[R] how to find index of maximum?
Prof Brian D Ripley
ripley at stats.ox.ac.uk
Mon Jul 12 17:42:45 CEST 1999
On Mon, 12 Jul 1999, Martin Maechler wrote:
> Peter> Dear R-users, Is there a simple way to find the index of the
> Peter> maximum of a vector?
> Peter meant:
> Something more efficient than
> x == max(x)
> which(x == max(x))
> match(max(x), x)
The first does not give what Peter asked for. I usually use
seq(along=x)[x == max(x)]
Also, watch out for duplicates, as match only gives the first one.
Now, one can be more efficient than seq(along=x)[x == max(x)], but you
need an enormous vector to need to do so. The usual problem is that
one wants to do this for each row of a matrix, and my library nnet
(VR5.x) or MASS (VR6.x) has a function in C to do that (and break ties).
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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