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Re: some multi-dimensional matrix problem

From: Bart Vandewoestyne
Subject: Re: some multi-dimensional matrix problem
Date: Wed, 26 May 2004 02:53:13 -0500
User-agent: Mutt/1.3.28i

On Tue, May 25, 2004 at 04:03:37PM +0200, David Bateman wrote:
> All the functions squeeze and reshape do is change the dim_vector for
> the octave_value, without touching the underlying data which is stored
> essentially as a vector. So there is no transpose involved, and it is
> thus a low cost operation. If what you want is the transpose of this
> then you have no choice you must touch the underlying data, thus
> "transpose(squeeze(x))" is probably optimal.

Thanks for the tips, but still, my problem was not solved and it was
because of one exceptional case.  Let me explain a bit more...

I have M s-dimensional pointsets of length N, which i represent in a
multidimensional sxNxM matrix:

p = rand(s, N, M);

Now I want to evaluate these M pointsets for certain functions.  To
make things easy, suppose the function evaluation for an s-dimensional
pointset of length N is just taking the product along the dimension s for
each of the N points:

myprod = prod(p, 1);

As a result, i would like to have an MxN matrix, representing in each
row the function evaluated at N s-dimensional points.

Therefore, I THOUGHT could do:


But there was one exceptional case which gave me wrong output,
being the case when I only use 1 point (N=1), then I got my results as
1xM where i expected Mx1.

After some more experimenting, i *think* I've found that

transpose(reshape(myprod, N, M))

does what I want.  If i now enter a sx1xM dimensional pointset, i do get
my result as an Mx1 matrix.

I guess this is the best way to solve my 'problem', and i cannot avoid
taking the transpose?  Or does anybody see any better ways of doing

The test-script i used is:

%%%%%%%%%%%%%%%%%%%%%%%%%%% BEGIN SCRIPT %%%%%%%%%%%%%%%%%%%%%%%%%

function squeeze_test(s, N, M)

% Create M s-dimensional pointsets with each N points.
p = rand(s, N, M);

% Calculate the N function values at each of these M pointsets
myprod = prod(p, 1)

% Show the result as a MxN matrix.

% This goes wrong when N=1...

% This seems correct when N=1
transpose(reshape(myprod, N, M))

%%%%%%%%%%%%%%%%%%%%%%%%%%%%% END SCRIPT %%%%%%%%%%%%%%%%%%%%%%%%%%


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