Properties and methods in MATLAB - Step-by-Step Execution

Concept Flow - Properties and methods

Define Class

↓

Add Properties

↓

Add Methods

↓

Create Object

↓

Access Properties

↓

Call Methods

↓

Use Object Behavior

This flow shows how a MATLAB class is defined with properties and methods, then how an object is created and used by accessing properties and calling methods.

Execution Sample

MATLAB

classdef Car
  properties
    Color
    Speed = 0;
  end
  methods
    function obj = accelerate(obj, amount)
      obj.Speed = obj.Speed + amount;
    end
  end
end

myCar = Car;
myCar.Color = 'Red';
myCar = myCar.accelerate(10);
myCar.Speed

Defines a Car class with properties Color and Speed, a method accelerate to increase Speed, creates a Car object, sets Color, accelerates by 10, then outputs Speed.

Execution Table

Step	Action	Object Property	Value Before	Value After	Output
1	Create myCar object	Speed	N/A	0	N/A
2	Set myCar.Color = 'Red'	Color	N/A	'Red'	N/A
3	Call myCar.accelerate(10)	Speed	0	10	N/A
4	Access myCar.Speed	Speed	10	10	10
5	End of execution	-	-	-	-

💡 Reached end of code, final Speed is 10

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
myCar.Speed	N/A	0	0	10	10
myCar.Color	N/A	N/A	'Red'	'Red'	'Red'

Key Moments - 2 Insights

Why do we assign the result of accelerate back to myCar?

In MATLAB, methods that modify object properties often return a new object. See execution_table step 3 where accelerate returns the updated object, so we must assign it back to myCar.

Why is Speed 0 when the object is created?

Because Speed has a default value of 0 in the properties block. See execution_table step 1 where Speed is initialized to 0.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the Speed value after calling accelerate?

A 0

B 10

C Undefined

D 5

💡 Hint

Check the 'Value After' column for Speed in step 3 of execution_table.

At which step is the Color property set to 'Red'?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column in execution_table for setting Color.

If we did not assign the result of accelerate back to myCar, what would myCar.Speed be after step 3?

A 0

B Undefined

C 10

D Error

💡 Hint

Refer to key_moments about why assignment is needed after calling accelerate.

Concept Snapshot

MATLAB classes have properties (data) and methods (functions).
Properties can have default values.
Methods can change properties but often return a new object.
Create objects with ClassName; set properties with dot notation.
Call methods with obj = obj.method(args) to update object.
Access properties with obj.PropertyName.

Full Transcript

This example shows how to define a MATLAB class with properties and methods. The Car class has Color and Speed properties. Speed starts at 0 by default. The accelerate method adds to Speed and returns the updated object. We create a Car object called myCar, set its Color to 'Red', then call accelerate(10) to increase Speed. We must assign the result back to myCar because MATLAB methods return a new object. Finally, accessing myCar.Speed shows 10. This teaches how properties hold data and methods change it, and how to use objects in MATLAB.

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