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

From: Uwe Ligges <ligges_at_statistik.tu-dortmund.de>
Date: Mon, 14 Apr 2008 13:39:53 +0200

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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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Mon 14 Apr 2008 - 13:39:03 GMT

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