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Re: Efficient multiplication by a diagonal matrix

From: John W. Eaton
Subject: Re: Efficient multiplication by a diagonal matrix
Date: Wed, 13 Nov 1996 00:18:11 -0600

On 13-Nov-1996, Ted Harding <address@hidden> wrote:

: A good while ago I wrote to John Eaton suggesting an extension of the
: octave ".*" multiplication operator so that, if A (m x n) is a matrix,
: u (m x 1) and v (1 x n) are vectors, then the following could be written:


: John reacted favourably at the time, but I have heard no more since.

I do want to add this feature.  Someone has even offered patches.  The
problem is mostly lack of time on my part, though I expect that
eventually Octave will have this feature.

: I certainly have frequent call for such operations: in Statistics,

I also found just yesterday that I could have used this feature, but
for a different operator.  What I wanted to do was subtract the mean
of each column of a matrix from the elements of the corresponding
column.  What I ended up doing was

  x - ones (rows (x), :) * mean (x)

but it would have been much simpler to be able to just write

  x .- mean (x)


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