From matloff at cs.ucdavis.edu  Mon Apr  5 02:27:35 2010
From: matloff at cs.ucdavis.edu (Norm Matloff)
Date: Sun, 4 Apr 2010 17:27:35 -0700
Subject: [Rd] full copy on assignment?
In-Reply-To: <mailman.13.1270375206.14388.r-devel@r-project.org>
References: <mailman.13.1270375206.14388.r-devel@r-project.org>
Message-ID: <20100405002734.GA28962@laura>

Thanks very much.

By the way, I tried setting a GDB breakpoint at duplicate1(), with the
following:

  > x <- 1:10000000
  > x[3] <- 8
  > x[33] <- 88

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 <jmc at r-project.org>
> To: r-devel at 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.
...



[Rd] full copy on assignment?
Norm Matloff