# Re: [Rd] order of levels in interaction()

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Thu 30 Nov 2006 - 21:48:35 GMT

On Thu, 30 Nov 2006, Deepayan Sarkar wrote:

> Hi,
>
> ?interaction says:
>
>
> 'factor'; ':' where 'f:g' is the similar to 'interaction(f, g,
> sep=":")' when 'f' and 'g' are factors.
>
> This is not quite true (or maybe 'similar' is the operative word), since:
>
>> f <- factor(1:3); g <- factor(letters[1:3])
>> levels(f:g)
> [1] "1:a" "1:b" "1:c" "2:a" "2:b" "2:c" "3:a" "3:b" "3:c"

>> levels(interaction(f, g, sep = ":"))
> [1] "1:a" "2:a" "3:a" "1:b" "2:b" "3:b" "1:c" "2:c" "3:c"
>
> Note the different order of the levels. I like the order in f:g better
> (and at least one other person agrees [1] ), but I also want the
> flexibility of having a 'sep' argument. Would it be possible to change
> interaction to allow this? A simple fix that makes interaction behave
> like ":" is to replace

No, for it also says

```      The levels are ordered so the level of the first factor varies
fastest, then the second and so on.  This is the reverse of
lexicographic ordering, and differs from ':'.  (It is done this
way for compatibility with S.)

```

and reversing it would break things. (It does, I tried it a while back.)

> as.vector(outer(l, lvs, paste, sep = sep))
>
> with
>
> as.vector(t(outer(l, lvs, paste, sep = sep)))
>
> in interaction.
>
> -Deepayan
>
> [1] http://tolstoy.newcastle.edu.au/R/help/06/08/32638.html
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

```--
Brian D. Ripley,                  ripley@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

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Received on Fri Dec 01 20:04:59 2006

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