Class definition syntax in MATLAB

Introduction

Classes help you organize data and actions together, like a blueprint for objects.

When you want to group related data and functions in one place.

When you need to create multiple similar objects with their own data.

When you want to model real-world things like cars or books in your code.

Syntax

MATLAB

classdef ClassName
    properties
        % Define variables here
    end
    methods
        % Define functions here
    end
end

classdef starts the class definition.

properties block holds data variables.

Examples

A simple class with two properties: Name and Age.

MATLAB

classdef Person
    properties
        Name
        Age
    end
end

This class has a constructor method to set Name and Age when creating an object.

MATLAB

classdef Person
    properties
        Name
        Age
    end
    methods
        function obj = Person(name, age)
            obj.Name = name;
            obj.Age = age;
        end
    end
end

Sample Program

This program defines a Car class with Make and Year properties, a constructor to set them, and a method to display the info. Then it creates a Car object and shows its details.

MATLAB

classdef Car
    properties
        Make
        Year
    end
    methods
        function obj = Car(make, year)
            obj.Make = make;
            obj.Year = year;
        end
        function displayInfo(obj)
            fprintf('Car make: %s, Year: %d\n', obj.Make, obj.Year);
        end
    end
end

% Create a Car object
myCar = Car('Toyota', 2020);
myCar.displayInfo();

Important Notes

Class names must start with a letter and contain only letters, digits, or underscores.

Methods inside the class can access and change properties.

Summary

Use classdef to start a class.

Put data inside properties and functions inside methods.

Classes help organize related data and actions together.

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