[R] Bug in colnames of data.frames?

[Peter Dalgaard]

Arne Henningsen <ahenningsen at email.uni-kiel.de> writes:

> Hi,
> 
> I am using R 1.9.1 on on i686 PC with SuSE Linux 9.0.
> 
> I have a data.frame, e.g.:
> 
> > myData <- data.frame( var1 = c( 1:4 ), var2 = c (5:8 ) )
> 
> If I add a new column by
> 
> > myData$var3 <- myData[ , "var1" ] + myData[ , "var2" ]
> 
> everything is fine, but if I omit the commas:
> 
> > myData$var4 <- myData[ "var1" ] + myData[ "var2" ]
> 
> the name shown above the 4th column is not "var4":
> 
> > myData
>   var1 var2 var3 var1
> 1    1    5    6    6
> 2    2    6    8    8
> 3    3    7   10   10
> 4    4    8   12   12
> 
> but names() and colnames() return the expected name:
> 
> > names( myData )
> [1] "var1" "var2" "var3" "var4"
> > colnames( myData )
> [1] "var1" "var2" "var3" "var4"
> 
> And it is even worse: I am not able to change the name shown above the 4th 
> column:
> > names( myData )[ 4 ] <- "var5"
> > myData
>   var1 var2 var3 var1
> 1    1    5    6    6
> 2    2    6    8    8
> 3    3    7   10   10
> 4    4    8   12   12
> 
> I guess that this is a bug, isn't it?

Nope:

> str(myData)
`data.frame':   4 obs. of  4 variables:
 $ var1: int  1 2 3 4
 $ var2: int  5 6 7 8
 $ var3: int  6 8 10 12
 $ var4:`data.frame':   4 obs. of  1 variable:
  ..$ var1: int  6 8 10 12

It's slightly peculiar, but if a column of a data frame is itself a
rectangular structure (data frame or matrix), then the "innermost" names
are used. Cf.

> myData[,"var4"] <- cbind(xyzzy=5:2)
> myData
  var1 var2 var3 xyzzy
1    1    5    6     5
2    2    6    8     4
3    3    7   10     3
4    4    8   12     2


Arguably, one might prefer

  var1 var2 var3  var4
                 xyzzy
1    1    5    6     5
2    2    6    8     4
3    3    7   10     3
4    4    8   12     2

or something like that, but it's hardly a bug.


-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907