From maintainers-request at octave dot org Thu Feb 10 08:56:02 2005
Subject: Re: Moving code from octave-forge to octave [Was: polyderiv problem?]
From: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
To: Michael Creel <michael dot creel at uab dot es>
Cc: maintainers at octave dot org, Paul Kienzle <pkienzle@users.sourceforge.net>,         Joe Koski <jkoski11@comcast.net>
Date: Thu, 10 Feb 2005 10:00:42 -0500

On  9-Feb-2005, Michael Creel <michael dot creel at uab dot es> wrote:

| On Wednesday 09 February 2005 16:50, David Bateman wrote:
| > The only
| > remaining patch to consider is M-v-ops-2.1.31.patch that allows the
| > extension of ./, .*, etc to allow Matrix by vector arguments.
|
| I sent this message to David, but I meant to send it to all, so here it is:
| 
| This is a behavior that I have found very useful in languages like Gauss and 
| Ox.

Do these languages have special syntax for these operations?

Although it has been discussed before, and I have probably even
suggested implementing it in the past, I now hesitate to overload
operators like .* for this purpose because (as David mentioned) it
means that a dimensioning error can lead to bad results with no
warning.  As another example, you might mean to multiply elements of
M1 by elements of M2, but M2 accidentally becomes a vector, so
suddenly you are scaling columns instead of seeing an error about
non-conforming operands.  That seems bad to me.  But it would be
useful to have this capability implemented efficiently, even if we do
not have special operators for it.

| Maybe it would be possible to allow it only for matrices and vectors, but 
| not NDArrays?

There is probably some way to generalize to N-d arrays when the
dimension of one operand is one less than the dimension of the other.

| Or maybe a function would do the job. I attach a quck hack for 
| consideration. Michael
| m_x_v.m
| function result = m_x_v(m,v)
| 	if !ismatrix(m)
| 		error("m_x_v: first arg must be a matrix");
| 	endif
| 	if !isvector(v)
| 		error("m_x_v: second arg must be a vector");
| 	endif
| 
| 	[rm, cm] = size(m);
| 	[rv, cv] = size(v);
| 	
| 	if (rm == rv)
| 		v = kron(v, ones(1,cm));
| 		result = m .* v;
| 	elseif (cm == cv)
| 		v = kron(v, ones(rm, 1));	
| 		result = m .* v;
| 	else
| 		error("m_x_v: dimension of vector must match one of the dimensions of the matr
| ix");	
| 	endif
| endfunction

This kind of approach will work, but it will use a lot of memory, so
it would be better to do this in a built-in function where we could
use loops and not sacrifice speed or memory efficiency.

jwe