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
boolean1 is not a logical value, it is considered true if its value
is nonzero, and false if its value is zero. If boolean1 is an array,
it is considered true only if it is non-empty and all elements are
non-zero. If boolean1 evaluates to false, the result of the overall
expression is false. If it is true, the expression boolean2 is
evaluated in the same way as boolean1. If it is true, the result of
the overall expression is true. Otherwise the result of the overall
expression is false.
Warning: the one exception to the equivalence with evaluating
all (boolean1(:)) is when boolean1 an the empty array.
For MATLAB compatibility, the truth value of an empty array 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
boolean1 is not a logical value, it is considered true if its value
is nonzero, and false if its value is zero. If boolean1 is an array,
it is considered true only if it is non-empty and all elements are
non-zero. If boolean1 evaluates to true, the result of the overall
expression is true. If it is false, the expression boolean2 is
evaluated in the same way as boolean1. If it is true, the result of
the overall expression is true. Otherwise the result of the overall
expression is false.
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.
M = merge (mask, tval, fval) ¶M = 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.