[R] [External] Re: Why is R making a copy-on-modification after using str?

[iuke-tier@ey m@iii@g oii uiow@@edu]

On Mon, 30 Nov 2020, Duncan Murdoch wrote:

> On 30/11/2020 5:12 a.m., Georg Kindermann wrote:
>> Dear list members,
>> 
>> I was wondering why R is making a copy-on-modification after using str.
>
> This isn't really an explanation, but adds a bit more data.  If you inspect m 
> before and after str(), you'll see that str(m) leaves it with two references:
>
>> m <- matrix(1:12, 3)
>>   .Internal(inspect(m))
> @7fcb0ad682c8 13 INTSXP g0c4 [REF(1),ATT] (len=12, tl=0) 1,2,3,4,5,...
> ATTRIB:
>  @7fcaf5f7b8a0 02 LISTSXP g0c0 [REF(1)]
>    TAG: @7fcafd091a80 01 SYMSXP g1c0 [MARK,REF(65535),LCK,gp=0x6000] "dim" 
> (has value)
>    @7fcb0afecc80 13 INTSXP g0c1 [REF(65535)] (len=2, tl=0) 3,4
>>   str(m)
> int [1:3, 1:4] 1 2 3 4 5 6 7 8 9 10 ...
>>   .Internal(inspect(m))
> @7fcb0ad682c8 13 INTSXP g0c4 [REF(2),ATT] (len=12, tl=0) 1,2,3,4,5,...
> ATTRIB:
>  @7fcaf5f7b8a0 02 LISTSXP g0c0 [REF(1)]
>    TAG: @7fcafd091a80 01 SYMSXP g1c0 [MARK,REF(65535),LCK,gp=0x6000] "dim" 
> (has value)
>    @7fcb0afecc80 13 INTSXP g0c1 [REF(65535)] (len=2, tl=0) 3,4
>
>
> It's not just str():  this sequence also does it.
>
>  m <- matrix(1:12, 3)
>  debug(mean)
>  mean(m)

This is due to the restarts needed by do_browser() creating links to
the environment that could be cleaned up in endcontext. There is a
runtime cost to doing so, but probably worth looking into.

Best,

luke

>
> Hit c a couple of times now to get out of debugging, and m is left with two 
> references to it.  I don't see that if mean isn't being debugged.
>
> Duncan Murdoch
>
>
>> 
>> m <- matrix(1:12, 3)
>> tracemem(m)
>> #[1] "<0x559df861af28>"
>> dim(m) <- 4:3
>> m[1,1] <- 0L
>> m[] <- 12:1
>> str(m)
>> # int [1:4, 1:3] 12 11 10 9 8 7 6 5 4 3 ...
>> dim(m) <- 3:4  #Here after str a copy is made
>> #tracemem[0x559df861af28 -> 0x559df838e4a8]:
>> dim(m) <- 3:4
>> str(m)
>> # int [1:3, 1:4] 12 11 10 9 8 7 6 5 4 3 ...
>> dim(m) <- 3:4 #Here again after str a copy
>> #tracemem[0x559df838e4a8 -> 0x559df82c9d78]:
>> 
>> Also I was wondering why a copy is made when having a Task Callback.
>> 
>> TCB <- addTaskCallback(function(...) TRUE)
>> m <- matrix(1:12, nrow = 3)
>> tracemem(m)
>> #[1] "<0x559dfa79def8>"
>> dim(m) <- 4:3  #Copy on modification
>> #tracemem[0x559dfa79def8 -> 0x559dfa8998e8]:
>> removeTaskCallback(TCB)
>> #[1] TRUE
>> dim(m) <- 4:3  #No copy
>> 
>> I am using R version 4.0.3.
>> 
>> Kind regards,
>> Georg
>> 
>> ______________________________________________
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>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>> 
>
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
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
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