AW: [R] constructing specially ordered factor
Prof Brian Ripley
ripley at stats.ox.ac.uk
Mon Oct 4 15:41:19 CEST 2004
On Mon, 4 Oct 2004, Khamenia, Valery wrote:
> Hi Dimitris,
> thank you for your reply,
> > does the following work in your data:
> > levs <- unique.default(Names)
> > factor(Names, levs[order(unique.default(Weights))])
> your solution is really shorter, but has two issues to be
> meant here:
> 1. "unique.default" is applied twice, what might
> be a bit expensive for strings.
> 2. your solution brings an implicit prerequisit on
> "unique.default". Indeed, "unique.default" should
> preserve the same order for output both working
> with strings and with numbers. In other words,
> correspondence must be kept by "unique.default".
> If "unique.default" implementation uses (or will use)
> lexicographic strings sorting for acceleration
> then this approach fails.
Please follow the posting guide and do your homework before posting, in
this case the help page for unique.default.
'unique' returns a vector, data frame or array like 'x' but with
duplicate elements removed.
An object of the same type of 'x'. but if an element is equal to
one with a smaller index, it is removed.
so the order is preserved, by definition.
BTW it uses hashing for `acceleration', not something as slow as sorting.
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
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