1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
|
<!DOCTYPE html>
<html>
<!-- Created by GNU Texinfo 7.1.1, https://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Ignoring Arguments (GNU Octave (version 10.3.0))</title>
<meta name="description" content="Ignoring Arguments (GNU Octave (version 10.3.0))">
<meta name="keywords" content="Ignoring Arguments (GNU Octave (version 10.3.0))">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta name="viewport" content="width=device-width,initial-scale=1">
<link href="index.html" rel="start" title="Top">
<link href="Concept-Index.html" rel="index" title="Concept Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Functions-and-Scripts.html" rel="up" title="Functions and Scripts">
<link href="Default-Arguments.html" rel="next" title="Default Arguments">
<link href="Variable_002dlength-Argument-Lists.html" rel="prev" title="Variable-length Argument Lists">
<style type="text/css">
<!--
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
div.example {margin-left: 3.2em}
span:hover a.copiable-link {visibility: visible}
strong.def-name {font-family: monospace; font-weight: bold; font-size: larger}
-->
</style>
<link rel="stylesheet" type="text/css" href="octave.css">
</head>
<body lang="en">
<div class="section-level-extent" id="Ignoring-Arguments">
<div class="nav-panel">
<p>
Next: <a href="Default-Arguments.html" accesskey="n" rel="next">Default Arguments</a>, Previous: <a href="Variable_002dlength-Argument-Lists.html" accesskey="p" rel="prev">Variable-length Argument Lists</a>, Up: <a href="Functions-and-Scripts.html" 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" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<h3 class="section" id="Ignoring-Arguments-1"><span>11.7 Ignoring Arguments<a class="copiable-link" href="#Ignoring-Arguments-1"> ¶</a></span></h3>
<p>In the formal argument list, it is possible to use the dummy placeholder
<code class="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">
<div class="group"><pre class="example-preformatted">function val = pick2nd (~, arg2)
val = arg2;
endfunction
</pre></div></div>
<p>The value of <code class="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 class="code">~</code>
placeholder.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">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></div>
<p>When using the <code class="code">~</code> placeholder, commas—not whitespace—must be used
to separate output arguments. Otherwise, the interpreter will view <code class="code">~</code> as
the logical not operator.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">[~ i] = sort (x)
parse error:
invalid left hand side of assignment
</pre></div></div>
<p>Functions may take advantage of ignored outputs to reduce the number of
calculations performed. To do so, use the <code class="code">isargout</code> function to query
whether the output argument is wanted. For example:
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">function [out1, out2] = long_function (x, y, z)
if (isargout (1))
## Long calculation
...
out1 = result;
endif
...
endfunction
</pre></div></div>
<a class="anchor" id="XREFisargout"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-isargout"><span><code class="def-type"><var class="var">tf</var> =</code> <strong class="def-name">isargout</strong> <code class="def-code-arguments">(<var class="var">k</var>)</code><a class="copiable-link" href="#index-isargout"> ¶</a></span></dt>
<dd><p>Within a function, return a logical value indicating whether the argument
<var class="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 class="code">isargout</code> to avoid performing unnecessary calculations for
outputs which are unwanted.
</p>
<p>If <var class="var">k</var> is outside the range <code class="code">1:max (nargout)</code>, the function returns
false. <var class="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 class="code">isargout</code> returns an error.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="Multiple-Return-Values.html#XREFnargout">nargout</a>, <a class="ref" href="Variable_002dlength-Return-Lists.html#XREFvarargout">varargout</a>, <a class="ref" href="Multiple-Return-Values.html#XREFnthargout">nthargout</a>.
</p></dd></dl>
</div>
<hr>
<div class="nav-panel">
<p>
Next: <a href="Default-Arguments.html">Default Arguments</a>, Previous: <a href="Variable_002dlength-Argument-Lists.html">Variable-length Argument Lists</a>, Up: <a href="Functions-and-Scripts.html">Functions and Scripts</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
</div>
</body>
</html>
|
