Constructor methods in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a constructor method named MyClass.

MATLAB

classdef MyClass
    methods
        function obj = [1]()
            obj.value = 0;
        end
    end
end

Drag options to blanks, or click blank then click option'

AMyConstructor

Bconstructor

Cinit

DMyClass

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different function name than the class name for the constructor.

Forgetting to define the constructor as a method inside the class.

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

2fill in blank

medium

Complete the constructor to accept an input argument val and assign it to the property value.

MATLAB

classdef MyClass
    properties
        value
    end
    methods
        function obj = MyClass([1])
            if nargin > 0
                obj.value = val;
            else
                obj.value = 0;
            end
        end
    end
end

Drag options to blanks, or click blank then click option'

Aval

Bvalue

Cobj

Dinput

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different input argument name than the one assigned to the property.

Not including any input argument in the constructor.

The constructor takes 'val' as input to assign it to the property 'value'.

3fill in blank

hard

Fix the error in the constructor method to properly initialize the object.

MATLAB

classdef MyClass
    properties
        value
    end
    methods
        function obj = MyClass(val)
            if nargin == 0
                obj.value = 0;
            else
                obj.[1] = val;
            end
        end
    end
end

Drag options to blanks, or click blank then click option'

Aobj

Bval

Cvalue

Dvalue()

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning to obj.val instead of obj.value.

Using parentheses after the property name.

The property 'value' must be assigned the input 'val', so use obj.value = val.

4fill in blank

hard

Fill both blanks to create a constructor that sets value and a new property name.

MATLAB

classdef MyClass
    properties
        value
        name
    end
    methods
        function obj = MyClass([1], [2])
            if nargin == 2
                obj.value = val;
                obj.name = nm;
            else
                obj.value = 0;
                obj.name = '';
            end
        end
    end
end

Drag options to blanks, or click blank then click option'

Aval

Bnm

Cname

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using property names as input argument names.

Mixing up the order of input arguments.

The constructor takes two inputs: 'val' for value and 'nm' for name.

5fill in blank

hard

Fill all three blanks to create a constructor that sets value, name, and active properties.

MATLAB

classdef MyClass
    properties
        value
        name
        active
    end
    methods
        function obj = MyClass([1], [2], [3])
            if nargin == 3
                obj.value = val;
                obj.name = nm;
                obj.active = act;
            else
                obj.value = 0;
                obj.name = '';
                obj.active = false;
            end
        end
    end
end

Drag options to blanks, or click blank then click option'

Aval

Bnm

Cact

Dactive

Attempts:

3 left

💡 Hint

Common Mistakes

Using property names as input argument names.

Forgetting to include all three input arguments.

The constructor inputs are 'val', 'nm', and 'act' to set the properties value, name, and active.

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