Re: short circuit & and | in if and while

[John W. Eaton]

On 09/14/2017 05:11 AM, Ernst Reissner wrote:

> This explains also why the operators | and & in conditions in until
> statements are not shortcut,
>
> i.e. implicitly replaced by && and ||: because until does not exist in
> matlab.

The short-circuit behavior of | and & inside IF and WHILE conditions is 
not identical to the short-circuit behavior of || and &&.

With || and &&, the operands are evaluated and determined whether they 
are true or false.  In Octave, this means that if an operand is an array 
(for example) it is considered to true if all of the elements are 
non-zero.  In Matlab, I believe it is an error if the operands are not 
scalar values.

In the | and & expressions inside an IF or WHILE condition, short 
circuiting can only happen if the first argument is a scalar value.  It 
might seem that these are equivalent, at least in Matlab, but consider 
this case:

  if (1 | x) ...; end

vs.

  cond = 1 | x;
  if (cond) ...; end

Are these always equivalent?  Unfortunately no, because in the first 
case, the operator always short-circuits because the first operand is a 
scalar and is true, but in the second case, short-circuiting doesn't 
happen and if X is an empty matrix, then cond will also be an empty 
matrix and in the IF condition it is considered false.

It seems to me that because of this bizarre behavior, it's best to NEVER 
use | or & inside IF or WHILE conditions, and only use them as 
element-wise array operators.

Note that using || instead, there is no difference in behavior, so

  if (1 || x) ...; end

and

  cond = 1 || x;
  if (cond) ...; end

should always do the same thing, regardless of the value of X.

jwe