanek]

On Mar 12, 2009, at 11:12 , Wacek Kusnierczyk wrote:

> Simon Urbanek wrote:
>>
>> On Mar 11, 2009, at 10:52 , Simon Urbanek wrote:
>>
>>> Wacek,
>>>
>>> Peter gave you a full answer explaining it very well. If you really
>>> want to be able to trace each instance yourself, you have to learn
>>> far more about R internals than you apparently know (and Peter  
>>> hinted
>>> at that). Internally x=1 an x=c(1) are slightly different in that  
>>> the
>>> former has NAMED(x) = 2 whereas the latter has NAMED(x) = 0 which is
>>> what causes the difference in behavior as Peter explained. The  
>>> reason
>>> is that c(1) creates a copy of the 1 (which is a constant
>>> [=unmutable] thus requiring a copy) and the new copy has no other
>>> references and thus can be modified and hence NAMED(x) = 0.
>>>
>>
>> Errata: to be precise replace NAMED(x) = 0 with NAMED(x) = 1 above --
>> since NAMED(c(1)) = 0 and once it's assigned to x it becomes NAMED(x)
>> = 1 -- this is just a detail on how things work with assignment, the
>> explanation above is still correct since duplication happens
>> conditional on NAMED == 2.
>
> there is an interesting corollary.  self-assignment seems to  
> increase the reference count:
>
>    x = 1;  'names<-'(x, 'foo'); names(x)
>    # NULL
>
>    x = 1;  x = x;  'names<-'(x, 'foo'); names(x)
>    # "foo"
>

Not for me, at least in current R:

 > x = 1;  'names<-'(x, 'foo'); names(x)
foo
   1
NULL
 > x = 1;  x = x;  'names<-'(x, 'foo'); names(x)
foo
   1
NULL

(both R 2.8.1 and R-devel 3/11/09, darwin 9.6)

In addition, you still got it backwards - your output suggests that  
the assignment created a new, clean copy. Functional call of `names<-`  
(whose side-effect on x is undefined BTW) is destructive when you get  
a clean copy (e.g. as a result of the c function) and non-destructive  
when the object was referenced. It is left as an exercise to the  
reader to reason why constants such as 1 are referenced.

Cheers,
Simon