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

From: <Richard.Cotton_at_hsl.gov.uk>
Date: Mon, 14 Apr 2008 12:50:54 +0100


> 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"

The (imho) unintuitive behaviour is to do with the subsetting function [.factor, not which. There are a couple of workarounds:

  1. Call factor to recreate the levels, and get rid of "A" factor(d$group)
  2. Redefine [.factor; see dropUnusedLevels in the Hmisc package.

Regards,
Richie.

Mathematical Sciences Unit
HSL



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