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

[Duncan Murdoch]

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)

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
> 
> ______________________________________________
> 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.
>