On 04/04/2010 05:27 PM, Norm Matloff wrote:
> Thanks very much.
>
> By the way, I tried setting a GDB breakpoint at duplicate1(), with the
> following:
>
> > x <- 1:1000
> > x[3] <- 8
> > x[33] <- 88
Here's how I investigated this, with the last line somewhat surprising
R -d gdb
gdb> r
... cntrl-C
gdb> break duplicate1
gdb> commands
> call Rf_PrintValue(s)
> c
> end
gdb> c
then
x=5:1
x[1L] = 1L # no copy
x[1L] = 10 # type coercion, new alloc but no copy
x[1] = 20 # copy of index (!)
Martin
>
> I found that duplicate1() was called on both of the latter two lines.
> I was a bit surprised, since change-on-write would seem to imply that
> copying would be done in that second line but NOT on the third.
> Moreover, system.time() gave 0.284 user time for the second and 0 on
> the third. YET duplicate1() WAS called on the third, and in stepping
> through the code, there didn't seem to be an immediate exit.
>
> Thanks to both John and Duncan for their comment on the fact that using
> [<- directly is a very different situation. That's not what I asked,
> but the comment is useful to me for other reasons.
>
> Norm
>
>> Message: 4
>> Date: Sat, 03 Apr 2010 17:54:58 -0700
>> From: John Chambers
>> To: r-devel@r-project.org
>> Subject: Re: [Rd] full copy on assignment?
> ...
> ...
>> How often does y get duplicated? Hopefully not a million times. One can
>> look at this in gdb, by trapping calls to duplicate1. The answer is:
>> just once, to ensure that the object is local. Then the duplicated
>> version has only one reference and the primitive replacement doesn't
>> copy it.
> ...
>
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--
Martin Morgan
Computational Biology / Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N.
PO Box 19024 Seattle, WA 98109
Location: Arnold Building M1 B861
Phone: (206) 667-2793
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