order of levels in interaction()

Hi,

?interaction says:

See Also:

    '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

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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
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order of levels in interaction()

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