Previous: Element-by-element Boolean Operators, Up: Boolean Expressions [Contents][Index]

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 (`

. If it is false, the result of the overall expression is 0. If it is true, the expression`boolean1`(:))`boolean2`is evaluated and converted to a scalar using the equivalent of the operation`all (`

. If it is true, the result of the overall expression is 1. Otherwise, the result of the overall expression is 0.`boolean2`(:))**Warning:**there is one exception to the rule of evaluating`all (`

, which is when`boolean1`(:))`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 (`

. If it is true, the result of the overall expression is 1. If it is false, the expression`boolean1`(:))`boolean2`is evaluated and converted to a scalar using the equivalent of the operation`all (`

. If it is true, the result of the overall expression is 1. Otherwise, the result of the overall expression is 0.`boolean2`(:))**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 also behaves the same way by default,
though the use of the ‘`&`’ and ‘`|`’ operators in this way is
strongly discouraged. 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.

- Built-in Function:
**merge***(*`mask`,`tval`,`fval`) - Built-in Function:
**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.

Previous: Element-by-element Boolean Operators, Up: Boolean Expressions [Contents][Index]