[R] Is this an artifact of using "which"?

[Uwe Ligges]

Tania Oh wrote:
> Dear all,
> 
> I used "which" to obtain a subset of values from my data.frame.  
> however, I find that there is a "trace" of the values I  have removed.  
> Any suggestions would be greatly appreciate.
> 
> Below is my data:
> 
> d <- data.frame( val   = 1:10,
>                  group = sample(LETTERS[1:5], 10, repl=TRUE) )
> 
>  >d
>     val group
> 1    1     B
> 2    2     E
> 3    3     B
> 4    4     C
> 5    5     A
> 6    6     B
> 7    7     A
> 8    8     E
> 9    9     E
> 10  10     A
> 
> ## selecting everything that is not group "A"
>   d<-d[which(d$group !="A"),]
> 
>  > d
>    val group
> 1   1     B
> 2   2     E
> 3   3     B
> 4   4     C
> 6   6     B
> 8   8     E
> 9   9     E
> 
>  > levels(d$group)
> [1] "A" "B" "C" "E"
> 
> ## why is group A still reflected here?


Because you have removed elements from a factor objects that has 
particular levels. You remove elements (=observations), but the factor 
still knows that all levels are possible (stired in attributes of the 
object).

If you want to remove all levels without corresponding observations, use 
explicit drop=TRUE as the help page suggests, e.g.:


d <- d[d$group != "A", ]
d$group <- d$group[ , drop = TRUE]

Uwe Ligges



> Many thanks in advance,
> tania
> 
> D.phil student
> Department of Physiology, Anatomy and Genetics
> Oxford University
> 
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