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
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