Constructor methods in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constructor method in MATLAB classes?

A constructor method is a special function inside a MATLAB class that creates and initializes new objects of that class.

beginner

How do you name a constructor method in MATLAB?

The constructor method must have the same name as the class itself.

intermediate

What happens if you do not define a constructor method in a MATLAB class?

MATLAB provides a default constructor that creates an object with default property values.

intermediate

How can you pass input arguments to a constructor method in MATLAB?

You define input parameters in the constructor method to accept values when creating an object, then assign those values to object properties.

beginner

Show a simple example of a MATLAB class with a constructor method that sets a property 'Name'.

classdef Person properties Name end methods function obj = Person(name) if nargin > 0 obj.Name = name; end end end end

What must the constructor method name match in MATLAB classes?

A The superclass name

B The first property name

C The file name

D The class name

In MATLAB, the constructor method must have the same name as the class.

What does the default constructor do if you don't define one?

A Deletes the class

B Throws an error

C Creates an object with default property values

D Requires manual initialization

MATLAB automatically creates a default constructor that initializes properties with default values.

How do you check if input arguments are passed to a constructor in MATLAB?

A Using nargin inside the constructor

B Using nargout inside the constructor

C Using isempty on the class name

D Using ischar on the input

The nargin function tells how many input arguments were passed to the constructor.

Which keyword defines a constructor method in MATLAB?

A constructor

B function with the class name

C new

D init

The constructor is defined as a function with the same name as the class.

What is the purpose of a constructor method?

A To create and initialize an object

B To delete an object

C To display object properties

D To inherit from another class

Constructors create new objects and set their initial state.

Explain what a constructor method is and how it is used in MATLAB classes.

Think about how you make a new object and set its starting values.

Describe how you would write a constructor that accepts input to set a property value.

Consider how to pass information when creating the object.

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