## 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) ``` ### `TensileData` Class Synopsis

Example CodeDiscussion
`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 ```
``` properties (Dependent) Modulus end ```

Calculate `Modulus` when queried. For information about this code, see Calculate Data on Demand.

For general information, see Set and Get 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 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 general information about property set methods, see Property 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 general information about property get methods, see Property Get 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 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.

``` 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.
``` end end```
`end` statements for `methods` and for `classdef`.