On Oct 3, 2011, at 2:43 PM, Terry Therneau wrote:
>
> On Mon, 2011-10-03 at 12:31 -0400, Simon Urbanek wrote:
>>> Thanks. I was hoping that x[,sorted] would act like "double(n)"
>> does in a .C call, and have no extra copies made since it has no local
>> assignment.
>>
>> Yes it does act the same way, you get an extra copy with double(n) as
>> well - there is no difference.
>>
>
> That is surprising. This is not true of Splus. Since Chambers mentions it
> as a specific case as well (Programming with Data, p421) I assumed that R
> would be at least as intellegent. He also used the unset() function to
> declare that something could be treated like double(n), i.e., need no further
> copies. But that always looked like a dangerous assertion to me and unset has
> disappeared, perhaps deservedly, from R.
>
Ok, let me clarify - there are two entirely separate issues: one issue is to
avoid duplication of closure arguments in the duplicate() sense -- and that has
to do with the reference count and closures in general. The other is
duplication in the .C() call which is entirely separate.
If you call
.C("foo", double(n))
then the double(n) object doesn't get duplicated in the call, because it has no
references [other than the frame], so there is no duplicate() call on it, but
.C will still create a duplicate double* object to pass to the C function
because of DUP=TRUE. Then a new R object is created for the result from that
double* array.
For comparison, in .Call you would use allocVector(REALSXP, n) and thus avoid
two copies.
You can try to avoid that with .C(..., DUP=FALSE) but you'll be treading on
eggs, because you can't check NAMED or call duplicate() yourself in C code for
cases that may need it, so have to be very, very careful to not modify
something inadvertently (this has been a known source of bugs that are hard to
track, that's why DUP=FALSE is flagged as dangerous in bold face).
Cheers,
Simon
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