From owner-bug-octave at bevo dot che dot wisc dot edu  Fri Dec  1 14:55:42 1995
Subject: Mathlab-compatibility
From: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
To: Michael Boss <bossm at wsr dot ac dot at>
Cc: bug-octave at bevo dot che dot wisc dot edu
Date: Fri, 1 Dec 1995 14:55:40 -0600

Michael Boss <bossm at wsr dot ac dot at> wrote:

: It seems to me that octave is incompatible with matlab in the following
: way:
: 
: In Matlab:
: 
: x=[x 1 2 3]
: 
: results in
: 
: x=[1 2 3]
: 
: In Octave:
: 
: x=[x 1 2 3]
: 
: results in
: 
: x undefined

I can see no good reason for allowing previously undefined variables
to be ignored, even in this context.  I suspect that for assignments,
Matlab is first initializing the LHS of the expression to `[]' and
then proceeding.  For example, Matlab complains if you try

  x = [y, 1, 2, 3];

but not for

  y = [];
  x = [y, 1, 2, 3];

Further evidence is that Matlab also does this:

  >> x
  ??? Undefined function or variable x.

but allows

  >> x = x

to set x to an empty matrix.  This looks more like a bug to me than a
feature worth imitating.

I think the best solution is to never reference undefined variables
in your programs.

A solution to your problem would be to set `empty_list_elements_ok' to
"true" and then initialize x to an empty matrix before trying to use
it.

jwe