Properties and methods in MATLAB

Introduction

Properties store information about an object. Methods are actions the object can do. Together, they help organize code around real-world things.

When you want to group related data and functions about something, like a car or a bank account.

When you want to create reusable code that models real-world objects.

When you want to keep data safe and control how it is changed.

When you want to perform actions related to the data inside an object.

When you want to make your code easier to understand by bundling data and behavior.

Syntax

MATLAB

classdef ClassName
    properties
        Property1
        Property2
    end
    methods
        function obj = ClassName(input1, input2)
            obj.Property1 = input1;
            obj.Property2 = input2;
        end
        function output = methodName(obj, input)
            % method code here
        end
    end
end

Properties hold data about the object.

Methods are functions that work with the object data.

Examples

This defines a Person class with properties Name and Age, and a method greet that says hello.

MATLAB

classdef Person
    properties
        Name
        Age
    end
    methods
        function obj = Person(name, age)
            obj.Name = name;
            obj.Age = age;
        end
        function greet(obj)
            disp(["Hello, my name is " obj.Name]);
        end
    end
end

This Circle class has a Radius property and a method to calculate the area.

MATLAB

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

Sample Program

This program creates a Car object with make and year, then shows its information using a method.

MATLAB

classdef Car
    properties
        Make
        Year
    end
    methods
        function obj = Car(make, year)
            obj.Make = make;
            obj.Year = year;
        end
        function displayInfo(obj)
            disp(["Car make: " obj.Make]);
            disp(["Year: " num2str(obj.Year)]);
        end
    end
end

% Create a Car object
myCar = Car("Toyota", 2020);
% Call the method to display info
myCar.displayInfo();

Important Notes

Use obj.PropertyName to access properties inside methods.

Constructor methods have the same name as the class and create new objects.

Methods can change properties or perform calculations using them.

Summary

Properties hold data about an object.

Methods are functions that act on the object's data.

Together, they help model real-world things in code.

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