MATLAB - User-Defined Classes

Quiz

MATLAB is traditionally known for its powerful matrix operations and built-in functions, but it also supports object-oriented programming (OOP). User-defined classes in MATLAB allow you to create custom data types that encapsulate data and functions (methods) that operate on that data. This approach can help organize code, promote reusability, and model complex systems more naturally.

What is a Class?

A class is a blueprint for creating objects (instances). It defines properties (data) and methods (functions) that the objects created from the class will have.

Defining a Class

In MATLAB, you define a class using the classdef keyword. Heres a simple example −

classdef MyClass
    properties
        Property1
    end
    
    methods
        function obj = MyClass(val)
            if nargin > 0
                obj.Property1 = val;
            end
        end
        
        function output = myMethod(obj, input)
            output = obj.Property1 + input;
        end
    end
end

Breakdown of the Example

1. Class Definition

classdef MyClass

This line declares a new class named MyClass.

2. Properties Block

properties
    Property1
end

Properties are variables that hold data for objects of the class. In this example, MyClass has one property, Property1.

3. Methods Block

methods

Methods are functions that define the behavior of the class. They operate on objects of the class.

4. Constructor Method

function obj = MyClass(val)
    if nargin > 0
        obj.Property1 = val;
    end
end

The constructor method is a special function that MATLAB calls when creating a new object. The name of the constructor method is the same as the class name. Here, the constructor initializes Property1 if an input value val is provided.

5. Other Methods

function output = myMethod(obj, input)
    output = obj.Property1 + input;
end

This is a regular method named myMethod. It takes two inputs: obj (the object itself) and input. It returns the sum of Property1 and input.

Creating Objects

To create an object of MyClass, you use the constructor method:

obj = MyClass(10);
disp(obj.Property1);  % Here Output: 10

Using Methods

You can call methods on an object using dot notation

result = obj.myMethod(5);
disp(result);  % Here Output: 15

Advantages of Using Classes

Encapsulation − Group related data and functions together, making the code more modular and easier to manage.
Reusability − Once a class is defined, it can be reused in different parts of a project or in different projects.
Abstraction − Hide the internal implementation details and expose only the necessary interface.
Inheritance − Create new classes that inherit properties and methods from existing classes, promoting code reuse and hierarchical class structures.

Example 1: A Point Class

Heres a more practical example, defining a class to represent a 2D point.

classdef Point
    properties
        X
        Y
    end
    methods
        function obj = Point(x, y)
            if nargin > 0
                obj.X = x;
                obj.Y = y;
            end
        end
        
        function obj = set.X(obj, x)
            if x >= 0
                obj.X = x;
            else
                error('X must be non-negative');
            end
        end
        
        function obj = set.Y(obj, y)
            if y >= 0
                obj.Y = y;
            else
                error('Y must be non-negative');
            end
        end
        
        function distance = distanceToOrigin(obj)
            distance = sqrt(obj.X^2 + obj.Y^2);
        end
    end
end

Using the Point Class

p = Point(3, 4);
disp(p.X);  % Output: 3
disp(p.Y);  % Output: 4

dist = p.distanceToOrigin();
disp(dist);  % Output: 5

In this example −

The Point class has properties X and Y to store coordinates.
The constructor initializes these properties.
Set methods (set.X and set.Y) validate that coordinates are non-negative.
The distanceToOrigin method calculates the Euclidean distance from the origin to the point.

Example 2: A Circle Class

This class will include properties for the radius and center of the circle, and methods to calculate the area, circumference, and to check if a point is inside the circle.

Circle Class Definition

classdef Circle
    properties
        Radius
        Center
    end
    methods
        % Constructor
        function obj = Circle(radius, center)
            if nargin > 0
                obj.Radius = radius;
                obj.Center = center;
            end
        end
        
        % Method to calculate area
        function area = area(obj)
            area = pi * obj.Radius^2;
        end
        
        % Method to calculate circumference
        function circumference = circumference(obj)
            circumference = 2 * pi * obj.Radius;
        end
        
        % Method to check if a point is inside the circle
        function isIn = isPointInside(obj, point)
            distance = sqrt((point(1) - obj.Center(1))^2 + (point(2) - obj.Center(2))^2);
            isIn = distance <= obj.Radius;
        end
    end
end

Breakdown of the Circle Class

1. Class Definition

classdef Circle

2. Properties

properties
    Radius
    Center
end

The Circle class has two properties: Radius (the radius of the circle) and Center (a 2-element vector representing the x and y coordinates of the circle's center).

3. Constructor Method

function obj = Circle(radius, center)
    if nargin > 0
        obj.Radius = radius;
        obj.Center = center;
    end
end

The constructor initializes the Radius and Center properties if input arguments are provided.

4. Area Method

function area = area(obj)
    area = pi * obj.Radius^2;
end

This method calculates the area of the circle using the formula x Radius2

5. Circumference Method

function circumference = circumference(obj)
    circumference = 2 * pi * obj.Radius;
end

This method calculates the circumference of the circle using the formula 2 x x Radius.

6. Point Inside Check Method.

function isIn = isPointInside(obj, point)
    distance = sqrt((point(1) - obj.Center(1))^2 + (point(2) - obj.Center(2))^2);
    isIn = distance <= obj.Radius;
end

This method checks if a given point is inside the circle. It calculates the distance from the point to the center of the circle and checks if this distance is less than or equal to the radius.

Using the Circle Class

To create a Circle object and use its methods, you can do the following.

% Create a Circle object with radius 5 and center at (0, 0)
c = Circle(5, [0, 0]);

% Calculate the area
a = c.area();
disp(['Area: ', num2str(a)]);  % Output: Area: 78.5398

% Calculate the circumference
circ = c.circumference();
disp(['Circumference: ', num2str(circ)]);  % Output: Circumference: 31.4159

% Check if a point is inside the circle
point = [3, 4];
isInside = c.isPointInside(point);
disp(['Is the point inside the circle? ', num2str(isInside)]);  % Output: Is the point inside the circle? 1

% Check if another point is inside the circle
point = [6, 0];
isInside = c.isPointInside(point);
disp(['Is the point inside the circle? ', num2str(isInside)]);  % Output: Is the point inside the circle? 0

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