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<h4 class="subsection" id="Short_002dcircuit-Boolean-Operators-1"><span>8.5.2 Short-circuit Boolean Operators<a class="copiable-link" href="#Short_002dcircuit-Boolean-Operators-1"> ¶</a></span></h4>
<a class="index-entry-id" id="index-short_002dcircuit-evaluation"></a>
<p>Combined with the implicit conversion to scalar values in <code class="code">if</code> and
<code class="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 class="dfn">short-circuit</em> boolean operators work this way.
</p>
<dl class="table">
<dt><a id="index-_0026_0026"></a><span><code class="code"><var class="var">boolean1</var> && <var class="var">boolean2</var></code><a class="copiable-link" href="#index-_0026_0026"> ¶</a></span></dt>
<dd><p>The expression <var class="var">boolean1</var> is evaluated and converted to a scalar using
the equivalent of the operation <code class="code">all (<var class="var">boolean1</var>(:))</code>. If
<var class="var">boolean1</var> is not a logical value, it is considered true if its value
is nonzero, and false if its value is zero. If <var class="var">boolean1</var> is an array,
it is considered true only if it is non-empty and all elements are
nonzero. If <var class="var">boolean1</var> evaluates to false, the result of the overall
expression is false. If it is true, the expression <var class="var">boolean2</var> is
evaluated in the same way as <var class="var">boolean1</var>. If it is true, the result of
the overall expression is true. Otherwise the result of the overall
expression is false.
</p>
<p><strong class="strong">Warning:</strong> the one exception to the equivalence with evaluating
<code class="code">all (<var class="var">boolean1</var>(:))</code> is when <code class="code">boolean1</code> an the empty array.
For <small class="sc">MATLAB</small> compatibility, the truth value of an empty array is always
<code class="code">false</code> so <code class="code">[] && true</code> evaluates to <code class="code">false</code> even though
<code class="code">all ([])</code> is <code class="code">true</code>.
</p>
</dd>
<dt><a id="index-_007c_007c"></a><span><code class="code"><var class="var">boolean1</var> || <var class="var">boolean2</var></code><a class="copiable-link" href="#index-_007c_007c"> ¶</a></span></dt>
<dd><p>The expression <var class="var">boolean1</var> is evaluated and converted to a scalar using
the equivalent of the operation <code class="code">all (<var class="var">boolean1</var>(:))</code>. If
<var class="var">boolean1</var> is not a logical value, it is considered true if its value
is nonzero, and false if its value is zero. If <var class="var">boolean1</var> is an array,
it is considered true only if it is non-empty and all elements are
nonzero. If <var class="var">boolean1</var> evaluates to true, the result of the overall
expression is true. If it is false, the expression <var class="var">boolean2</var> is
evaluated in the same way as <var class="var">boolean1</var>. If it is true, the result of
the overall expression is true. Otherwise the result of the overall
expression is false.
</p>
<p><strong class="strong">Warning:</strong> the truth value of an empty matrix is always
<code class="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-preformatted">a && b++
</pre></div>
<p>the value of the variable <var class="var">b</var> is only incremented if the variable
<var class="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">
<div class="group"><pre class="example-preformatted">function f (a, b, c)
if (nargin > 2 && ischar (c))
...
</pre></div></div>
<p>instead of having to use two <code class="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">
<div class="group"><pre class="example-preformatted">function f (a, b, c)
if (nargin > 2)
if (ischar (c))
...
</pre></div></div>
<p>Writing
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">function f (a, b, c)
if (nargin > 2 & ischar (c))
...
</pre></div></div>
<p>would result in an error if <code class="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 class="samp">&</samp>’.
</p>
<p><small class="sc">MATLAB</small> has special behavior that allows the operators ‘<samp class="samp">&</samp>’ and
‘<samp class="samp">|</samp>’ to short-circuit when used in the truth expression for <code class="code">if</code>
and <code class="code">while</code> statements. Octave behaves the same way for
compatibility, however, the use of the ‘<samp class="samp">&</samp>’ and ‘<samp class="samp">|</samp>’ operators in
this way is strongly discouraged and a warning will be issued. Instead,
you should use the ‘<samp class="samp">&&</samp>’ and ‘<samp class="samp">||</samp>’ operators that always have
short-circuit behavior.
</p>
<p>Finally, the ternary operator (?:) is not supported in Octave. If
short-circuiting is not important, it can be replaced by the <code class="code">ifelse</code>
function.
</p>
<a class="anchor" id="XREFmerge"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-merge"><span><code class="def-type"><var class="var">M</var> =</code> <strong class="def-name">merge</strong> <code class="def-code-arguments">(<var class="var">mask</var>, <var class="var">tval</var>, <var class="var">fval</var>)</code><a class="copiable-link" href="#index-merge"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-ifelse"><span><code class="def-type"><var class="var">M</var> =</code> <strong class="def-name">ifelse</strong> <code class="def-code-arguments">(<var class="var">mask</var>, <var class="var">tval</var>, <var class="var">fval</var>)</code><a class="copiable-link" href="#index-ifelse"> ¶</a></span></dt>
<dd><p>Merge elements of <var class="var">true_val</var> and <var class="var">false_val</var>, depending on the
value of <var class="var">mask</var>.
</p>
<p>If <var class="var">mask</var> is a logical scalar, the other two arguments can be arbitrary
values. Otherwise, <var class="var">mask</var> must be a logical array, and <var class="var">tval</var>,
<var class="var">fval</var> should be arrays of matching class, or cell arrays. In the
scalar mask case, <var class="var">tval</var> is returned if <var class="var">mask</var> is true, otherwise
<var class="var">fval</var> is returned.
</p>
<p>In the array mask case, both <var class="var">tval</var> and <var class="var">fval</var> must be either
scalars or arrays with dimensions equal to <var class="var">mask</var>. The result is
constructed as follows:
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">result(mask) = tval(mask);
result(! mask) = fval(! mask);
</pre></div></div>
<p><var class="var">mask</var> can also be arbitrary numeric type, in which case it is first
converted to logical.
</p>
<p>Programming Note: <code class="code">ifelse</code> is an alias for <code class="code">merge</code> and can be used
interchangeably.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="Logical-Values.html#XREFlogical">logical</a>, <a class="ref" href="Finding-Elements-and-Checking-Conditions.html#XREFdiff">diff</a>.
</p></dd></dl>
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