<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ --> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <title>Ignoring Arguments (GNU Octave (version 5.1.0))</title> <meta name="description" content="Ignoring Arguments (GNU Octave (version 5.1.0))"> <meta name="keywords" content="Ignoring Arguments (GNU Octave (version 5.1.0))"> <meta name="resource-type" content="document"> <meta name="distribution" content="global"> <meta name="Generator" content="makeinfo"> <link href="index.html#Top" rel="start" title="Top"> <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> <link href="Functions-and-Scripts.html#Functions-and-Scripts" rel="up" title="Functions and Scripts"> <link href="Variable_002dlength-Return-Lists.html#Variable_002dlength-Return-Lists" rel="next" title="Variable-length Return Lists"> <link href="Variable_002dlength-Argument-Lists.html#Variable_002dlength-Argument-Lists" rel="prev" title="Variable-length Argument Lists"> <style type="text/css"> <!-- a.summary-letter {text-decoration: none} blockquote.indentedblock {margin-right: 0em} blockquote.smallindentedblock {margin-right: 0em; font-size: smaller} blockquote.smallquotation {font-size: smaller} div.display {margin-left: 3.2em} div.example {margin-left: 3.2em} div.lisp {margin-left: 3.2em} div.smalldisplay {margin-left: 3.2em} div.smallexample {margin-left: 3.2em} div.smalllisp {margin-left: 3.2em} kbd {font-style: oblique} pre.display {font-family: inherit} pre.format {font-family: inherit} pre.menu-comment {font-family: serif} pre.menu-preformatted {font-family: serif} pre.smalldisplay {font-family: inherit; font-size: smaller} pre.smallexample {font-size: smaller} pre.smallformat {font-family: inherit; font-size: smaller} pre.smalllisp {font-size: smaller} span.nolinebreak {white-space: nowrap} span.roman {font-family: initial; font-weight: normal} span.sansserif {font-family: sans-serif; font-weight: normal} ul.no-bullet {list-style: none} --> </style> <link rel="stylesheet" type="text/css" href="octave.css"> </head> <body lang="en"> <a name="Ignoring-Arguments"></a> <div class="header"> <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> </div> <hr> <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> <hr> <div class="header"> <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> </div> </body> </html>