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8.5.2 Shortcircuit Boolean Operators
Combined with the implicit conversion to scalar values in if
and
while
conditions, Octave's elementbyelement 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
shortcircuit 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 operationall (boolean1(:))
. 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 whenboolean1
is the empty matrix. The truth value of an empty matrix is alwaysfalse
so[] && true
evaluates tofalse
even thoughall ([])
istrue
. 
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 operationall (boolean1(:))
. 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
shortcircuit 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 ‘&’.
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