On Mon, 3 Mar 2008, Nordlund, Dan (DSHS/RDA) wrote:
[..., quoting Hadley Wickham]
>>>> gc()
>>> used (Mb) gc trigger (Mb) max used (Mb)
>>> Ncells 133095 3.6 350000 9.4 350000 9.4
>>> Vcells 87049 0.7 786432 6.0 478831 3.7
>>>> a <- runif(1e7)
>>>> gc()
>>> used (Mb) gc trigger (Mb) max used (Mb)
>>> Ncells 133112 3.6 350000 9.4 350000 9.4
>>> Vcells 10087364 77.0 11458389 87.5 10087374 77.0
>>>> b <- a
>>>> gc()
>>> used (Mb) gc trigger (Mb) max used (Mb)
>>> Ncells 133117 3.6 350000 9.4 350000 9.4
>>> Vcells 10087365 77.0 12111308 92.5 10087476 77.0
>>>
>>> R will only create a copy if either of a or b is modified.
> But, the OP should know that in the above scenario, if a or b is changed
> the copy will be created, doubling the storage requirements. Of course,
> this can be prevented by removing vector a after the assignment.
Hadley was correct: it is not prevented by removing 'a', as R does not
have reference counting. E.g.
rm(a)
b[1] <- 1
gc()
used (Mb) gc trigger (Mb) max used (Mb)
Ncells 133947 3.6 350000 9.4 350000 9.4
Vcells 10087573 77.0 21337085 162.8 20087562 153.3
Note the 'max used' Vcells.
There's a fairly complete explanation of what happens in the 'R Internals'
manual.
I think the most common source of confusion is over the term 'objects'. R
does not have 'objects' in this sense: 'a' and 'b' are symbols with
bindings to values. So you cannot change 'b', but you can change its
binding. When you do b[1] <- 1 you may create a new C-level structure as
the new value, or you may change the existing one. In this case it
created a new structure (by copying the old one and altering that).
Certain replacement functions are the only way to avoid making a new
value: a <- a+0 for example always creates a new value (at a different
address in memory) even though its contents will be identical.
Once we get away from the simplest vectors more sharing can be done: e.g.
character vectors with duplicate elements will share storage for those
elements.
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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