[R] bug in interaction order when using drop?
Petr Pikal
petr.pikal at precheza.cz
Fri Aug 11 14:07:44 CEST 2006
On 11 Aug 2006 at 12:31, Prof Brian Ripley wrote:
Date sent: Fri, 11 Aug 2006 12:31:55 +0100 (BST)
From: Prof Brian Ripley <ripley at stats.ox.ac.uk>
To: Petr Pikal <petr.pikal at precheza.cz>
Copies to: r-help at stat.math.ethz.ch
Subject: Re: [R] bug in interaction order when using drop?
> On Thu, 10 Aug 2006, Petr Pikal wrote:
>
> > Ooops, my first suggestion reorders factor itself but
> >
> > if (drop) factor(ans) else ans
> >
> > instead of whole drop construction shall preserve levels order
> > without changing order of factor
>
> Even easier would be to return ans[,drop=drop]. It seems to me that
> there is an argument for expecting interaction(..., drop=TRUE) to give
> the same result as interaction(...)[,drop=TRUE], but little argument
> that any ordering is a *bug*.
Maybe bug was an *exaggeration*, but what surprised me was different
order in using interaction with and without drop. Well, I would call
it not consistent behaviour as omitting unused levels silently change
an order of factor levels.
> set.seed(1)
> DF<-data.frame(x=sample(LETTERS[1:3],10, replace=T),
y=sample(letters[1:3],10, replace=T))
> interaction(DF$x,DF$y)
[1] A.a B.a B.c C.b A.c C.b C.c B.c B.b A.c
Levels: A.a B.a C.a A.b B.b C.b A.c B.c C.c
Here is neat ordering, however as you said first level varying
fastest.
> interaction(DF$x,DF$y, drop=T)
[1] A.a B.a B.c C.b A.c C.b C.c B.c B.b A.c
Levels: A.a B.a B.c C.b A.c C.c B.b
This seems to me chaotic, but I will be glad if you explain to me
some rational pattern in it.
> my.int(DF$x,DF$y,drop=T) # changed as suggested
[1] A.a B.a B.c C.b A.c C.b C.c B.c B.b A.c
Levels: A.a B.a B.b C.b A.c B.c C.c
>
Same ordering as without drop, with unused levels omitted.
Best regards.
Petr Pikal
>
> The order of the levels of a factor are arbitrary, and in fact they
> seem to me to be in a strange order, with the levels of the first
> factor varying fastest (reverse lexiographic order).
>
> > levels(interaction(c("A", "A", "B"), letters[1:3]))
> [1] "A.a" "B.a" "A.b" "B.b" "A.c" "B.c"
>
> so the existing
>
> > levels(interaction(c("A", "A", "B"), letters[1:3], drop=T))
> [1] "A.a" "A.b" "B.c"
>
> looks more sensible in this case.
>
> >
> > Petr
> >
> > On 10 Aug 2006 at 16:32, Petr Pikal wrote:
> >
> > From: "Petr Pikal" <petr.pikal at precheza.cz>
> > To: r-help at stat.math.ethz.ch
> > Date sent: Thu, 10 Aug 2006 16:32:54 +0200
> > Priority: normal
> > Subject: [R] bug in interaction order when using drop?
<snip>
> >
> > ______________________________________________
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> > PLEASE do read the posting guide
> > http://www.R-project.org/posting-guide.html and provide commented,
> > minimal, self-contained, reproducible code.
> >
>
> --
> Brian D. Ripley, ripley at stats.ox.ac.uk
> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> University of Oxford, Tel: +44 1865 272861 (self) 1 South
> Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG,
> UK Fax: +44 1865 272595
Petr Pikal
petr.pikal at precheza.cz
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