[R] about the function order()
Prof Brian Ripley
ripley at stats.ox.ac.uk
Tue Nov 27 15:41:27 CET 2001
On Tue, 27 Nov 2001, Emmanuel Charpentier wrote:
> According to Thomas Lumley :
> >In a sense it's doing the opposite of what you thought.
> >The definition of order() is basically that
> > a[order(a)]
> >is in increasing order. This works with your example, where the correct
> >order is the fourth, second, first, then third element.
> >You may have been looking for rank(), which returns the rank of the
> />R> a <- c(4.1, 3.2, 6.1, 3.1) /
> />R> order(a) /
> > 4 2 1 3
> />R> rank(a) /
> > 3 2 4 1
> >so rank() tells you what order the numbers are in, order() tells you how
> >to get them in ascending order.
> Hmmm ... meaning that order behaves like the (gradeup) APL function,
> right ? (Yes, I'm *that* old ...).
> That should imply that, barring possible ties, rank(x) ==
> order(order(x)). Right ?
> So : why distinc implementations ? Are there efficiency considerations
> I'm missing ?
Well, ties, as you mentioned. rank averages ties.
Also, porting S code needs both, as S has both. We've even added
sort.list to facilitate that. (sort.list in S is a lighter-weight version
of order for just one vector as used here.)
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 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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