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<h4 class="subsection">8.5.2 Short-circuit Boolean Operators</h4>
<p><a name="index-short_002dcircuit-evaluation-523"></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_0026-524"></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-525"></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>’.
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Mandriva
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2010.0
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i586
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media
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contrib-release
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366
