(octave.info)Short-circuit Boolean Operators

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8.5.2 Short-circuit Boolean Operators
-------------------------------------

Combined with the implicit conversion to scalar values in ‘if’ and
‘while’ conditions, Octave’s element-by-element boolean operators are
often sufficient for performing most logical operations.  However, it is
sometimes desirable to stop evaluating a boolean expression as soon as
the overall truth value can be determined.  Octave’s “short-circuit”
boolean operators work this way.

‘BOOLEAN1 && BOOLEAN2’
     The expression BOOLEAN1 is evaluated and converted to a scalar
     using the equivalent of the operation ‘all (BOOLEAN1(:))’.  If it
     is false, the result of the overall expression is 0.  If it is
     true, the expression BOOLEAN2 is evaluated and converted to a
     scalar using the equivalent of the operation ‘all (BOOLEAN2(:))’.
     If it is true, the result of the overall expression is 1.
     Otherwise, the result of the overall expression is 0.

     *Warning:* there is one exception to the rule of evaluating ‘all
     (BOOLEAN1(:))’, which is when ‘boolean1’ is the empty matrix.  The
     truth value of an empty matrix is always ‘false’ so ‘[] && true’
     evaluates to ‘false’ even though ‘all ([])’ is ‘true’.

‘BOOLEAN1 || BOOLEAN2’
     The expression BOOLEAN1 is evaluated and converted to a scalar
     using the equivalent of the operation ‘all (BOOLEAN1(:))’.  If it
     is true, the result of the overall expression is 1.  If it is
     false, the expression BOOLEAN2 is evaluated and converted to a
     scalar using the equivalent of the operation ‘all (BOOLEAN2(:))’.
     If it is true, the result of the overall expression is 1.
     Otherwise, the result of the overall expression is 0.

     *Warning:* the truth value of an empty matrix is always ‘false’,
     see the previous list item for details.

   The fact that both operands may not be evaluated before determining
the overall truth value of the expression can be important.  For
example, in the expression

     a && b++

the value of the variable B is only incremented if the variable A is
nonzero.

   This can be used to write somewhat more concise code.  For example,
it is possible write

     function f (a, b, c)
       if (nargin > 2 && ischar (c))
         ...

instead of having to use two ‘if’ statements to avoid attempting to
evaluate an argument that doesn’t exist.  For example, without the
short-circuit feature, it would be necessary to write

     function f (a, b, c)
       if (nargin > 2)
         if (ischar (c))
           ...

Writing

     function f (a, b, c)
       if (nargin > 2 & ischar (c))
         ...

would result in an error if ‘f’ were called with one or two arguments
because Octave would be forced to try to evaluate both of the operands
for the operator ‘&’.

   MATLAB has special behavior that allows the operators ‘&’ and ‘|’ to
short-circuit when used in the truth expression for ‘if’ and ‘while’
statements.  Octave behaves the same way for compatibility, however, the
use of the ‘&’ and ‘|’ operators in this way is strongly discouraged and
a warning will be issued.  Instead, you should use the ‘&&’ and ‘||’
operators that always have short-circuit behavior.

   Finally, the ternary operator (?:) is not supported in Octave.  If
short-circuiting is not important, it can be replaced by the ‘ifelse’
function.

 -- : merge (MASK, TVAL, FVAL)
 -- : ifelse (MASK, TVAL, FVAL)
     Merge elements of TRUE_VAL and FALSE_VAL, depending on the value of
     MASK.

     If MASK is a logical scalar, the other two arguments can be
     arbitrary values.  Otherwise, MASK must be a logical array, and
     TVAL, FVAL should be arrays of matching class, or cell arrays.  In
     the scalar mask case, TVAL is returned if MASK is true, otherwise
     FVAL is returned.

     In the array mask case, both TVAL and FVAL must be either scalars
     or arrays with dimensions equal to MASK.  The result is constructed
     as follows:

          result(mask) = tval(mask);
          result(! mask) = fval(! mask);

     MASK can also be arbitrary numeric type, in which case it is first
     converted to logical.

     See also: Note: logical, Note: diff.