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<a name="Manipulating-Classes"></a>
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Next: <a href="Indexing-Objects.html#Indexing-Objects" accesskey="n" rel="next">Indexing Objects</a>, Previous: <a href="Creating-a-Class.html#Creating-a-Class" accesskey="p" rel="prev">Creating a Class</a>, Up: <a href="Object-Oriented-Programming.html#Object-Oriented-Programming" accesskey="u" rel="up">Object Oriented Programming</a> &nbsp; [<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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<hr>
<a name="Manipulating-Classes-1"></a>
<h3 class="section">34.2 Manipulating Classes</h3>

<p>There are a number of basic classes methods that can be defined to allow
the contents of the classes to be queried and set.  The most basic of
these is the <code>display</code> method.  The <code>display</code> method is used
by Octave when displaying a class on the screen, due to an expression
that is not terminated with a semicolon.  If this method is not defined,
then Octave will printed nothing when displaying the contents of a class.
</p>
<a name="XREFdisplay"></a><dl>
<dt><a name="index-display"></a>Function File: <em></em> <strong>display</strong> <em>(<var>a</var>)</em></dt>
<dd><p>Display the contents of an object.  If <var>a</var> is an object of the
class <code>&quot;myclass&quot;</code>, then <code>display</code> is called in a case like
</p>
<div class="example">
<pre class="example">myclass (&hellip;)
</pre></div>

<p>where Octave is required to display the contents of a variable of the
type <code>&quot;myclass&quot;</code>.
</p>

<p><strong>See also:</strong> <a href="Built_002din-Data-Types.html#XREFclass">class</a>, <a href="Defining-Indexing-And-Indexed-Assignment.html#XREFsubsref">subsref</a>, <a href="Defining-Indexing-And-Indexed-Assignment.html#XREFsubsasgn">subsasgn</a>.
</p></dd></dl>


<p>An example of a display method for the polynomial class might be
</p>
<div class="example">
<pre class="verbatim">function display (p)
  a = p.poly;
  first = true;
  fprintf (&quot;%s =&quot;, inputname (1));
  for i = 1 : length (a);
    if (a(i) != 0)
      if (first)
        first = false;
      elseif (a(i) &gt; 0)
        fprintf (&quot; +&quot;);
      endif
      if (a(i) &lt; 0)
        fprintf (&quot; -&quot;);
      endif
      if (i == 1)
        fprintf (&quot; %g&quot;, abs (a(i)));
      elseif (abs(a(i)) != 1)
        fprintf (&quot; %g *&quot;, abs (a(i)));
      endif
      if (i &gt; 1)
        fprintf (&quot; X&quot;);
      endif
      if (i &gt; 2)
        fprintf (&quot; ^ %d&quot;, i - 1);
      endif
    endif
  endfor
  if (first)
    fprintf (&quot; 0&quot;);
  endif
  fprintf (&quot;\n&quot;);
endfunction
</pre><pre class="example">
</pre></div>

<p>Note that in the display method, it makes sense to start the method
with the line <code>fprintf (&quot;%s =&quot;, inputname (1))</code> to be consistent
with the rest of Octave and print the variable name to be displayed
when displaying the class. 
</p>
<p>To be consistent with the Octave graphic handle classes, a class
should also define the <code>get</code> and <code>set</code> methods.  The
<code>get</code> method should accept one or two arguments, and given one
argument of the appropriate class it should return a structure with
all of the properties of the class.  For example:
</p>
<div class="example">
<pre class="verbatim">function s = get (p, f)
  if (nargin == 1)
    s.poly = p.poly;
  elseif (nargin == 2)
    if (ischar (f))
      switch (f)
        case &quot;poly&quot;
          s = p.poly;
        otherwise
          error (&quot;get: invalid property %s&quot;, f);
      endswitch
    else
      error (&quot;get: expecting the property to be a string&quot;);
    endif
  else
    print_usage ();
  endif
endfunction
</pre><pre class="example">
</pre></div>

