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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 (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 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 (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 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 ‘&’.


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