[R] multivariate distance and apply
Prof Brian Ripley
ripley at stats.ox.ac.uk
Mon Jul 23 18:17:29 CEST 2001
On Mon, 23 Jul 2001, Agustin Lobo wrote:
> I'm computing a mutivariate distance that is
> not among the different options of dist. Hence
> I've written
> where v1 and v2 are vectors.
> Now, I have to calculate the distances between
> one given vector (V1) and a number of vectors
> arranged as the rows of a matrix (stat). I'm
> apply(stat, 1, midistancia, V1)
> to apply midstancia to each row of stat.
> The question is: is this the most efficient method
> in R or is there a faster way to calculate midistancia
> between V1 and the rows of stat?
> (stat can become ~80000 rows x 7 cols)
It entirely depends on what the distance actually is and if midistancia()
could be vectorized. apply() in R just runs a for loop, and using for()
directly is probably slightly more efficient when the result (as here) is
a simple vector.
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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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