<p>Similarly, the <code>set</code> method should taken as its first argument an
object to modify, and then take property/value pairs to be modified. 
</p>
<div class="example">
<pre class="verbatim">function s = set (p, varargin)
  s = p;
  if (length (varargin) &lt; 2 || rem (length (varargin), 2) != 0)
    error (&quot;set: expecting property/value pairs&quot;);
  endif
  while (length (varargin) &gt; 1)
    prop = varargin{1};
    val = varargin{2};
    varargin(1:2) = [];
    if (ischar (prop) &amp;&amp; strcmp (prop, &quot;poly&quot;))
      if (isvector (val) &amp;&amp; isreal (val))
        s.poly = val(:).';
      else
        error (&quot;set: expecting the value to be a real vector&quot;);
      endif
    else
      error (&quot;set: invalid property of polynomial class&quot;);
    endif
  endwhile
endfunction
</pre><pre class="example">
</pre></div>

<p>Note that as Octave does not implement pass by reference, than the
modified object is the return value of the <code>set</code> method and it
must be called like
</p>
<div class="example">
<pre class="example">p = set (p, &quot;a&quot;, [1, 0, 0, 0, 1]);
</pre></div>

<p>Also the <code>set</code> method makes use of the <code>subsasgn</code> method of
the class, and this method must be defined.  The <code>subsasgn</code> method
is discussed in the next section.
</p>
<p>Finally, user classes can be considered as a special type of a
structure, and so they can be saved to a file in the same manner as a
structure.  For example:
</p>
<div class="example">
<pre class="example">p = polynomial ([1, 0, 1]);
save userclass.mat p
clear p
load userclass.mat
</pre></div>

<p>All of the file formats supported by <code>save</code> and <code>load</code> are
supported.  In certain circumstances, a user class might either contain
a field that it makes no sense to save or a field that needs to be
initialized before it is saved.  This can be done with the
<code>saveobj</code> method of the class
</p>
<a name="XREFsaveobj"></a><dl>
<dt><a name="index-saveobj"></a>Function File: <em><var>b</var> =</em> <strong>saveobj</strong> <em>(<var>a</var>)</em></dt>
<dd><p>Method of a class to manipulate an object prior to saving it to a file.
The function <code>saveobj</code> is called when the object <var>a</var> is saved
using the <code>save</code> function.  An example of the use of <code>saveobj</code>
might be to remove fields of the object that don&rsquo;t make sense to be saved
or it might be used to ensure that certain fields of the object are
initialized before the object is saved.  For example:
</p>
<div class="example">
<pre class="example">function b = saveobj (a)
  b = a;
  if (isempty (b.field))
     b.field = initfield (b);
  endif
endfunction
</pre></div>


<p><strong>See also:</strong> <a href="#XREFloadobj">loadobj</a>, <a href="Built_002din-Data-Types.html#XREFclass">class</a>.
</p></dd></dl>


<p><code>saveobj</code> is called just prior to saving the class to a
file.  Likely, the <code>loadobj</code> method is called just after a class
is loaded from a file, and can be used to ensure that any removed
fields are reinserted into the user object.
</p>
<a name="XREFloadobj"></a><dl>
<dt><a name="index-loadobj"></a>Function File: <em><var>b</var> =</em> <strong>loadobj</strong> <em>(<var>a</var>)</em></dt>
<dd><p>Method of a class to manipulate an object after loading it from a file.
The function <code>loadobj</code> is called when the object <var>a</var> is loaded
using the <code>load</code> function.  An example of the use of <code>saveobj</code>
might be to add fields to an object that don&rsquo;t make sense to be saved.
For example:
</p>
<div class="example">
<pre class="example">function b = loadobj (a)
  b = a;
  b.addmissingfield = addfield (b);
endfunction
</pre></div>


<p><strong>See also:</strong> <a href="#XREFsaveobj">saveobj</a>, <a href="Built_002din-Data-Types.html#XREFclass">class</a>.
</p></dd></dl>


<hr>
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<p>
Next: <a href="Indexing-Objects.html#Indexing-Objects" accesskey="n" rel="next">Indexing Objects</a>, Previous: <a href="Creating-a-Class.html#Creating-a-Class" accesskey="p" rel="prev">Creating a Class</a>, Up: <a href="Object-Oriented-Programming.html#Object-Oriented-Programming" accesskey="u" rel="up">Object Oriented Programming</a> &nbsp; [<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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