Class definition syntax in MATLAB - Step-by-Step Execution

Concept Flow - Class definition syntax

Start class file

↓

Write classdef keyword

↓

Name the class

↓

Define properties block

↓

Define methods block

↓

End classdef

↓

Save and use class

This flow shows how MATLAB reads a class file: starting with the classdef keyword, naming the class, defining properties and methods, then ending the class.

Execution Sample

MATLAB

classdef MyClass
    properties
        Value
    end
    methods
        function obj = MyClass(val)
            obj.Value = val;
        end
    end
end

Defines a simple MATLAB class named MyClass with one property and a constructor method.

Execution Table

Step	Code Line	Action	Result
1	classdef MyClass	Start class definition	Class named MyClass created
2	properties	Begin properties block	Ready to define properties
3	Value	Define property 'Value'	Property 'Value' added
4	end	End properties block	Properties block closed
5	methods	Begin methods block	Ready to define methods
6	function obj = MyClass(val)	Define constructor method	Constructor method started
7	obj.Value = val;	Assign input val to property Value	Property 'Value' set to val
8	end	End constructor method	Constructor method closed
9	end	End methods block	Methods block closed
10	end	End classdef	Class definition complete

💡 All class components defined and classdef ended, class ready to use

Variable Tracker

Variable	Start	After Step 7	Final
obj.Value	undefined	val (input value)	val (input value)

Key Moments - 3 Insights

Why do we use 'classdef' at the start?

The 'classdef' keyword tells MATLAB this file defines a class. See execution_table step 1 where classdef starts the class.

What is the purpose of the 'properties' block?

The 'properties' block lists data the class holds. Step 3 shows adding 'Value' as a property.

How does the constructor method work?

The constructor method sets initial property values when creating an object. Step 7 assigns the input 'val' to 'obj.Value'.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 3?

A The class definition ends

B A property named 'Value' is defined

C The constructor method starts

D The methods block ends

💡 Hint

Check the 'Action' column at step 3 in the execution_table

At which step does the constructor assign the input value to the property?

A Step 6

B Step 4

C Step 7

D Step 9

💡 Hint

Look for 'Assign input val to property Value' in the execution_table

If we remove the 'end' at step 10, what happens?

A Class definition is incomplete and causes error

B Properties block will not close

C Constructor method will not run

D No effect, code runs fine

💡 Hint

Step 10 marks the end of classdef; missing it breaks the class closure

Concept Snapshot

MATLAB class syntax:
classdef ClassName
  properties
    % data fields
  end
  methods
    function obj = ClassName(args)
      % constructor
    end
  end
end
Use 'classdef' to start, 'properties' for data, 'methods' for functions, and 'end' to close blocks.

Full Transcript

This visual trace shows how MATLAB reads a class definition. It starts with the 'classdef' keyword naming the class. Then it reads the 'properties' block where data fields like 'Value' are defined. Next, it reads the 'methods' block, including the constructor function that sets initial property values. Each block ends with 'end'. Finally, the classdef ends with 'end', completing the class. Variables like obj.Value start undefined and get assigned in the constructor. Common confusions include why 'classdef' is needed, what properties do, and how the constructor works. The quiz checks understanding of these steps and the importance of closing 'end' statements.

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