[datatable-help] setnames changes names of other data.table

[Arunkumar Srinivasan]

Thanks for reporting. That's expected behaviour. Use an explicit copy.

In short, when you do: DT1 <- DT2, there's no copy being made. They still reference/point to the same location (try doing tracemem(DT1) and tracemem(DT2)).

So when you change the names of one DT by reference, the other one will get changed as well - they're both pointing to the same location.

To overcome this, when you want to duplicate a DT, explicitly use copy. That is, DT1 <- copy(DT2). Now if you setnames(DT1, c("x", "y")), then DT2 names won't get changed.

I think there's a FR somewhere on documenting this… Thanks again for reporting (with nice example).


Arun
From: Holger Kirsten Holger Kirsten
Reply: Holger Kirsten hkirsten at imise.uni-leipzig.de
Date: January 11, 2014 at 3:19:02 PM
To: datatable-help at lists.r-forge.r-project.org datatable-help at lists.r-forge.r-project.org
Subject:  [datatable-help] setnames changes names of other data.table  
In a debugging session, I found that setnames changed the names of an identical data.table although having a different name>

> ############### using setnames()
> require(data.table)
> mytab = data.table(a = letters[1:4], b = 1:4 )
> str(mytab)
Classes ‘data.table’ and 'data.frame':    4 obs. of  2 variables:
 $ a: chr  "a" "b" "c" "d"
 $ b: int  1 2 3 4
 - attr(*, ".internal.selfref")=<externalptr>
> mytab
   a b
1: a 1
2: b 2
3: c 3
4: d 4
>
> othertab = mytab
> othertab
   a b
1: a 1
2: b 2
3: c 3
4: d 4
> setnames(othertab, c("a", "b"), c("aa","bb"))
> othertab
   aa bb
1:  a  1
2:  b  2
3:  c  3
4:  d  4
> mytab ## names have unexpectedly changed too
   aa bb
1:  a  1
2:  b  2
3:  c  3
4:  d  4
>
> ############### using names()
> mytab = data.table(a = letters[1:4], b = 1:4 )
> str(mytab)
Classes ‘data.table’ and 'data.frame':    4 obs. of  2 variables:
 $ a: chr  "a" "b" "c" "d"
 $ b: int  1 2 3 4
 - attr(*, ".internal.selfref")=<externalptr>
> mytab
   a b
1: a 1
2: b 2
3: c 3
4: d 4
>
> othertab = mytab
> othertab
   a b
1: a 1
2: b 2
3: c 3
4: d 4
> names(othertab) = c("aa","bb")
Warning message:
In `names<-.data.table`(`*tmp*`, value = c("aa", "bb")) :
  The names(x)<-value syntax copies the whole table. This is due to <- in R itself. Please change to setnames(x,old,new) which does not copy and is faster. See help('setnames'). You can safely ignore this warning if it is inconvenient to change right now. Setting options(warn=2) turns this warning into an error, so you can then use traceback() to find and change your names<- calls.
> othertab
   aa bb
1:  a  1
2:  b  2
3:  c  3
4:  d  4
> mytab ## names unchanged as expected
   a b
1: a 1
2: b 2
3: c 3
4: d 4
>
> sessionInfo()
R version 3.0.1 (2013-05-16)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252    LC_MONETARY=German_Germany.1252 LC_NUMERIC=C                    LC_TIME=German_Germany.1252   

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base    

other attached packages:
[1] data.table_1.8.10

loaded via a namespace (and not attached):
[1] tools_3.0.1
_______________________________________________  
datatable-help mailing list  
datatable-help at lists.r-forge.r-project.org  
https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.r-forge.r-project.org/pipermail/datatable-help/attachments/20140111/f44cc281/attachment.html>