[R] Transformation of dissimilarity or distance matrix

[Prof Brian Ripley]

On Wed, 30 May 2001 Jan_Svatos at eurotel.cz wrote:

>
> Dear List,
>
> is there an elegant (or even not elegant) way how to transform
> dissimilarity or distance matrix A
> (or, in general, arbitrary symmetrical matrix) by transposition of rows and
> columns into a form
> closest to "block diagonal" matrix B?
> The matrix A is adjusted the following way
>
> A[A<epsilon] <-0 #(epsilon is given "small" number)
>
> B: (in its ideal form)
>
> b_{11}...b_{1i} 0...0
> ...
> b_{i1}...b_{ii}
> 0...0          b_{i+1, i+1}
>
> etc,
> with "reasonable" number of blocks.
> Dimensions of this problem: about 3000 rows (given) and about 30-45 blocks
> (expected).
>
> If there is some function for this task in "Matrix, multiv,..." packages,
> then RTFM is a perfectly good answer.

This makes no sense to me.  In your `approximation' most pairs of objects
are at zero distance from each other.  With dissimilarity or distance
matrices the entries are small within clusters and large between clusters,
and in particular the diagonal is zero.

For the reverse problem, with blocks of zeroes on the diagonal, any
reasonable clustering algorithm will given you a permutation with
the small entries near and on the diagonal.  For example, hclust in
package mva.  *However*, I doubt if anything in R will be very happy with
a distance matrix on 3000 rows unless you have a lot of memory.

-- 
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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