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<h3 class="section">33.2 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.
<!-- ./general/display.m -->
<p><a name="doc_002ddisplay"></a>
<div class="defun">
— Function File: <b>display</b> (<var>a</var>)<var><a name="index-display-2258"></a></var><br>
<blockquote><p>Display the contents of an object. If <var>a</var> is an object of the
class "myclass", then <code>display</code> is called in a case like
<pre class="example"> myclass (...)
</pre>
<p class="noindent">where Octave is required to display the contents of a variable of the
type "myclass".
<!-- Texinfo @sp should work but in practice produces ugly results for HTML. -->
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<!-- @sp 1 -->
<p class="noindent"><strong>See also:</strong> <a href="doc_002dclass.html#doc_002dclass">class</a>, <a href="doc_002dsubsref.html#doc_002dsubsref">subsref</a>, <a href="doc_002dsubsasgn.html#doc_002dsubsasgn">subsasgn</a>.
</p></blockquote></div>
<p class="noindent">An example of a display method for the polynomial class might be
<pre class="example"><pre class="verbatim"> function display (p)
a = p.poly;
first = true;
fprintf("%s =", inputname(1));
for i = 1 : length (a);
if (a(i) != 0)
if (first)
first = false;
elseif (a(i) > 0)
fprintf (" +");
endif
if (a(i) < 0)
fprintf (" -");
endif
if (i == 1)
fprintf (" %g", abs (a(i)));
elseif (abs(a(i)) != 1)
fprintf (" %g *", abs (a(i)));
endif
if (i > 1)
fprintf (" X");
endif
if (i > 2)
fprintf (" ^ %d", i - 1);
endif
endif
endfor
if (first)
fprintf(" 0");
endif
fprintf("\n");
endfunction
</pre>
</pre>
<p class="noindent">Note that in the display method, it makes sense to start the method
with the line <code>fprintf("%s =", inputname(1))</code> to be consistent
with the rest of Octave and print the variable name to be displayed
when displaying the class.
<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
<pre 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 "poly"
s = p.poly;
otherwise
error ("get: invalid property %s", f);
endswitch
else
error ("get: expecting the property to be a string");
endif
else
print_usage ();
endif
endfunction
</pre>
</pre>
<p class="noindent">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.
<pre class="example"><pre class="verbatim"> function s = set (p, varargin)
s = p;
if (length (varargin) < 2 || rem (length (varargin), 2) != 0)
error ("set: expecting property/value pairs");
endif
while (length (varargin) > 1)
prop = varargin{1};
val = varargin{2};
varargin(1:2) = [];
if (ischar (prop) && strcmp (prop, "poly"))
if (isvector (val) && isreal (val))
s.poly = val(:).';
else
error ("set: expecting the value to be a real vector");
endif
else
error ("set: invalid property of polynomial class");
endif
endwhile
endfunction
</pre>
</pre>
<p class="noindent">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
<pre class="example"> p = set (p, "a", [1, 0, 0, 0, 1]);
</pre>
<p class="noindent">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>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
<pre class="example"> p = polynomial ([1, 0, 1]);
save userclass.mat p
clear p
load userclass.mat
</pre>
<p class="noindent">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
<!-- ./general/saveobj.m -->
<p><a name="doc_002dsaveobj"></a>
<div class="defun">
— Function File: <var>b</var> = <b>saveobj</b> (<var>a</var>)<var><a name="index-saveobj-2259"></a></var><br>
<blockquote><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'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
<pre class="example"> function b = saveobj (a)
b = a;
if (isempty (b.field))
b.field = initfield(b);
endif
endfunction
</pre>
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dloadobj.html#doc_002dloadobj">loadobj</a>, <a href="doc_002dclass.html#doc_002dclass">class</a>.
</p></blockquote></div>
<p class="noindent"><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.
<!-- ./general/loadobj.m -->
<p><a name="doc_002dloadobj"></a>
<div class="defun">
— Function File: <var>b</var> = <b>loadobj</b> (<var>a</var>)<var><a name="index-loadobj-2260"></a></var><br>
<blockquote><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't make sense to be saved.
For example
<pre class="example"> function b = loadobj (a)
b = a;
b.addmissingfield = addfield (b);
endfunction
</pre>
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<p class="noindent"><strong>See also:</strong> <a href="doc_002dsaveobj.html#doc_002dsaveobj">saveobj</a>, <a href="doc_002dclass.html#doc_002dclass">class</a>.
</p></blockquote></div>
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