On Thu, 16 Nov 2006, Hong Ooi wrote: > Not sure if this counts as a bug or not, but I just noticed in R 2.4.0 > that : and interaction are not quite equivalent. For example:
Well, that depends on what you mean by 'equivalent', a word that is not used in the help you quote but is used in help(":"). If you view a factor as an enumeration type, they are equivalent. This difference seems to be long-standing (it was there in R 2.0.0), and I'd say it was unclear documentation which I will amend. (As a historical note, interaction() behaves the same as S, whereas ':' in S does not have this alternative meaning but had been that way in R for many years.) > >> x <- factor(letters[1:4]) >> y <- factor(letters[11:14]) > >> x:y > [1] a:k b:l c:m d:n > Levels: a:k a:l a:m a:n b:k b:l b:m b:n c:k c:l c:m c:n d:k d:l d:m d:n > >> interaction(x, y, sep=":") > [1] a:k b:l c:m d:n > Levels: a:k b:k c:k d:k a:l b:l c:l d:l a:m b:m c:m d:m a:n b:n c:n d:n > > The ordering of the levels is different between the two, although > ?interaction says > > > f:g is the same as interaction(f,g, sep=":") when f and g are > factors. > > I came across this when fitting multinom models, and the column order of > the predicted probabilities shifted around depending on whether I used > interaction or : on the LHS of the formula. Which seems fine, as it is the labels and not the order which mattered. -- Brian D. Ripley, [EMAIL PROTECTED] 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 ______________________________________________ R-help@stat.math.ethz.ch mailing list 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.