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<h4 class="subsection">8.5.2 Short-circuit Boolean Operators</h4>
<p><a name="index-short_002dcircuit-evaluation-652"></a>
Combined with the implicit conversion to scalar values in <code>if</code> and
<code>while</code> 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
<dfn>short-circuit</dfn> boolean operators work this way.
<dl>
<dt><var>boolean1</var><code> && </code><var>boolean2</var><dd><a name="index-g_t_0026_0026-653"></a>The expression <var>boolean1</var> is evaluated and converted to a scalar
using the equivalent of the operation <code>all (</code><var>boolean1</var><code>(:))</code>.
If it is false, the result of the overall expression is 0. If it is
true, the expression <var>boolean2</var> is evaluated and converted to a
scalar using the equivalent of the operation <code>all
(</code><var>boolean1</var><code>(:))</code>. If it is true, the result of the overall expression
is 1. Otherwise, the result of the overall expression is 0.
<p><strong>Warning:</strong> there is one exception to the rule of evaluating
<code>all (</code><var>boolean1</var><code>(:))</code>, which is when <code>boolean1</code> is the
empty matrix. The truth value of an empty matrix is always <code>false</code>
so <code>[] && true</code> evaluates to <code>false</code> even though
<code>all ([])</code> is <code>true</code>.
<br><dt><var>boolean1</var><code> || </code><var>boolean2</var><dd><a name="index-g_t_007c_007c-654"></a>The expression <var>boolean1</var> is evaluated and converted to a scalar
using the equivalent of the operation <code>all (</code><var>boolean1</var><code>(:))</code>.
If it is true, the result of the overall expression is 1. If it is
false, the expression <var>boolean2</var> is evaluated and converted to a
scalar using the equivalent of the operation <code>all
(</code><var>boolean1</var><code>(:))</code>. If it is true, the result of the overall expression
is 1. Otherwise, the result of the overall expression is 0.
<p><strong>Warning:</strong> the truth value of an empty matrix is always <code>false</code>,
see the previous list item for details.
</dl>
<p>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
<pre class="example"> a && b++
</pre>
<p class="noindent">the value of the variable <var>b</var> is only incremented if the variable
<var>a</var> is nonzero.
<p>This can be used to write somewhat more concise code. For example, it
is possible write
<pre class="example"> function f (a, b, c)
if (nargin > 2 && ischar (c))
...
</pre>
<p class="noindent">instead of having to use two <code>if</code> 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
<pre class="example"> function f (a, b, c)
if (nargin > 2)
if (ischar (c))
...
</pre>
<p class="noindent">Writing
<pre class="example"> function f (a, b, c)
if (nargin > 2 & ischar (c))
...
</pre>
<p class="noindent">would result in an error if <code>f</code> were called with one or two
arguments because Octave would be forced to try to evaluate both of the
operands for the operator ‘<samp><span class="samp">&</span></samp>’.
<p><span class="sc">matlab</span> has special behavior that allows the operators ‘<samp><span class="samp">&</span></samp>’ and
‘<samp><span class="samp">|</span></samp>’ to short-circuit when used in the truth expression for <code>if</code> and
<code>while</code> statements. The Octave parser may be instructed to behave in the
same manner, but its use is strongly discouraged.
<!-- do_braindead_shortcircuit_evaluation src/pt-binop.cc -->
<p><a name="doc_002ddo_005fbraindead_005fshortcircuit_005fevaluation"></a>
<div class="defun">
— Built-in Function: <var>val</var> = <b>do_braindead_shortcircuit_evaluation</b> ()<var><a name="index-do_005fbraindead_005fshortcircuit_005fevaluation-655"></a></var><br>
— Built-in Function: <var>old_val</var> = <b>do_braindead_shortcircuit_evaluation</b> (<var>new_val</var>)<var><a name="index-do_005fbraindead_005fshortcircuit_005fevaluation-656"></a></var><br>
— Built-in Function: <b>do_braindead_shortcircuit_evaluation</b> (<var>new_val, "local"</var>)<var><a name="index-do_005fbraindead_005fshortcircuit_005fevaluation-657"></a></var><br>
<blockquote><p>Query or set the internal variable that controls whether Octave will
do short-circuit evaluation of ‘<samp><span class="samp">|</span></samp>’ and ‘<samp><span class="samp">&</span></samp>’ operators inside the
conditions of if or while statements.
<p>This feature is only provided for compatibility with <span class="sc">matlab</span> and should
not be used unless you are porting old code that relies on this feature.
<p>To obtain short-circuit behavior for logical expressions in new programs,
you should always use the ‘<samp><span class="samp">&&</span></samp>’ and ‘<samp><span class="samp">||</span></samp>’ operators.
<p>When called from inside a function with the "local" option, the variable is
changed locally for the function and any subroutines it calls. The original
variable value is restored when exiting the function.
</p></blockquote></div>
<p>Finally, the ternary operator (?:) is not supported in Octave. If
short-circuiting is not important, it can be replaced by the <code>ifelse</code>
function.
<!-- merge src/data.cc -->
<p><a name="doc_002dmerge"></a>
<div class="defun">
— Built-in Function: <b>merge</b> (<var>mask, tval, fval</var>)<var><a name="index-merge-658"></a></var><br>
— Built-in Function: <b>ifelse</b> (<var>mask, tval, fval</var>)<var><a name="index-ifelse-659"></a></var><br>
<blockquote><p>Merge elements of <var>true_val</var> and <var>false_val</var>, depending on the
value of <var>mask</var>. If <var>mask</var> is a logical scalar, the other two
arguments can be arbitrary values. Otherwise, <var>mask</var> must be a logical
array, and <var>tval</var>, <var>fval</var> should be arrays of matching class, or
cell arrays. In the scalar mask case, <var>tval</var> is returned if <var>mask</var>
is true, otherwise <var>fval</var> is returned.
<p>In the array mask case, both <var>tval</var> and <var>fval</var> must be either
scalars or arrays with dimensions equal to <var>mask</var>. The result is
constructed as follows:
<pre class="example"> result(mask) = tval(mask);
result(! mask) = fval(! mask);
</pre>
<p><var>mask</var> can also be arbitrary numeric type, in which case
it is first converted to logical.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dlogical.html#doc_002dlogical">logical</a>.
</p></blockquote></div>
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