Constructor methods in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple MATLAB class constructor

What is the output of the following MATLAB code when creating an object of class MyClass?

MATLAB

classdef MyClass
    properties
        Value
    end
    methods
        function obj = MyClass(val)
            if nargin > 0
                obj.Value = val;
            else
                obj.Value = 0;
            end
        end
    end
end

obj = MyClass(5);
disp(obj.Value);

A Empty array []

B 0

C Error: Undefined function or variable 'val'.

D 5

💡 Hint

Look at how the constructor assigns the property Value when an argument is passed.

The constructor checks if an input argument is given (nargin > 0). Since 5 is passed, it sets Value to 5. The disp prints 5.

❓ Predict Output

intermediate

Default property value with no constructor input

What will be displayed when the following MATLAB code runs?

MATLAB

classdef Sample
    properties
        Number = 10;
    end
    methods
        function obj = Sample(num)
            if nargin > 0
                obj.Number = num;
            end
        end
    end
end

obj = Sample();
disp(obj.Number);

A Error: Not enough input arguments.

B 0

C 10

D Empty array []

💡 Hint

Consider the default value assigned to the property and what happens if no input is given.

Since no input is passed, the constructor does not change the default property value 10. So disp prints 10.

🔧 Debug

advanced

Identify the error in this MATLAB constructor

What error will occur when running this MATLAB class code?

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');

A Error: Not enough input arguments.

B Error: Too many input arguments.

C No error, object created with Name='Alice' and Age=[]

D Error: Undefined function or variable 'age'.

💡 Hint

Check how many arguments the constructor expects versus how many are given.

The constructor expects two inputs but only one is provided, causing a 'Not enough input arguments' error.

🧠 Conceptual

advanced

Behavior of handle class constructor in MATLAB

Consider this MATLAB class that inherits from handle. What will be the output of the code below?

MATLAB

classdef Counter < handle
    properties
        Count = 0;
    end
    methods
        function obj = Counter(start)
            if nargin > 0
                obj.Count = start;
            end
        end
        function increment(obj)
            obj.Count = obj.Count + 1;
        end
    end
end

c1 = Counter(5);
c2 = c1;
c2.increment();
disp(c1.Count);

A Error: Undefined method 'increment' for input arguments.

B 6

C 5

D 0

💡 Hint

Remember that handle classes share the same object reference.

Since Counter inherits from handle, c1 and c2 point to the same object. Incrementing c2 changes c1.Count to 6.

❓ Predict Output

expert

Output of constructor with varargin and property assignment

What is the output of this MATLAB code when creating an object with obj = Config('Mode','Auto','Level',3);?

MATLAB

classdef Config
    properties
        Mode = 'Manual';
        Level = 1;
    end
    methods
        function obj = Config(varargin)
            for k = 1:2:length(varargin)
                prop = varargin{k};
                val = varargin{k+1};
                if isprop(obj, prop)
                    obj.(prop) = val;
                end
            end
        end
    end
end

obj = Config('Mode','Auto','Level',3);
disp([obj.Mode, ' ', num2str(obj.Level)]);

A Auto 3

B Manual 1

C Error: Invalid property name.

D Auto 1

💡 Hint

Look at how the constructor uses varargin to assign properties dynamically.

The constructor loops through pairs of property names and values, updating Mode to 'Auto' and Level to 3. The disp prints 'Auto 3'.

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