Catalogue

Using classes to represent structured data

Object as data structure

Data structure

TensileData class

Create an instance and assign it to data

Restrict attributes to specific values

Simplify interfaces with constructors

On demand calculation data

Display TensileData object

Method of drawing stress versus strain diagram

TensileData} class summary

Using classes to represent structured data

Object as data structure

The following example defines a class for storing data with a specific structure. Using a consistent structure to store data makes it easier to create functions to manipulate data. MATLAB ® struct} can contain field names that describe specific data elements, which is a useful way to organize data. However, classes can define data stores (properties) and operations (Methods) performed on that data. This example illustrates these advantages.

Example background

For this example, the data represents tensile stress / strain measurements. These data are used to calculate the elastic modulus of various materials. In short, stress is the force applied to a material, and strain is the resulting deformation. Its ratio determines the properties of the material. Although this method is a greatly simplified process, it is enough to meet the purpose of this example.

Data structure

The following table describes the structure of the data.

TensileData class

The following example starts with a simple implementation of a class and uses this implementation as a basis to illustrate how features enhance the usefulness of a class. The first version of this class provides only data storage. This class defines an attribute for each required data element.

classdef TensileData
   properties
      Material
      SampleNumber
      Stress
      Strain
      Modulus
   end
end

Create an instance and assign it to data

The following statement will 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 class over structure

Process the # TensileData # object (in the previous statement, # td) like any matlab structure. However, defining a specialized data structure as a class has the following advantages over using a general data structure (such as MATLAB # struct):

• When users accidentally misspell the field name, they will receive an error message. For example, type the following:

  td.Modulis = ...

  A field is added directly to the structure. However, if td is an instance of the TensileData class, an error is returned.

• Classes are easy to reuse. Once a class is defined, you can easily extend it with subclasses that add new properties.

• Classes are easy to identify. Classes have names, so you can use whos and class functions and workspace browsers to identify objects. Using class names, you can easily reference records with meaningful names.

• When assigning values, the class can verify the values of various fields, including classes or values.

• Class can restrict access to a field, for example, allowing a specific field to be read but not changed.

Restrict attributes to specific values

You can limit attributes to specific values by defining attribute set access methods. MATLAB will call the set access method every time it sets the attribute value.

Material property set function

The Material # property set method limits the assignment of the property to one of the following strings: aluminum, stainless steel # or # carbon steel. Add this function definition to the method code 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 trying to set the material property, MATLAB calls set before setting the property value Material method. If the value matches the acceptable value, the function sets the property to that value. The code in the set method can directly access the property to avoid calling the property set method recursively.

For example:

td = TensileData;
td.Material = 'brass';
Error using TensileData/set.Material
Invalid Material

Simplify interfaces with constructors

Simplify the interface with TensileData class by adding a constructor:

• Enables you to pass data as parameters to the constructor

• Assign values to attributes

The constructor is a method with 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 that is fully populated with data using the following statement:

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

On demand calculation data

If the value of a property depends on the value of other properties, use the Dependent attribute to define the property. MATLAB does not store values of Dependent attributes. When accessing a Dependent property, the Dependent property get method is used to determine the property value. It can be accessed when displaying object properties or as the result of an explicit query.

Calculation modulus

TensileData object does not store the value of the module property. The constructor does not have an input parameter for the value of the Modulus property. Value of module:

• Calculated from the values of the Stress and string attributes

• When the value of the Stress or string attribute changes, it must also change

Therefore, it is best to calculate the value of the module attribute only when needed. Use the property get access method to calculate the value of Modulus.

Module property get method

The "module" attribute depends on "Stress" and "Strain", so its "Dependent" attribute is "true". Put the 'module' attribute in a separate 'properties' code block and set the' Dependent 'attribute.

        get. The module} method calculates and returns the value of the} module} property.

properties (Dependent)
   Modulus
end

Only the default attribute is used to define the property get method in the methods code block.

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 and strain data after eliminating zero in denominator data. When querying properties, MATLAB will call

get.Modulus

Methods. 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 the Dependent property, the class must implement the property set method. You do not need to allow the module property to be set explicitly. However, you can use the set method to provide custom error messages. The module set method refers to the current property value and 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

Display TensileData object

TensileData class overloads the {disp} method. This method controls the display of objects in the command line window. The disp ￠ method displays the values of the ￠ Material, SampleNumber, and module ￠ properties. It does not display the ， Stress ， and ， Strain ， attribute data. These properties contain raw data that is not easy to view in the command line window.

The disp # method uses fprintf # to display formatted text in the command line 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 of drawing stress versus strain diagram

Viewing the stress / strain data graph helps to determine the behavior of the material within the range of applied tension. The TensileData} class overloads the MATLAB plot function.

The plot # method creates a linear graph of stress strain data and adds a title and axis label to generate a standardized graph of tension data record:

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 parameter of this method is the TensileData object that contains the data. This method passes the variable list (varargin) of the parameter directly to the built-in {plot} function. The TensileData # plot # method allows you to pass a line setter parameter or attribute name value pair group.

For example:

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

TensileData} class summary

classdef TensileData

   properties
      Material
      SampleNumber
      Stress
      Strain
   end

   properties (Dependent)
      Modulus
   end 

   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

   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

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

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

   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
end

Expand class code:

classdef TensileData
   properties
      Material
      SampleNumber
      Stress
      Strain
   end
   properties (Dependent)
      Modulus
   end
   
   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
      
      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
      
      function m = get.Modulus(obj)
         ind = find(obj.Strain > 0);
         m = mean(obj.Stress(ind)./obj.Strain(ind));
      end
      
      function obj = set.Modulus(obj,~)
         fprintf('%s%d\n','Modulus is: ',obj.Modulus)
         error('You cannot set Modulus property');
      end
      
      function disp(td)
         sprintf('Material: %s\nSample Number: %g\nModulus: %1.5g\n',...
            td.Material,td.SampleNumber,td.Modulus)
      end
      
      function plot(td,varargin)
         plot(td.Strain,td.Stress,varargin{:})
         title(['Stress/Strain plot for Sample ',...
            num2str(td.SampleNumber)])
         xlabel('Strain %')
         ylabel('Stress (psi)')
      end
   end
end