[R] distance between two matrices

[Roger Bivand]

On Tue, 27 Jan 2004, Sundar Dorai-Raj wrote:

> Hi all,
>    Say I have a matrix A with dimension m x 2 and matrix B with 
> dimension n x 2. I would like to find the row in A that is closest to 
> the each row in B. Here's an example (using a loop):
> 
> set.seed(1)
> A <- matrix(runif(12), 6, 2) # 6 x 2
> B <- matrix(runif(6), 3, 2)  # 3 x 2
> m <- vector("numeric", nrow(B))
> for(j in 1:nrow(B)) {
>    d <- (A[, 1] - B[j, 1])^2 + (A[, 2] - B[j, 2])^2
>    m[j] <- which.min(d)
> }
> 
> All I need is m[]. I would like to accomplish this without using the 
> loop if possible, since for my real data n > 140K and m > 1K. I hope 
> this makes sense.

I think you need a quadtree of the larger set of points, the do lookup for 
buckets of the smaller one. There is a good deal of information on

http://www.cs.umd.edu/~brabec/quadtree/

This isn't an answer within R, the functionality in the gstat contributed
package doesn't seem to be at the user level, but it does point to the
same site at UMD. 

Roger


> 
> Thanks,
> Sundar
> 
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-- 
Roger Bivand
Econonic Geography Section, Department of Economics, Norwegian School of 
Economics and Business Administration, Breiviksveien 40, N-5045 Bergen, 
Norway, voice: +47-55959355, fax: +47-55959393; Roger.Bivand at nhh.no