Constructor methods in MATLAB - Mini Project: Build & Apply

Constructor Methods in MATLAB Classes

📖 Scenario: You are creating a simple MATLAB class to represent a Book in a library system. Each Book has a title, author, and year of publication.

🎯 Goal: Build a MATLAB class with a constructor method that initializes the Book's title, author, and year properties when a new Book object is created.

📋 What You'll Learn

Create a MATLAB class named Book.

Add properties title, author, and year to the class.

Write a constructor method named Book that takes three inputs: title, author, and year.

Inside the constructor, assign the input values to the corresponding properties.

Create an instance of the Book class with the title 'The Great Gatsby', author 'F. Scott Fitzgerald', and year 1925.

Display the properties of the created Book object.

💡 Why This Matters

🌍 Real World

Constructor methods are used in MATLAB to create objects with initial values, which is common in engineering and scientific applications to model real-world entities.

💼 Career

Understanding constructors is essential for MATLAB developers working on simulations, data modeling, or any object-oriented programming tasks in research and industry.

1

Create the Book class with properties

Create a MATLAB class named Book with public properties title, author, and year.

MATLAB

classdef Book
    properties
        % Define properties here
    end
end

Hint

Use classdef Book to start the class and list the properties inside a properties block.

2

Add the constructor method

Inside the Book class, add a constructor method named Book that takes inputs title, author, and year. Assign these inputs to the corresponding properties.

MATLAB

classdef Book
    properties
        title
        author
        year
    end
    methods
        % Add constructor method here
    end
end

Hint

The constructor method has the same name as the class. Use obj as the object variable and assign inputs to properties.

3

Create a Book object

Create a variable named myBook that is an instance of the Book class with title set to 'The Great Gatsby', author set to 'F. Scott Fitzgerald', and year set to 1925.

MATLAB

classdef Book
    properties
        title
        author
        year
    end
    methods
        function obj = Book(title, author, year)
            obj.title = title;
            obj.author = author;
            obj.year = year;
        end
    end
end

% Create the Book object here
% Your code here

Hint

Use the constructor by calling Book(...) with the exact values and assign it to myBook.

4

Display the Book properties

Write code to display the title, author, and year properties of the myBook object using disp or fprintf.

MATLAB

classdef Book
    properties
        title
        author
        year
    end
    methods
        function obj = Book(title, author, year)
            obj.title = title;
            obj.author = author;
            obj.year = year;
        end
    end
end

myBook = Book('The Great Gatsby', 'F. Scott Fitzgerald', 1925);

% Display the properties below
% Your code here

Hint

Use fprintf to print each property on its own line with labels.

Practice

1. What is the main purpose of a constructor method in a MATLAB class?

easy

A. To inherit properties from another class

B. To delete an object

C. To display object properties

D. To create an object with initial values

Solution

1. Step 1: Understand the role of constructors

   Constructor methods are special functions that create objects and set their initial state.

2. Step 2: Identify the correct purpose

   Among the options, only creating an object with initial values matches the constructor's role.

3. Final Answer:

   To create an object with initial values -> Option D

4. Quick Check:

   Constructor = create object [OK]

Hint: Constructors initialize objects when created [OK]

Common Mistakes:

• Confusing constructors with destructors
• Thinking constructors display properties
• Mixing constructors with inheritance

2. Which of the following is the correct way to define a constructor method in a MATLAB class named Car?

easy

A. function Car = obj()
obj.color = 'red';
end

B. function obj = Car(color)
obj.color = color;
end

C. function obj = car()
obj.color = 'red';
end

D. function obj = Car()
color = 'red';
end

Solution

1. Step 1: Check constructor name matches class name

   The constructor must have the same name as the class, which is 'Car' with uppercase C.

2. Step 2: Verify syntax and parameters

   function obj = Car(color) obj.color = color; end correctly defines a constructor with input argument to set the color property.

3. Final Answer:

   function obj = Car(color) obj.color = color; end -> Option B

4. Quick Check:

   Constructor name = class name [OK]

Hint: Constructor name must match class name exactly [OK]

Common Mistakes:

• Using lowercase class name in constructor
• Swapping function and variable names
• Missing input arguments for initialization

3. Consider this MATLAB class constructor code:

classdef Book
  properties
    Title
    Author
  end
  methods
    function obj = Book(title, author)
      obj.Title = title;
      obj.Author = author;
    end
  end
end

b = Book('1984', 'Orwell');
disp(b.Title);

What will be the output when this code runs?

medium

A. Error: Missing input arguments

B. Orwell

C. 1984

D. Book object

Solution

1. Step 1: Understand object creation with constructor

   The constructor sets Title to '1984' and Author to 'Orwell' when creating object b.

2. Step 2: Check the displayed property

   The code displays b.Title, which was set to '1984'.

3. Final Answer:

   1984 -> Option C

4. Quick Check:

   Displayed Title = '1984' [OK]

Hint: disp shows the property assigned in constructor [OK]

Common Mistakes:

• Confusing Title and Author properties
• Expecting object display instead of property
• Assuming error due to missing arguments

4. What is wrong with this MATLAB constructor method?

classdef Person
  properties
    Name
  end
  methods
    function obj = Person(name)
      Name = name;
    end
  end
end

medium

A. The property Name is not assigned to the object

B. Constructor name does not match class name

C. Missing semicolon after assignment

D. Properties block is missing

Solution

1. Step 1: Check property assignment in constructor

   The code assigns 'name' to a variable 'Name' but not to obj.Name, so the object property remains empty.

2. Step 2: Identify correct assignment syntax

   Correct syntax is obj.Name = name; to assign the input to the object's property.

3. Final Answer:

   The property Name is not assigned to the object -> Option A

4. Quick Check:

   Assign to obj.Name, not just Name [OK]

Hint: Always assign properties with obj.Property = value [OK]

Common Mistakes:

• Assigning to local variable instead of object property
• Ignoring case sensitivity in property names
• Forgetting to return obj from constructor

5. You want to create a MATLAB class Rectangle with properties Length and Width. Write a constructor that sets these properties and also calculates the Area property automatically. Which constructor method is correct?

hard

A. function obj = Rectangle(length, width)
obj.Length = length;
obj.Width = width;
obj.Area = length * width;
end

B. function obj = Rectangle(length, width)
obj.Length = length;
obj.Width = width;
Area = length * width;
end

C. function obj = Rectangle(length, width)
obj.Length = length;
obj.Width = width;
obj.Area = obj.Length + obj.Width;
end

D. function obj = Rectangle(length, width)
Length = length;
Width = width;
Area = length * width;
end

Solution

1. Step 1: Assign Length and Width to object properties

   function obj = Rectangle(length, width) obj.Length = length; obj.Width = width; obj.Area = length * width; end correctly assigns length and width to obj.Length and obj.Width.

2. Step 2: Calculate Area as product and assign to obj.Area

   function obj = Rectangle(length, width) obj.Length = length; obj.Width = width; obj.Area = length * width; end calculates area as length * width and assigns it to obj.Area, which is correct.

3. Final Answer:

   function obj = Rectangle(length, width) obj.Length = length; obj.Width = width; obj.Area = length * width; end -> Option A

4. Quick Check:

   Assign all properties to obj with correct calculation [OK]

Hint: Calculate and assign all properties to obj inside constructor [OK]

Common Mistakes:

• Assigning Area to local variable instead of obj.Area
• Adding Length and Width instead of multiplying for Area
• Not assigning Length and Width to obj