MATLAB®

Example — Class for Graphing Functions

The class block is the code that starts with the classdef key word and terminates with the end key word. The following example illustrated a simple class definition that uses:

• Handle class

• Property set and get functions

• Use of a delete method for the handle object

• Static method syntax

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Class Definition Block

The following code defines a class called topo. It is derived from handle so it is a handle class, which means it references the data it contains. See Using the topo Class for information on how this class behaves.

classdef topo < handle                 % topo is a subclass of handle
   properties
      FigHandle = [];
      FofXY
      Lm = [-2*pi 2*pi];                 % Initial property value
   end % properties 

   properties (Dependent = true)        % Do not store the Data property 
      Data
   end % properties Dependent = true

   methods
      function obj = topo(fnc,limits)   % Constructor returns object
         obj.FofXY = fnc;                 % Assign property values
         obj.Lm = limits;
      end % topo

      function lm = set.Lm(obj,lim)     % Lm property set function
         if  ~(lim(1) < lim(2))
            error('Limits must be monotonically increasing')
         else
               obj.Lm = lim;
         end
      end % set.Lm

      function data = get.Data(obj)      % get function calculates Data
         [x,y] = topo.grid(obj.Lm);        % Use class name to call static method
         matrix = obj.FofXY(x,y);
         data.X = x;
         data.Y = y;
         data.Matrix = matrix;             % Return value of property
      end % get.Data

      function surflight(obj)            % Graph function as surface
         surf(obj.Data.X,obj.Data.Y,obj.Data.Matrix,...
            'FaceColor',[.8 .8 0],'EdgeColor',[0 .2 0],...
            'FaceLighting','phong');
         camlight left; material shiny; grid off
         colormap copper
         obj.FigHandle = gcf;
      end % surflight method
   
      function delete(obj)
         h = obj.FigHandle;
         if ishandle(h)
            delete(h); % Delete the figure
         else
            return
         end
      end % delete
   end % methods

   methods (Static = true)            % Define static method 
     function [x,y] = grid(lim)        
        inc = (lim(2)-lim(1))/35;
        [x,y] = meshgrid(lim(1):inc:lim(2));
     end % grid
   end % methods Static = true
end % topo class

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Using the topo Class

This class is designed to display a combination surface/contour graph of mathematical functions of two variables evaluated on a rectangular domain of x and y. For example, any of the following functions can be evaluated over the specified domain (note that x and y have the same range of values in this example just for simplicity).

x.*exp(-x.^2 - y.^2); [-2 2]
sin(x).*sin(y); [-2*pi 2*pi]
sqrt(x.^2 + y.^2); [-2*pi 2*pi]

To create an instance of the class, passing a function handle and a vector of limits to the constructor. The easiest way to create a function handle for these functions is to use an anonymous function:

tobj = topo(@(x,y) x.*exp(-x.^2-y.^2),[-2 2]);

The class surflight method uses the object to create a graph of the function. The actual data required to create the graph is not stored. When the surflight method accesses the Data property, the property's get function performs the evaluation and returns the data in the Data property structure fields. This data is then plotted. The advantage of not storing the data is the reduced size of the object.

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Behavior of the Handle Class

The topo class is defined as a handle class. This means that instances of this class are handle objects that reference the underlying data store created by constructing the object. For example, suppose you create an instance of the class and create a copy of the object:

tobj = topo(@(x,y) x.*exp(-x.^2-y.^2),[-2 2]);
a = tobj;
surflight(a) % Call class method to create a graph

Now suppose you change the FofXY property so that it contains a function handle that points to another function:

tobj.FofXY = @(x,y) y.*exp(-x.^2-y.^2); % now multiply exp by y instead of x
surflight(a)

Because a is a copy of the handle object tobj, changes to the data referenced by tobj also change the data referenced by a.

How a Value Class Differs

If topo were a value class, the objects tobj and a would not share data; each would have its own copy of the property values.

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Example — Implementing Linked Lists

Working with Classes