[Rd] allocVector reference
Andrew Piskorski
atp at piskorski.com
Thu Mar 15 13:01:49 CET 2007
On Wed, Mar 14, 2007 at 05:44:04PM -0000, Matthew Dowle wrote:
>
> Hi,
>
> I'm trying to write a function to return an R vector which points
> directly to a contiguous subset of another vector, without taking a
> copy.
Matthew, I don't know the answer to your question, but this all seems
related to support for references in R. I've included my notes on R
references below.
Ah, I think Jens Oehlschlaegel's "ref" package is what you want:
http://tolstoy.newcastle.edu.au/R/packages/04/0008.html
Class refdata is a transparent wrapper to matrices and data.frames
which allows for memory efficient nested subsetting. I.e. you can
create a subset of a subset ... of a data.frame without duplicating
the data in memory, instead only indexing information is duplicated.
> Since SEXPREC is a header followed by the data, rather than the
> header plus a pointer to the data, I'm not sure what I'm trying to
> do is possible. Is there a way of doing this? Similar in spirit to
> how the R assignment "x=y" does not copy y until x is modified,
Is you last statement above in fact true? I was under the impression
that R does NOT do copy on write, that when you make a copy of a
variable, R immediately allocates memory and makes a deep copy of the
value.
But you're using old deprecated "=" for assignment, which is weird, so
maybe you mean when you pass named arguments to a function? Function
args are evaluated lazily, and I think that is used (I don't know how
exactly) to give copy on write behavior - but only for function
arguments.
My (older) notes on R references:
R does not support reference variables and does not do copy on write -
when you copy a variable, it eagerly allocates all memory and makes a
deep copy. (I believe S-Plus has the same behavior but have not
checked.) This can be annoying...
There are however specific exceptions. For example, R environments
are treated as references, NOT values like most everything else in R.
The July 2006 "attributes of environments" thread makes that clear:
https://stat.ethz.ch/pipermail/r-devel/2006-July/038352.html
Jens Oehlschlaegel's ref package implements references for both R and
S-Plus:
http://cran.r-project.org/src/contrib/Descriptions/ref.html
http://www.maths.lth.se/help/R/.R/library/ref/html/00Index.html
http://tolstoy.newcastle.edu.au/~rking/R/packages/04/0008.html
Henrik Bengtsson's R.oo package emulates reference variables via R
environments (but perhaps only in the context of his OO framework, I'm
not sure).
http://www.braju.com/R/
http://cran.r-project.org/src/contrib/Descriptions/R.oo.html
http://www.maths.lth.se/help/R/R.oo/
Bengtsson also wrote a 2002 paper, "Implementing support for
references in R":
http://www.maths.lth.se/help/R/ImplementingReferences/
Further out, see also Tierny's (mostly old) R development notes
Notes on References, External Objects, or Mutable State for R:
http://www.stat.uiowa.edu/~luke/R/references.html
Simple References with Finalization:
http://www.stat.uiowa.edu/~luke/R/simpleref.html
Finalization and Weak References in R:
http://www.stat.uiowa.edu/~luke/R/references/weakfinex.html
--
Andrew Piskorski <atp at piskorski.com>
http://www.piskorski.com/
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