Instable evaluation of combined logical expressions

[John W. Eaton]

On 13-Feb-2009, Jörg Bretschneider wrote:

| Bug report for Octave 3.0.3 configured for i686-pc-msdosmsvc
| 
| Description:
| -----------
| 
| Octave seems to evaluate all expressions in combined logical condition
| expressions,
| i.e.    cond1 LOGICALOPERATOR cond2 (LOGICALOPERATOR ...)
| even if the logical result of the condition is already determined by the
| first condition
| An error occurs if the syntactical validity of the second or further
| conditions depends
| on a FALSE value of the first condition, e.g. an index request into an
| empty matrix.
| It is not unusual to check the length of an array first in order to
| request a condition
| on some of its elements. The example works with Matlab for years, but
| not Octave 3.0.x
| 
| Repeat-By:
| ---------
| 
|   x=[];   if ~isempty(x)   &   x(1) ~= 0,   disp('do something'),   end
| 
| Fix:
| ---
| 	A work around for this was to pose multiple separate IF conditions,
| 	but this is not the preferred way, as octave can easily understand
| 	that an AND expression is FALSE if its first condition is, as well
| 	as an OR expression is TRUE if its first condition is, there may be
| 	other instances but the first one is the most important case.
| 	I had to adapt hundreds of lines of M-code to make it compatible.

We consider the way that | and & short-circuit to be a bug in Matlab,
because they only short-circuit in some instances (IF and WHILE
conditions) and the behavior is different if you do

  t = a & b;
  if (t) ...

vs

  if (a & b) ...

A better solution for you is to use the && and || operators which
always do short-circuiting.

jwe