Class definition syntax in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MATLAB class property access?

Consider this MATLAB class definition and usage:

classdef MyClass
    properties
        Value = 10;
    end
end
obj = MyClass();
disp(obj.Value);

What will be printed?

MATLAB

classdef MyClass
    properties
        Value = 10;
    end
end
obj = MyClass();
disp(obj.Value);

A Empty array []

B 0

C 10

D Error: Undefined property 'Value'.

💡 Hint

Look at the default value assigned to the property.

The property Value is initialized to 10. Creating an object sets this property to 10, so disp(obj.Value) prints 10.

❓ Predict Output

intermediate

What error does this MATLAB class code produce?

Examine this MATLAB class code:

classdef Test
    properties
        x
    end
    methods
        function obj = Test(val)
            obj.x = val;
        end
    end
end
obj = Test();

What happens when running this code?

MATLAB

classdef Test
    properties
        x
    end
    methods
        function obj = Test(val)
            obj.x = val;
        end
    end
end
obj = Test();

A Error: Not enough input arguments.

B Error: Undefined function or variable 'val'.

C No error, obj.x is empty.

D Error: Invalid class constructor syntax.

💡 Hint

Check the constructor call and its input arguments.

The constructor Test(val) requires one input argument. Calling Test() without arguments causes a 'Not enough input arguments' error.

❓ Predict Output

advanced

What is the output of this MATLAB class method call?

Given this MATLAB class:

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

What is printed?

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();
disp(c.Count);

A 1

B 0

C Error: Attempt to reference field of non-structure array.

D Error: Method must return an object.

💡 Hint

Remember MATLAB classes are value classes by default.

The method increment returns the updated object. Assigning it back to c updates the property. So disp(c.Count) prints 1.

❓ Predict Output

advanced

What error does this MATLAB class definition cause?

Look at this MATLAB class snippet:

classdef Sample
    properties (Constant)
        PI = 3.14;
    end
end
obj = Sample();
disp(obj.PI);

What happens when running this code?

MATLAB

classdef Sample
    properties (Constant)
        PI = 3.14;
    end
end
obj = Sample();
disp(obj.PI);

A Error: Missing semicolon after property definition.

B Error: Constant properties must be initialized with constant expressions.

C Error: Cannot access constant property through object.

D 3.14

💡 Hint

How do you access constant properties in MATLAB?

Constant properties can be accessed via object or class name. Here, disp(obj.PI) prints 3.14.

❓ Predict Output

expert

What is the output of this MATLAB class inheritance example?

Consider these MATLAB classes:

classdef Base
    methods
        function greet(~)
            disp('Hello from Base');
        end
    end
end

classdef Derived < Base
    methods
        function greet(obj)
            disp('Hello from Derived');
            greet@Base(obj);
        end
    end
end

obj = Derived();
obj.greet();

What is printed?

MATLAB

classdef Base
    methods
        function greet(~)
            disp('Hello from Base');
        end
    end
end

classdef Derived < Base
    methods
        function greet(obj)
            disp('Hello from Derived');
            greet@Base(obj);
        end
    end
end

obj = Derived();
obj.greet();

A

Hello from Base
Hello from Derived

B

Hello from Derived
Hello from Base

C Error: Method greet@Base not found.

D Hello from Derived

💡 Hint

Check how superclass methods are called from subclass.

The greet method in Derived first prints its message, then calls the greet method of Base. So both messages print in order.

Practice

1. Which keyword is used to start a class definition in MATLAB?

easy

A. class

B. classdef

C. defineclass

D. objectdef

Solution

1. Step 1: Recall MATLAB class syntax

   In MATLAB, classes always start with the keyword classdef.

2. Step 2: Compare options

   Only classdef uses classdef, the correct keyword for defining classes.

3. Final Answer:

   classdef -> Option B

4. Quick Check:

   Class start keyword = classdef [OK]

Hint: Remember: MATLAB classes start with 'classdef' keyword [OK]

Common Mistakes:

• Using 'class' instead of 'classdef'
• Confusing with other languages' syntax
• Forgetting to use any keyword

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

easy

A. attributes
x
end

B. property
x
end

C. properties
x
end

D. props
x
end

Solution

1. Step 1: Recall properties block syntax

   In MATLAB, class data members are defined inside a properties block.

2. Step 2: Check each option

   Only properties x end uses the correct keyword properties to define data.

3. Final Answer:

   properties block -> Option C

4. Quick Check:

   Data inside 'properties' block [OK]

Hint: Use 'properties' keyword to define class data [OK]

Common Mistakes:

• Using 'property' instead of 'properties'
• Using 'props' or 'attributes' which are invalid
• Placing properties outside the classdef block

3. What will be the output of this MATLAB code?

classdef MyClass
    properties
        Value = 10;
    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(25);
disp(obj.getValue());

medium

A. 25

B. 10

C. Error: Cannot assign value

D. 0

Solution

1. Step 1: Understand property initialization

   The property Value starts at 10.

2. Step 2: Follow method calls

   The method setValue sets Value to 25, then getValue returns it.

3. Final Answer:

   25 -> Option A

4. Quick Check:

   Value updated to 25 and displayed [OK]

Hint: Setter updates property; getter returns updated value [OK]

Common Mistakes:

• Forgetting to assign returned object from setter
• Expecting initial value instead of updated
• Assuming direct property change without methods

4. Identify the error in this MATLAB class definition:

classdef Sample
    properties
        Data
    end
    methods
        function obj = Sample(val)
            Data = val;
        end
    end
end

medium

A. Missing semicolon after Data property

B. Incorrect classdef keyword usage

C. Properties block should be inside methods

D. Constructor does not assign value to object property

Solution

1. Step 1: Check constructor assignment

   The constructor assigns val to Data without obj., so it does not update the object property.

2. Step 2: Understand correct assignment

   It should be obj.Data = val; to update the property.

3. Final Answer:

   Constructor does not assign value to object property -> Option D

4. Quick Check:

   Use 'obj.Data = val;' in constructor [OK]

Hint: Use 'obj.Property = value;' inside methods [OK]

Common Mistakes:

• Assigning property without 'obj.' prefix
• Placing properties inside methods block
• Missing constructor function name matching class

5. How can you define a MATLAB class with a property Count that starts at 0 and a method increment that adds 1 to Count each time it is called? Choose the correct complete class definition.

hard

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

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

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

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

Solution

1. Step 1: Initialize property correctly

   The property Count must start at 0, so initialize it as Count = 0;.

2. Step 2: Define method with object output and update property

   The method increment must accept and return the object (obj) and update obj.Count by adding 1.

3. Step 3: Check each option

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

   correctly initializes Count, defines increment with obj input/output, and updates obj.Count. Others miss one or more of these.

4. Final Answer:

   Option A is the correct full class definition -> Option A

5. Quick Check:

   Property init + method with obj input/output =

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

   [OK]

Hint: Methods changing properties must return updated object [OK]

Common Mistakes:

• Not returning updated object from method
• Updating property without 'obj.' prefix
• Not initializing property value