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<a name="Ignoring-Arguments"></a>
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<p>
Next: <a href="Variable_002dlength-Return-Lists.html#Variable_002dlength-Return-Lists" accesskey="n" rel="next">Variable-length Return Lists</a>, Previous: <a href="Variable_002dlength-Argument-Lists.html#Variable_002dlength-Argument-Lists" accesskey="p" rel="prev">Variable-length Argument Lists</a>, Up: <a href="Functions-and-Scripts.html#Functions-and-Scripts" accesskey="u" rel="up">Functions and Scripts</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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<a name="Ignoring-Arguments-1"></a>
<h3 class="section">11.5 Ignoring Arguments</h3>
<p>In the formal argument list, it is possible to use the dummy placeholder
<code>~</code> instead of a name. This indicates that the corresponding argument
value should be ignored and not stored to any variable.
</p>
<div class="example">
<pre class="example">function val = pick2nd (~, arg2)
val = arg2;
endfunction
</pre></div>
<p>The value of <code>nargin</code> is not affected by using this declaration.
</p>
<p>Return arguments can also be ignored using the same syntax. For example, the
sort function returns both the sorted values, and an index vector for the
original input which will result in a sorted output. Ignoring the second
output is simple—don’t request more than one output. But ignoring the first,
and calculating just the second output, requires the use of the <code>~</code>
placeholder.
</p>
<div class="example">
<pre class="example">x = [2, 3, 1];
[s, i] = sort (x)
⇒
s =
1 2 3
i =
3 1 2
[~, i] = sort (x)
⇒
i =
3 1 2
</pre></div>
<p>When using the <code>~</code> placeholder, commas—not whitespace—must be used
to separate output arguments. Otherwise, the interpreter will view <code>~</code> as
the logical not operator.
</p>
<div class="example">
<pre class="example">[~ i] = sort (x)
parse error:
invalid left hand side of assignment
</pre></div>
<p>Functions may take advantage of ignored outputs to reduce the number of
calculations performed. To do so, use the <code>isargout</code> function to query
whether the output argument is wanted. For example:
</p>
<div class="example">
<pre class="example">function [out1, out2] = long_function (x, y, z)
if (isargout (1))
## Long calculation
…
out1 = result;
endif
…
endfunction
</pre></div>
<a name="XREFisargout"></a><dl>
<dt><a name="index-isargout"></a><em></em> <strong>isargout</strong> <em>(<var>k</var>)</em></dt>
<dd><p>Within a function, return a logical value indicating whether the argument
<var>k</var> will be assigned to a variable on output.
</p>
<p>If the result is false, the argument has been ignored during the function
call through the use of the tilde (~) special output argument. Functions
can use <code>isargout</code> to avoid performing unnecessary calculations for
outputs which are unwanted.
</p>
<p>If <var>k</var> is outside the range <code>1:max (nargout)</code>, the function returns
false. <var>k</var> can also be an array, in which case the function works
element-by-element and a logical array is returned. At the top level,
<code>isargout</code> returns an error.
</p>
<p><strong>See also:</strong> <a href="Multiple-Return-Values.html#XREFnargout">nargout</a>, <a href="Multiple-Return-Values.html#XREFvarargout">varargout</a>, <a href="Multiple-Return-Values.html#XREFnthargout">nthargout</a>.
</p></dd></dl>
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Next: <a href="Variable_002dlength-Return-Lists.html#Variable_002dlength-Return-Lists" accesskey="n" rel="next">Variable-length Return Lists</a>, Previous: <a href="Variable_002dlength-Argument-Lists.html#Variable_002dlength-Argument-Lists" accesskey="p" rel="prev">Variable-length Argument Lists</a>, Up: <a href="Functions-and-Scripts.html#Functions-and-Scripts" accesskey="u" rel="up">Functions and Scripts</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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