Representing Structured Data with Classes

Objects as Data Structures

This example defines a class for storing data with a specific structure. Using a consistent structure for data storage makes it easier to create functions that operate on the data. A MATLAB^® struct with field names describing the particular data element is a useful way to organize data. However, a class can define both the data storage (properties) and operations that you can perform on that data (methods). This example illustrates these advantages.

Background for the Example

For this example, the data represents tensile stress/strain measurements. These data are used to calculate the elastic modulus of various materials. In simple terms, stress is the force applied to a material and strain is the resulting deformation. Their ratio defines a characteristic of the material. While this approach is an over simplification of the process, it suffices for this example.

Structure of the Data

This table describes the structure of the data.

Data	Description
`Material`	`char` vector identifying the type of material tested
`SampleNumber`	Number of a particular test sample
`Stress`	Vector of numbers representing the stress applied to the sample during the test.
`Strain`	Vector of numbers representing the strain at the corresponding values of the applied stress.
`Modulus`	Number defining an elastic modulus of the material under test, which is calculated from the stress and strain data

The `TensileData` Class

This example begins with a simple implementation of the class and builds on this implementation to illustrate how features enhance the usefulness of the class.

The first version of the class provides only data storage. The class defines a property for each of the required data elements.

classdef TensileData
   properties
      Material
      SampleNumber
      Stress
      Strain
      Modulus
   end
end

Create an Instance and Assign Data

The following statements create a TensileData object and assign data to it:

td = TensileData;
td.Material = 'Carbon Steel';
td.SampleNumber = 001;
td.Stress = [2e4 4e4 6e4 8e4];
td.Strain = [.12 .20 .31 .40];
td.Modulus = mean(td.Stress./td.Strain);

Advantages of a Class vs. a Structure

Treat the TensileData object (td in the previous statements) much as you would any MATLAB structure. However, defining a specialized data structure as a class has advantages over using a general-purpose data structure, like a MATLAB struct:

Users cannot accidentally misspell a field name without getting an error. For example, typing the following:
```
td.Modulus = ...
```
would simply add a field to a structure. However, it returns an error when td is an instance of the TensileData class.
A class is easy to reuse. Once you have defined the class, you can easily extend it with subclasses that add new properties.
A class is easy to identify. A class has a name so that you can identify objects with the whos and class functions and the Workspace browser. The class name makes it easy to refer to records with a meaningful name.
A class can validate individual field values when assigned, including class or value.
A class can restrict access to fields, for example, allowing a particular field to be read, but not changed.

Restrict Properties to Specific Values

Restrict properties to specific values by defining a property set access method. MATLAB calls the set access method whenever setting a value for a property.

Material Property Set Function

The Material property set method restricts the assignment of the property to one of the following strings: aluminum, stainless steel, or carbon steel.

Add this function definition to the methods block.

classdef TensileData
   properties
      Material
      SampleNumber
      Stress
      Strain
      Modulus
   end
   methods
      function obj = set.Material(obj,material)
         if (strcmpi(material,'aluminum') ||...
               strcmpi(material,'stainless steel') ||...
               strcmpi(material,'carbon steel'))
            obj.Material = material;
         else
            error('Invalid Material')
         end
      end
   end
end

When there is an attempt to set the Material property, MATLAB calls the set.Material method before setting the property value.

If the value matches the acceptable values, the function set the property to that value. The code within set method can access the property directly to avoid calling the property set method recursively.

For example:

td = TensileData;
td.Material = 'brass';

Error using TensileData/set.Material
Invalid Material

Simplifying the Interface with a Constructor

Simplify the interface to the TensileData class by adding a constructor that:

Enables you to pass the data as arguments to the constructor
Assigns values to properties

The constructor is a method having the same name as the class.

methods
   function td = TensileData(material,samplenum,stress,strain)
      if nargin > 0
         td.Material = material;
         td.SampleNumber = samplenum;
         td.Stress = stress;
         td.Strain = strain;
      end
   end 
end

Create a TensileData object fully populated with data using the following statement:

td = TensileData('carbon steel',1,...
      [2e4 4e4 6e4 8e4],...
      [.12 .20 .31 .40]);

Calculate Data on Demand

If the value of a property depends on the values of other properties, define that property using the Dependent attribute. MATLAB does not store the values of dependent properties. The dependent property get method determines the property value when the property is accessed. Access can occur when displaying object properties or as the result of an explicit query.

Calculating Modulus

TensileData objects do not store the value of the Modulus property. The constructor does not have an input argument for the value of the Modulus property. The value of the Modulus:

Is calculated from the Stress and Strain property values
Must change if the value of the Stress or Strain property changes

Therefore, it is better to calculate the value of the Modulus property only when its value is requested. Use a property get access method to calculate the value of the Modulus.

Modulus Property Get Method

The Modulus property depends on Stress and Strain, so its Dependent attribute is true. Place the Modulus property in a separate properties block and set the Dependent attribute.

