Properties and methods in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method accessing property

What is the output of this MATLAB code when calling the method getArea?

MATLAB

classdef Circle
    properties
        Radius
    end
    methods
        function obj = Circle(r)
            obj.Radius = r;
        end
        function a = getArea(obj)
            a = pi * obj.Radius^2;
        end
    end
end

c = Circle(3);
disp(c.getArea())

A Error: Undefined function or variable 'Radius'

B 28.2743

C 9

D 18.8496

💡 Hint

Recall the formula for the area of a circle is pi times radius squared.

The method getArea calculates the area using pi * Radius^2. With Radius = 3, area = pi * 9 = 28.2743 approximately.

❓ Predict Output

intermediate

Value of property after method call

What is the value of the property Count after running this code?

MATLAB

classdef Counter
    properties
        Count = 0;
    end
    methods
        function obj = increment(obj)
            obj.Count = obj.Count + 1;
        end
    end
end

c = Counter();
c = c.increment();
c = c.increment();
disp(c.Count)

A Error: Attempt to modify property of a value class without assignment

B 0

C 1

D 2

💡 Hint

Each call to increment adds 1 to Count.

The increment method adds 1 to Count each time. After two calls, Count is 2.

❓ Predict Output

advanced

Output of method with dependent property

What is the output of obj.Area after creating the object obj = Rectangle(4,5);?

MATLAB

classdef Rectangle
    properties
        Width
        Height
    end
    properties (Dependent)
        Area
    end
    methods
        function obj = Rectangle(w,h)
            obj.Width = w;
            obj.Height = h;
        end
        function a = get.Area(obj)
            a = obj.Width * obj.Height;
        end
    end
end

obj = Rectangle(4,5);
disp(obj.Area)

A Error: Property 'Area' is not settable

B 9

C 20

D 0

💡 Hint

Dependent properties compute their value on access.

The dependent property Area calculates Width * Height, which is 4 * 5 = 20.

❓ Predict Output

advanced

Error type when accessing private property

What error occurs when running this code?

MATLAB

classdef Secret
    properties (Access = private)
        Data = 42;
    end
    methods
        function val = getData(obj)
            val = obj.Data;
        end
    end
end

s = Secret();
disp(s.Data);

A Error: No appropriate method, property, or field 'Data' for class 'Secret'.

B Error: Undefined function or variable 'Data'.

C Error: Attempt to access private property 'Data' from outside the class.

D 42

💡 Hint

Private properties cannot be accessed directly from outside the class.

Accessing a private property directly from outside the class causes an error saying no such property is accessible.

🧠 Conceptual

expert

Number of items in property after method call

Consider this MATLAB class that stores names in a property. After running the code below, how many names are stored in the Names property?

MATLAB

classdef NameList
    properties
        Names = {};
    end
    methods
        function obj = addName(obj, newName)
            obj.Names{end+1} = newName;
        end
    end
end

list = NameList();
list = list.addName('Alice');
list = list.addName('Bob');
list = list.addName('Charlie');
disp(length(list.Names))

A 3

B 0

C 1

D Error: Cell contents reference from a non-cell array object.

💡 Hint

Each call to addName adds one name to the cell array.

The addName method appends a new name to the cell array Names. After three calls, there are 3 names stored.

Practice

1. In MATLAB, what is the main purpose of properties in a class definition?

easy

A. To define functions that perform actions

B. To create loops inside the class

C. To store data related to the object

D. To handle errors during execution

Solution

1. Step 1: Understand what properties represent

   Properties in MATLAB classes are used to hold data or attributes about the object.

2. Step 2: Differentiate properties from methods

   Methods are functions that act on the data, not the data itself.

3. Final Answer:

   To store data related to the object -> Option C

4. Quick Check:

   Properties = Data storage [OK]

Hint: Properties always hold data, methods perform actions [OK]

Common Mistakes:

• Confusing properties with methods
• Thinking properties are for functions
• Assuming properties handle errors

2. Which of the following is the correct way to define a method inside a MATLAB class?

easy

A. methods
function obj = setValue(obj, val)
obj.Value = val;
end
end

B. properties
function obj = setValue(obj, val)
obj.Value = val;
end
end

C. function setValue(obj, val)
obj.Value = val;
end

D. methods
obj.Value = val;
end

Solution

1. Step 1: Identify method block syntax

   Methods in MATLAB classes are defined inside a methods block.

