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<a name="Short_002dcircuit-Boolean-Operators"></a>
<div class="header">
<p>
Previous: <a href="Element_002dby_002delement-Boolean-Operators.html#Element_002dby_002delement-Boolean-Operators" accesskey="p" rel="prev">Element-by-element Boolean Operators</a>, Up: <a href="Boolean-Expressions.html#Boolean-Expressions" accesskey="u" rel="up">Boolean Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Short_002dcircuit-Boolean-Operators-1"></a>
<h4 class="subsection">8.5.2 Short-circuit Boolean Operators</h4>
<a name="index-short_002dcircuit-evaluation"></a>
<p>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
<em>short-circuit</em> boolean operators work this way.
</p>
<dl compact="compact">
<dt><code><var>boolean1</var> && <var>boolean2</var></code></dt>
<dd><a name="index-_0026_0026"></a>
<p>The expression <var>boolean1</var> is evaluated and converted to a scalar
using the equivalent of the operation <code>all (<var>boolean1</var>(:))</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
(<var>boolean1</var>(:))</code>. If it is true, the result of the overall expression
is 1. Otherwise, the result of the overall expression is 0.
</p>
<p><strong>Warning:</strong> there is one exception to the rule of evaluating
<code>all (<var>boolean1</var>(:))</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>.
</p>
</dd>
<dt><code><var>boolean1</var> || <var>boolean2</var></code></dt>
<dd><a name="index-_007c_007c"></a>
<p>The expression <var>boolean1</var> is evaluated and converted to a scalar
using the equivalent of the operation <code>all (<var>boolean1</var>(:))</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
(<var>boolean1</var>(:))</code>. If it is true, the result of the overall expression
is 1. Otherwise, the result of the overall expression is 0.
</p>
<p><strong>Warning:</strong> the truth value of an empty matrix is always <code>false</code>,
see the previous list item for details.
</p></dd>
</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
</p>
<div class="example">
<pre class="example">a && b++
</pre></div>
<p>the value of the variable <var>b</var> is only incremented if the variable
<var>a</var> is nonzero.
</p>
<p>This can be used to write somewhat more concise code. For example, it
is possible write
</p>
<div class="example">
<pre class="example">function f (a, b, c)
if (nargin > 2 && ischar (c))
…
</pre></div>
<p>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
</p>
<div class="example">
<pre class="example">function f (a, b, c)
if (nargin > 2)
if (ischar (c))
…
</pre></div>
<p>Writing
</p>
<div class="example">
<pre class="example">function f (a, b, c)
if (nargin > 2 & ischar (c))
…
</pre></div>
<p>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>&</samp>’.
</p>
<p><small>MATLAB</small> has special behavior that allows the operators ‘<samp>&</samp>’ and
‘<samp>|</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.
</p>
<a name="XREFdo_005fbraindead_005fshortcircuit_005fevaluation"></a><dl>
<dt><a name="index-do_005fbraindead_005fshortcircuit_005fevaluation"></a>Built-in Function: <em><var>val</var> =</em> <strong>do_braindead_shortcircuit_evaluation</strong> <em>()</em></dt>
<dt><a name="index-do_005fbraindead_005fshortcircuit_005fevaluation-1"></a>Built-in Function: <em><var>old_val</var> =</em> <strong>do_braindead_shortcircuit_evaluation</strong> <em>(<var>new_val</var>)</em></dt>
<dt><a name="index-do_005fbraindead_005fshortcircuit_005fevaluation-2"></a>Built-in Function: <em></em> <strong>do_braindead_shortcircuit_evaluation</strong> <em>(<var>new_val</var>, "local")</em></dt>
<dd><p>Query or set the internal variable that controls whether Octave will
do short-circuit evaluation of ‘<samp>|</samp>’ and ‘<samp>&</samp>’ operators inside the
conditions of if or while statements.
</p>
<p>This feature is only provided for compatibility with <small>MATLAB</small> and should
not be used unless you are porting old code that relies on this feature.
</p>
<p>To obtain short-circuit behavior for logical expressions in new programs,
you should always use the ‘<samp>&&</samp>’ and ‘<samp>||</samp>’ operators.
</p>
<p>When called from inside a function with the <code>"local"</code> 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></dd></dl>
<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.
</p>
<a name="XREFmerge"></a><dl>
<dt><a name="index-merge"></a>Built-in Function: <em></em> <strong>merge</strong> <em>(<var>mask</var>, <var>tval</var>, <var>fval</var>)</em></dt>
<dt><a name="index-ifelse"></a>Built-in Function: <em></em> <strong>ifelse</strong> <em>(<var>mask</var>, <var>tval</var>, <var>fval</var>)</em></dt>
<dd><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>
<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:
</p>
<div class="example">
<pre class="example">result(mask) = tval(mask);
result(! mask) = fval(! mask);
</pre></div>
<p><var>mask</var> can also be arbitrary numeric type, in which case
it is first converted to logical.
</p>
<p><strong>See also:</strong> <a href="Logical-Values.html#XREFlogical">logical</a>, <a href="Finding-Elements-and-Checking-Conditions.html#XREFdiff">diff</a>.
</p></dd></dl>
<hr>
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Previous: <a href="Element_002dby_002delement-Boolean-Operators.html#Element_002dby_002delement-Boolean-Operators" accesskey="p" rel="prev">Element-by-element Boolean Operators</a>, Up: <a href="Boolean-Expressions.html#Boolean-Expressions" accesskey="u" rel="up">Boolean Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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