[Rd] Extra copies of objects in environments when using $ operator?

[luke-tierney at uiowa.edu]

On Fri, 5 Aug 2016, Winston Chang wrote:

> My understanding is that R will not make copies of lists if there is
> only one reference to the object. However, I've encountered a case
> where R does make copies, even though (I think) there should be only
> one reference to the object. I hope that someone could shed some light
> on why this is happening.
>
> I'll start with a simple example. Below, x is a list with one element,
> and changing that element doesn't result in a copy. (We know this
> because nothing is printed when we do the assignment after the
> tracemem call.) This is as expected.
>  x <- list(1)
>  tracemem(x)
>  # [1] "<0x1149e08f8>"
>  x[[1]] <- 2
>  # (No output)
>
> Similarly, modifying a list contained in a list doesn't result in a copy:
>  e <- list(x = list(1))
>  tracemem(e$x)
>  # [1] "<0x11b3a4b38>"
>  e$x[[1]] <- 2
>  # (No output)
>
> However, modifying a list contained in an environment *does* result in
> a copy -- tracemem prints out some info when we do the assignment:
>  e <- new.env(parent = emptyenv())
>  e$x <- list(1)
>  tracemem(e$x)
>  # [1] "<0x1148c1708>"
>  e$x[[1]] <- 2
>  # tracemem[0x1148c1708 -> 0x11b2fc1b8]:

Currently e$x marks values as immutable if they have any references by
setting NAMED to 2. You can see this with

> e <- new.env(parent = emptyenv())
> e$x <- list(1)
> .Internal(inspect(e))
@30b2498 04 ENVSXP g0c0 [NAM(1)] <0x30b2498>
ENCLOS:
   @2600e98 04 ENVSXP g0c0 [MARK,NAM(2)] <R_EmptyEnv>
HASHTAB:
   @2e41540 19 VECSXP g0c7 [] (len=29, tl=1)
     @25c9628 00 NILSXP g0c0 [MARK,NAM(2)]
     @25c9628 00 NILSXP g0c0 [MARK,NAM(2)]
     @25c9628 00 NILSXP g0c0 [MARK,NAM(2)]
     @25c9628 00 NILSXP g0c0 [MARK,NAM(2)]
     @30b3370 02 LISTSXP g0c0 []
       TAG: @2637870 01 SYMSXP g0c0 [MARK,NAM(2)] "x"
       @3569488 19 VECSXP g0c1 [NAM(1)] (len=1, tl=0)   ## <--- NAM = 1
       @35694e8 14 REALSXP g0c1 [NAM(2)] (len=1, tl=0) 1
     ...
> e$x
[[1]]
[1] 1

> .Internal(inspect(e))
@30b2498 04 ENVSXP g0c0 [NAM(1)] <0x30b2498>
ENCLOS:
   @2600e98 04 ENVSXP g0c0 [MARK,NAM(2)] <R_EmptyEnv>
HASHTAB:
   @2e41540 19 VECSXP g0c7 [] (len=29, tl=1)
     @25c9628 00 NILSXP g0c0 [MARK,NAM(2)]
     @25c9628 00 NILSXP g0c0 [MARK,NAM(2)]
     @25c9628 00 NILSXP g0c0 [MARK,NAM(2)]
     @25c9628 00 NILSXP g0c0 [MARK,NAM(2)]
     @30b3370 02 LISTSXP g0c0 []
       TAG: @2637870 01 SYMSXP g0c0 [MARK,NAM(2)] "x"
       @3569488 19 VECSXP g0c1 [NAM(2)] (len=1, tl=0)   ## <--- NAM = 2
       @35694e8 14 REALSXP g0c1 [NAM(2)] (len=1, tl=0) 1
     ...

It is not clear if this is needed or just done in an abundance of
caution. If R is built to use reference counting for determining
sharing information this does not happen, so this is likely to change
and not force a copy by 3.4.0.

Best,

luke

> This is surprising to me. Why is a copy made in this case? It also
> results in slower performance for these situations.
>
> The most that I've been able to figure out is that it probably has
> something to do with how the $ operator works with environments (but
> not with lists). If you do the same operations without the $ operator,
> by evaluating code in environment e, then no copy is made:
>
>  e <- new.env(parent = globalenv())
>  eval(quote({
>    x <- list(1)
>    tracemem(x)
>    x[[1]] <- 2
>  }), envir = e)
>  # (No output)
>
>
> I'd appreciate it if someone could shed light on this. And if it's a
> bug, that would be good to know too.
>
> -Winston
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu