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

[Holger Kirsten]

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Thanks for the immediate answer! Now, everything is clear for me.
 


Arunkumar Srinivasan <aragorn168b at gmail.com> , 11.01.2014 15:31:


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
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