The get.Modulus method calculates and returns the value of the Modulus property.

properties (Dependent)
   Modulus
end

Define the property get method in a methods block using only default attributes.

methods
   function modulus = get.Modulus(obj)
      ind = find(obj.Strain > 0);
      modulus = mean(obj.Stress(ind)./obj.Strain(ind));
   end
end

This method calculates the average ratio of stress to strain data after eliminating zeros in the denominator data.

MATLAB calls the get.Modulus method when the property is queried. For example,

td = TensileData('carbon steel',1,...
      [2e4 4e4 6e4 8e4],...
      [.12 .20 .31 .40]);
td.Modulus

ans =
  1.9005e+005

Modulus Property Set Method

To set the value of a Dependent property, the class must implement a property set method. There is no need to allow explicit setting of the Modulus property. However, a set method enables you to provide a customized error message. The Modulus set method references the current property value and then returns an error:

methods
   function obj = set.Modulus(obj,~)
      fprintf('%s%d\n','Modulus is: ',obj.Modulus)
      error('You cannot set the Modulus property');
   end
end

Displaying `TensileData` Objects

The TensileData class overloads the disp method. This method controls object display in the command window.

The disp method displays the value of the Material, SampleNumber, and Modulus properties. It does not display the Stress and Strain property data. These properties contain raw data that is not easily viewed in the command window.

The disp method uses fprintf to display formatted text in the command window:

methods
   function disp(td)
      fprintf(1,...
         'Material: %s\nSample Number: %g\nModulus: %1.5g\n',...
         td.Material,td.SampleNumber,td.Modulus);
   end 
end

Method to Plot Stress vs. Strain

It is useful to view a graph of the stress/strain data to determine the behavior of the material over a range of applied tension. The TensileData class overloads the MATLAB plot function.

The plot method creates a linear graph of the stress versus strain data and adds a title and axis labels to produce a standardized graph for the tensile data records:

methods
   function plot(td,varargin)
      plot(td.Strain,td.Stress,varargin{:})
      title(['Stress/Strain plot for Sample',...
         num2str(td.SampleNumber)])
      ylabel('Stress (psi)')
      xlabel('Strain %')
   end 
end

The first argument to this method is a TensileData object, which contains the data.

The method passes a variable list of arguments (varargin) directly to the built-in plot function. The TensileData plot method allows you to pass line specifier arguments or property name-value pairs.

For example:

td = TensileData('carbon steel',1,...
      [2e4 4e4 6e4 8e4],[.12 .20 .31 .40]);
plot(td,'-+b','LineWidth',2)

Stress graphed as function of strain

`TensileData` Class Synopsis

Example Code	Discussion
classdef TensileData	Value class enables independent copies of object. For more information, see Comparison of Handle and Value Classes
properties Material SampleNumber Stress Strain end	See Structure of the Data
properties (Dependent) Modulus end	Calculate `Modulus` when queried. For information about this code, see Calculate Data on Demand. For general information, see Get and Set Methods for Dependent Properties
methods	For general information about methods, see Ordinary Methods
function td = TensileData(material,samplenum,... stress,strain) if nargin > 0 td.Material = material; td.SampleNumber = samplenum; td.Stress = stress; td.Strain = strain; end end	For information about this code, see Simplifying the Interface with a Constructor. For general information about constructors, see Class Constructor Methods
function obj = set.Material(obj,material) if (strcmpi(material,'aluminum') \|\|... strcmpi(material,'stainless steel') \|\|... strcmpi(material,'carbon steel')) obj.Material = material; else error('Invalid Material') end end	Restrict possible values for `Material` property. For information about this code, see Restrict Properties to Specific Values. For general information about property set methods, see Property Get and Set Methods.
function m = get.Modulus(obj) ind = find(obj.Strain > 0); m = mean(obj.Stress(ind)./obj.Strain(ind)); end	Calculate `Modulus` property when queried. For information about this code, see Modulus Property Get Method. For general information about property get methods, see Property Get and Set Methods.
function obj = set.Modulus(obj,~) fprintf('%s%d\n','Modulus is: ',obj.Modulus) error('You cannot set Modulus property'); end	Add set method for `Dependent` `Modulus` property. For information about this code, see Modulus Property Set Method. For general information about property set methods, see Property Get and Set Methods.
function disp(td) fprintf(1,'Material: %s\nSample Number: %g\nModulus: %1.5g\n',... td.Material,td.SampleNumber,td.Modulus) end	Overload `disp` method to display certain properties. For information about this code, see Displaying TensileData Objects For general information about overloading disp, see Overloading the disp Function
function plot(td,varargin) plot(td.Strain,td.Stress,varargin{:}) title(['Stress/Strain plot for Sample',... num2str(td.SampleNumber)]) ylabel('Stress (psi)') xlabel('Strain %') end	Overload `plot` function to accept `TensileData` objects and graph stress vs. strain. Method to Plot Stress vs. Strain
end end	`end` statements for `methods` and for `classdef`.

Expand for Class Code