[R-sig-Geo] nearest distance in matrix

[White.Denis at epamail.epa.gov]

Yup.  Space complexity is O((nrow(mat)*ncol(mat))^2).


r-sig-geo-bounces at stat.math.ethz.ch wrote on 2004-07-14 10:49:14:

> White.Denis at epamail.epa.gov wrote:
> >
> >
> >
> > Here's another version that probably can be simplified:
> >
> > nmatdist <- function (m)
> > {
> >     v <- as.vector (m)
> >     ind <- as.matrix (expand.grid (seq (nrow (m)), seq (ncol (m))))
> >     d <- as.matrix (dist (ind, upper=TRUE))
> >     ones <- as.numeric (dimnames(ind[v == 1,])[[1]])
> >     matrix (sapply (seq(nrow(ind)),
> >         function (x) min (d[x, ones])), nrow=nrow(m))
> > }
>
>   Eeek! Have you tried that on a 100x100 matrix? Darn near killed my
> machine!
>
>   > mt=matrix(runif(10000)>.1,100,100)
>   > nmatdist(mt)
>    ... near swap death experience...
>
>   The code used by Adrian Baddeley's spatstat routine uses a very neat

> method for working out the distances, which involves sweeping along
rows
> and columns or something. He did explain it to me when I was in Perth
> but I can't recall it now!
>
>   Anyway, its super-quick and uses next-to-no memory. Here's how long
my
> function that calls the spatstat routine takes:
>
>   > unix.time(nmatdist(mt))
>   [1] 0.02 0.01 0.02 0.00 0.00
>
>   it was so quick I thought I'd check I'd not done it on a small
matrix
> by mistake:
>
>   > dim(mt)
>   [1] 100 100
>
> Nope!
>
> Baz
>
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