<!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>Defining Indexing And Indexed Assignment (GNU Octave (version 5.1.0))</title>
<meta name="description" content="Defining Indexing And Indexed Assignment (GNU Octave (version 5.1.0))">
<meta name="keywords" content="Defining Indexing And Indexed Assignment (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="Indexing-Objects.html#Indexing-Objects" rel="up" title="Indexing Objects">
<link href="Indexed-Assignment-Optimization.html#Indexed-Assignment-Optimization" rel="next" title="Indexed Assignment Optimization">
<link href="Indexing-Objects.html#Indexing-Objects" rel="prev" title="Indexing Objects">
<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="Defining-Indexing-And-Indexed-Assignment"></a>
<div class="header">
<p>
Next: <a href="Indexed-Assignment-Optimization.html#Indexed-Assignment-Optimization" accesskey="n" rel="next">Indexed Assignment Optimization</a>, Up: <a href="Indexing-Objects.html#Indexing-Objects" accesskey="u" rel="up">Indexing Objects</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="Defining-Indexing-And-Indexed-Assignment-1"></a>
<h4 class="subsection">34.3.1 Defining Indexing And Indexed Assignment</h4>
<p>Objects can be indexed with parentheses or braces, either like
<code><var>obj</var>(<var>idx</var>)</code> or like <code><var>obj</var>{<var>idx</var>}</code>, or even
like <code><var>obj</var>(<var>idx</var>).<var>field</var></code>. However, it is up to the
programmer to decide what this indexing actually means. In the case of the
polynomial class <code><var>p</var>(<var>n</var>)</code> might mean either the coefficient of
the <var>n</var>-th power of the polynomial, or it might be the evaluation of the
polynomial at <var>n</var>. The meaning of this subscripted referencing is
determined by the <code>subsref</code> method.
</p>
<a name="XREFsubsref"></a><dl>
<dt><a name="index-subsref"></a><em></em> <strong>subsref</strong> <em>(<var>val</var>, <var>idx</var>)</em></dt>
<dd><p>Perform the subscripted element selection operation on <var>val</var> according
to the subscript specified by <var>idx</var>.
</p>
<p>The subscript <var>idx</var> must be a structure array with fields ‘<samp>type</samp>’
and ‘<samp>subs</samp>’. Valid values for ‘<samp>type</samp>’ are <code>"()"</code>,
<code>"{}"</code>, and <code>"."</code>. The ‘<samp>subs</samp>’ field may be either
<code>":"</code> or a cell array of index values.
</p>
<p>The following example shows how to extract the first two columns of a matrix
</p>
<div class="example">
<pre class="example">val = magic (3)
⇒ val = [ 8 1 6
3 5 7
4 9 2 ]
idx.type = "()";
idx.subs = {":", 1:2};
subsref (val, idx)
⇒ [ 8 1
3 5
4 9 ]
</pre></div>
<p>Note that this is the same as writing <code>val(:, 1:2)</code>.
</p>
<p>If <var>idx</var> is an empty structure array with fields ‘<samp>type</samp>’ and
‘<samp>subs</samp>’, return <var>val</var>.
</p>
<p><strong>See also:</strong> <a href="#XREFsubsasgn">subsasgn</a>, <a href="Manipulating-Structures.html#XREFsubstruct">substruct</a>.
</p></dd></dl>
<p>For example, this class uses the convention that indexing with <code>"()"</code>
evaluates the polynomial and indexing with <code>"{}"</code> returns the
<var>n</var>-th coefficient (of the <var>n</var>-th power). The code for the
<code>subsref</code> method looks like
</p>
<div class="example">
<pre class="verbatim">function r = subsref (p, s)
if (isempty (s))
error ("@polynomial/subsref: missing index");
endif
switch (s(1).type)
case "()"
idx = s(1).subs;
if (numel (idx) != 1)
error ("@polynomial/subsref: need exactly one index");
endif
r = polyval (fliplr (p.poly), idx{1});
case "{}"
idx = s(1).subs;
if (numel (idx) != 1)
error ("@polynomial/subsref: need exactly one index");
endif
if (isnumeric (idx{1}))
r = p.poly(idx{1}+1);
else
r = p.poly(idx{1});
endif
case "."
fld = s.subs;
if (! strcmp (fld, "poly"))
error ('@polynomial/subsref: invalid property "%s"', fld);
endif
r = p.poly;
otherwise
error ("@polynomial/subsref: invalid subscript type");
endswitch
if (numel (s) > 1)
r = subsref (r, s(2:end));
endif
endfunction
</pre></div>
<p>The equivalent functionality for subscripted assignments uses the
<code>subsasgn</code> method.
</p>
<a name="XREFsubsasgn"></a><dl>
<dt><a name="index-subsasgn"></a><em></em> <strong>subsasgn</strong> <em>(<var>val</var>, <var>idx</var>, <var>rhs</var>)</em></dt>
<dd><p>Perform the subscripted assignment operation according to the subscript
specified by <var>idx</var>.
</p>
<p>The subscript <var>idx</var> must be a structure array with fields ‘<samp>type</samp>’
and ‘<samp>subs</samp>’. Valid values for ‘<samp>type</samp>’ are <code>"()"</code>,
<code>"{}"</code>, and <code>"."</code>. The ‘<samp>subs</samp>’ field may be either
<code>":"</code> or a cell array of index values.
</p>
<p>The following example shows how to set the two first columns of a 3-by-3
matrix to zero.
</p>
<div class="example">
<pre class="example">val = magic (3);
idx.type = "()";
idx.subs = {":", 1:2};
subsasgn (val, idx, 0)
⇒ [ 0 0 6
0 0 7
0 0 2 ]
</pre></div>
<p>Note that this is the same as writing <code>val(:, 1:2) = 0</code>.
