Constructor methods in MATLAB - Step-by-Step Execution

Concept Flow - Constructor methods

Create object instance

↓

Call constructor method

↓

Initialize properties

↓

Return initialized object

↓

Use object

When you create an object, MATLAB calls the constructor method to set up initial property values and return the ready-to-use object.

Execution Sample

MATLAB

classdef Person
  properties
    Name
    Age
  end
  methods
    function obj = Person(name, age)
      obj.Name = name;
      obj.Age = age;
    end
  end
end

p = Person('Alice', 30);

This code defines a Person class with a constructor that sets Name and Age when creating a new Person object.

Execution Table

Step	Action	Input	Property Name	Property Age	Output
1	Call constructor Person('Alice', 30)	name='Alice', age=30	empty	empty	new Person object
2	Set obj.Name = 'Alice'	name='Alice'	Alice	empty	partially initialized object
3	Set obj.Age = 30	age=30	Alice	30	fully initialized object
4	Return object	obj with Name and Age	Alice	30	Person object with properties set

💡 Constructor finishes after setting all properties and returns the initialized object.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
obj.Name	empty	Alice	Alice	Alice
obj.Age	empty	empty	30	30

Key Moments - 2 Insights

Why do properties start as empty before the constructor runs?

Properties are empty initially because MATLAB creates the object shell first, then the constructor sets the actual values as shown in execution_table steps 1 and 2.

What happens if you forget to assign a property in the constructor?

That property remains empty or default because the constructor only sets what you assign, as seen in step 3 where Age is set explicitly.

Visual Quiz - 3 Questions

Look at the execution table, what is obj.Name after step 2?

A empty

B "30"

C "Alice"

D undefined

💡 Hint

Check the 'Property Name' column in row for step 2 in execution_table.

At which step does obj.Age get its value?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Property Age' column in execution_table rows.

If the constructor did not assign obj.Name, what would be its final value?

A empty

B "Alice"

C null

D error

💡 Hint

Refer to key_moments about unassigned properties and variable_tracker for obj.Name.

Concept Snapshot

Constructor methods in MATLAB:
- Defined inside classdef with same name as class
- Called automatically when creating object
- Initialize object properties
- Return fully set object
- Properties start empty until assigned

Full Transcript

In MATLAB, when you create an object from a class, the constructor method runs automatically. This method has the same name as the class and sets initial values for the object's properties. At first, properties are empty. Then the constructor assigns values step-by-step. Finally, the fully initialized object is returned and ready to use. If a property is not assigned, it stays empty. This process ensures your object starts with the right data.

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