2. Step 2: Check function definition inside methods

   The function must have the object as input and output if modifying properties.

3. Final Answer:

   methods function obj = setValue(obj, val) obj.Value = val; end end -> Option A

4. Quick Check:

   Methods block + function syntax = methods function obj = setValue(obj, val) obj.Value = val; end end [OK]

Hint: Methods go inside 'methods' block with function keyword [OK]

Common Mistakes:

• Placing functions inside properties block
• Omitting the methods keyword
• Not returning the modified object

3. Consider the following MATLAB class snippet:

classdef MyClass
  properties
    Value
  end
  methods
    function obj = setValue(obj, val)
      obj.Value = val;
    end
    function v = getValue(obj)
      v = obj.Value;
    end
  end
end

obj = MyClass();
obj = obj.setValue(10);
disp(obj.getValue());

What will be the output when this code runs?

medium

A. 0

B. obj.Value

C. Error: Undefined function or variable 'Value'

D. 10

Solution

1. Step 1: Understand method calls and property assignment

   The method setValue sets the property Value to 10 and returns the updated object.

2. Step 2: Check the output of getValue

   The getValue method returns the current value of the property Value, which is 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Property set and get returns 10 [OK]

Hint: Methods update and return object; get method returns stored value [OK]

Common Mistakes:

• Forgetting to assign returned object from setValue
• Expecting property access without method
• Confusing property name with variable

4. Identify the error in this MATLAB class method definition:

methods
  function setValue(obj, val)
    obj.Value = val;
  end
end

medium

A. The method must return the modified object

B. The method name is invalid

C. Properties block is missing

D. The function keyword is not allowed inside methods

Solution

1. Step 1: Check method signature for modifying properties

   In MATLAB, when a method modifies object properties, it must return the updated object.

2. Step 2: Identify missing return value

   The method setValue does not return obj, so changes won't persist.

3. Final Answer:

   The method must return the modified object -> Option A

4. Quick Check:

   Modify method must return object [OK]

Hint: Return updated object when changing properties [OK]

Common Mistakes:

• Not returning the object after modification
• Assuming properties change without return
• Confusing function keyword usage

5. You want to create a MATLAB class BankAccount with a property Balance and a method deposit that adds money to the balance only if the amount is positive. Which of the following method definitions correctly implements this?

hard

A. methods
function deposit(obj, amount)
if amount > 0
obj.Balance = obj.Balance + amount;
end
end
end

B. methods
function obj = deposit(obj, amount)
if amount > 0
obj.Balance = obj.Balance + amount;
end
end
end

C. methods
function obj = deposit(amount)
if amount > 0
obj.Balance = obj.Balance + amount;
end
end
end

D. methods
function obj = deposit(obj, amount)
obj.Balance = obj.Balance + amount;
end
end

Solution

1. Step 1: Check method signature for modifying object

   The method must accept the object and amount, and return the updated object.

2. Step 2: Verify condition for positive amount

   The method correctly adds amount only if it is greater than zero.

3. Step 3: Identify incorrect options

   methods function deposit(obj, amount) if amount > 0 obj.Balance = obj.Balance + amount; end end end does not return the object, so changes won't persist. methods function obj = deposit(amount) if amount > 0 obj.Balance = obj.Balance + amount; end end end misses the object input. methods function obj = deposit(obj, amount) obj.Balance = obj.Balance + amount; end end adds amount without checking if positive.

4. Final Answer:

   methods function obj = deposit(obj, amount) if amount > 0 obj.Balance = obj.Balance + amount; end end end -> Option B

5. Quick Check:

   Return updated object and check amount > 0 [OK]

Hint: Return object and check positive amount before adding [OK]

Common Mistakes:

• Not returning updated object
• Missing object input argument
• Not checking if amount is positive