</p>
<p>If <var>idx</var> is an empty structure array with fields ‘<samp>type</samp>’ and
‘<samp>subs</samp>’, return <var>rhs</var>.
</p>
<p><strong>See also:</strong> <a href="#XREFsubsref">subsref</a>, <a href="Manipulating-Structures.html#XREFsubstruct">substruct</a>, <a href="#XREFoptimize_005fsubsasgn_005fcalls">optimize_subsasgn_calls</a>.
</p></dd></dl>
<a name="XREFoptimize_005fsubsasgn_005fcalls"></a><dl>
<dt><a name="index-optimize_005fsubsasgn_005fcalls"></a><em><var>val</var> =</em> <strong>optimize_subsasgn_calls</strong> <em>()</em></dt>
<dt><a name="index-optimize_005fsubsasgn_005fcalls-1"></a><em><var>old_val</var> =</em> <strong>optimize_subsasgn_calls</strong> <em>(<var>new_val</var>)</em></dt>
<dt><a name="index-optimize_005fsubsasgn_005fcalls-2"></a><em></em> <strong>optimize_subsasgn_calls</strong> <em>(<var>new_val</var>, "local")</em></dt>
<dd><p>Query or set the internal flag for <code>subsasgn</code> method call
optimizations.
</p>
<p>If true, Octave will attempt to eliminate the redundant copying when calling
the <code>subsasgn</code> method of a user-defined class.
</p>
<p>When called from inside a function with the <code>"local"</code> option, the
variable is changed locally for the function and any subroutines it calls.
The original variable value is restored when exiting the function.
</p>
<p><strong>See also:</strong> <a href="#XREFsubsasgn">subsasgn</a>.
</p></dd></dl>
<p>Note that the <code>subsref</code> and <code>subsasgn</code> methods always receive the
whole index chain, while they usually handle only the first element. It is the
responsibility of these methods to handle the rest of the chain (if needed),
usually by forwarding it again to <code>subsref</code> or <code>subsasgn</code>.
</p>
<p>If you wish to use the <code>end</code> keyword in subscripted expressions of an
object, then there must be an <code>end</code> method defined. For example, the
<code>end</code> method for the polynomial class might look like
</p>
<div class="example">
<pre class="verbatim">function r = end (obj, index_pos, num_indices)
if (num_indices != 1)
error ("polynomial object may only have one index");
endif
r = length (obj.poly) - 1;
endfunction
</pre></div>
<p>which is a fairly generic <code>end</code> method that has a behavior similar to the
<code>end</code> keyword for Octave Array classes. An example using the polynomial
class is then
</p>
<div class="example">
<pre class="example">p = polynomial ([1,2,3,4]);
p{end-1}
⇒ 3
</pre></div>
<p>Objects can also be used themselves as the index in a subscripted expression
and this is controlled by the <code>subsindex</code> function.
</p>
<a name="XREFsubsindex"></a><dl>
<dt><a name="index-subsindex"></a><em><var>idx</var> =</em> <strong>subsindex</strong> <em>(<var>obj</var>)</em></dt>
<dd><p>Convert an object to an index vector.
</p>
<p>When <var>obj</var> is a class object defined with a class constructor, then
<code>subsindex</code> is the overloading method that allows the conversion of
this class object to a valid indexing vector. It is important to note that
<code>subsindex</code> must return a zero-based real integer vector of the class
<code>"double"</code>. For example, if the class constructor were
</p>
<div class="example">
<pre class="example">function obj = myclass (a)
obj = class (struct ("a", a), "myclass");
endfunction
</pre></div>
<p>then the <code>subsindex</code> function
</p>
<div class="example">
<pre class="example">function idx = subsindex (obj)
idx = double (obj.a) - 1.0;
endfunction
</pre></div>
<p>could be used as follows
</p>
<div class="example">
<pre class="example">a = myclass (1:4);
b = 1:10;
b(a)
⇒ 1 2 3 4
</pre></div>
<p><strong>See also:</strong> <a href="Built_002din-Data-Types.html#XREFclass">class</a>, <a href="#XREFsubsref">subsref</a>, <a href="#XREFsubsasgn">subsasgn</a>.
</p></dd></dl>
<p>Finally, objects can be used like ranges by providing a <code>colon</code> method.
</p>
<a name="XREFcolon"></a><dl>
<dt><a name="index-colon"></a><em><var>r</var> =</em> <strong>colon</strong> <em>(<var>base</var>, <var>limit</var>)</em></dt>
<dt><a name="index-colon-1"></a><em><var>r</var> =</em> <strong>colon</strong> <em>(<var>base</var>, <var>increment</var>, <var>limit</var>)</em></dt>
<dd><p>Return the result of the colon expression corresponding to <var>base</var>,
<var>limit</var>, and optionally, <var>increment</var>.
</p>
<p>This function is equivalent to the operator syntax
<code><var>base</var> : <var>limit</var></code><!-- /@w --> or
<code><var>base</var> : <var>increment</var> : <var>limit</var></code><!-- /@w -->.
</p>
<p><strong>See also:</strong> <a href="Special-Utility-Matrices.html#XREFlinspace">linspace</a>.
</p></dd></dl>
<hr>
<div class="header">
<p>
Next: <a href="Indexed-Assignment-Optimization.html#Indexed-Assignment-Optimization" accesskey="n" rel="next">Indexed Assignment Optimization</a>, Up: <a href="Indexing-Objects.html#Indexing-Objects" accesskey="u" rel="up">Indexing Objects</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>
