Access control in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MATLAB class property access?

Consider the following MATLAB class definition and usage:

classdef MyClass
    properties (Access = private)
        secret = 42;
    end
    methods
        function val = getSecret(obj)
            val = obj.secret;
        end
    end
end

obj = MyClass();
disp(obj.getSecret());

What will be printed when running this code?

MATLAB

classdef MyClass
    properties (Access = private)
        secret = 42;
    end
    methods
        function val = getSecret(obj)
            val = obj.secret;
        end
    end
end

obj = MyClass();
disp(obj.getSecret());

A 42

B Error: Property 'secret' is not accessible.

C 0

D Empty output

💡 Hint

Private properties can be accessed inside class methods.

The property secret is private, so it cannot be accessed directly outside the class. However, the method getSecret is inside the class and can access it. Thus, calling obj.getSecret() returns 42.

❓ Predict Output

intermediate

What happens when accessing a protected property from outside the class?

Given this MATLAB class:

classdef MyClass
    properties (Access = protected)
        data = 10;
    end
end

obj = MyClass();
disp(obj.data);

What will be the result of running this code?

MATLAB

classdef MyClass
    properties (Access = protected)
        data = 10;
    end
end

obj = MyClass();
disp(obj.data);

A 10

B Error: Property 'data' is not accessible.

C Empty output

D 0

💡 Hint

Protected properties are accessible only within the class and subclasses.

The property data is protected, so it cannot be accessed directly from outside the class. Attempting to do so causes an error.

🔧 Debug

advanced

Why does this code cause an error when accessing a private method?

Examine this MATLAB class and script:

classdef MyClass
    methods (Access = private)
        function secretMethod(obj)
            disp('Secret method called');
        end
    end
    methods
        function callSecret(obj)
            obj.secretMethod();
        end
    end
end

obj = MyClass();
obj.secretMethod();

Why does the last line cause an error?

MATLAB

classdef MyClass
    methods (Access = private)
        function secretMethod(obj)
            disp('Secret method called');
        end
    end
    methods
        function callSecret(obj)
            obj.secretMethod();
        end
    end
end

obj = MyClass();
obj.secretMethod();

A Because private methods cannot be called from outside the class.

B Because the method name is misspelled.

C Because the object was not created properly.

D Because the method is static and needs to be called differently.

💡 Hint

Think about where private methods can be accessed.

Private methods are only accessible inside the class methods. Calling obj.secretMethod() from outside the class causes an access error.

❓ Predict Output

advanced

What is the output of this subclass accessing a protected property?

Consider these MATLAB classes:

classdef ParentClass
    properties (Access = protected)
        value = 5;
    end
end

classdef ChildClass < ParentClass
    methods
        function v = getValue(obj)
            v = obj.value;
        end
    end
end

obj = ChildClass();
disp(obj.getValue());

What will be printed?

MATLAB

classdef ParentClass
    properties (Access = protected)
        value = 5;
    end
end

classdef ChildClass < ParentClass
    methods
        function v = getValue(obj)
            v = obj.value;
        end
    end
end

obj = ChildClass();
disp(obj.getValue());

A Empty output

B 0

C 5

D Error: Property 'value' is not accessible.

💡 Hint

Protected properties are accessible in subclasses.

The subclass ChildClass inherits the protected property value and can access it inside its methods. So getValue returns 5.

🧠 Conceptual

expert

Which access level allows property access only within the defining class and its subclasses?

In MATLAB, which property access level restricts access to the class itself and any subclasses, but not from outside these classes?

A global

B private

C public

D protected

💡 Hint

Think about access levels that allow inheritance but restrict external access.

Protected access allows properties and methods to be accessed within the class and subclasses only. Private restricts access to the class itself only. Public allows access from anywhere. Global is not an access level in MATLAB classes.

Practice

1. What does the private access modifier do in a MATLAB class?

easy

A. It makes properties or methods accessible only in the command window.

B. It allows access to properties or methods from anywhere in the program.

C. It allows access only to child classes and the class itself.

D. It restricts access to class properties or methods only within the class itself.

Solution

1. Step 1: Understand the role of private in MATLAB classes

   The private keyword restricts access to properties or methods so that only the class itself can use them.

2. Step 2: Compare with other access modifiers

   public allows access from anywhere, protected allows access from child classes, but private is the most restrictive.

3. Final Answer:

   It restricts access to class properties or methods only within the class itself. -> Option D

4. Quick Check:

   private means class-only access [OK]

Hint: Private means only the class itself can access [OK]

Common Mistakes:

• Confusing private with public access
• Thinking private allows child class access
• Assuming private is accessible from outside

2. Which of the following is the correct syntax to declare a protected property named data in a MATLAB class?

easy

A. properties protected
data
end

B. properties (protected)
data
end

C. properties (public)
data
end

D. properties (private)
data
end

Solution

1. Step 1: Recall MATLAB property access syntax

   Access modifiers are specified in parentheses after the properties keyword, e.g., properties (protected).

2. Step 2: Identify correct syntax for protected

   The correct syntax uses parentheses: properties (protected). properties protected data end misses parentheses, so it's invalid.

3. Final Answer:

   properties (protected) data end -> Option B

4. Quick Check:

   Use parentheses for access modifiers [OK]

Hint: Use parentheses after properties for access [OK]

Common Mistakes:

• Omitting parentheses around access modifier
• Using wrong access modifier for the question
• Placing access modifier outside properties block

3. Consider this MATLAB class snippet:

classdef MyClass
    properties (Access = private)
        secret = 42;
    end
    methods
        function val = getSecret(obj)
            val = obj.secret;
        end
    end
end

obj = MyClass();
disp(obj.secret);

What happens when you run this code?

medium

A. It causes an error because secret is private.

B. It displays 42.

C. It displays an empty value.

D. It displays the class name.

Solution

1. Step 1: Identify access level of secret

   The property secret is declared private, so it cannot be accessed directly outside the class.

2. Step 2: Analyze the code trying to access secret

   The line disp(obj.secret); tries to access the private property directly, which is not allowed and causes an error.

3. Final Answer:

   It causes an error because secret is private. -> Option A

4. Quick Check:

   Private properties block outside access [OK]

Hint: Private properties can't be accessed outside class [OK]

Common Mistakes:

• Assuming private properties can be accessed directly
• Confusing method access with property access
• Ignoring access modifiers in property declaration

4. This MATLAB class has an error. Find and fix it:

classdef Parent
    properties (Access = protected)
        value
    end
end

classdef Child < Parent
    methods
        function setValue(obj, val)
            obj.value = val;
        end
    end
end

c = Child();
c.setValue(10);
disp(c.value);

medium

A. Add a getter method in Child to access value.

B. Make setValue a static method.

C. Change value access to public.

D. Remove inheritance from Child.

Solution

1. Step 1: Understand protected access

   The property value is protected, so Child can set it, but outside code cannot access it directly.

2. Step 2: Identify why disp(c.value) fails

   Outside code tries to access value directly, which is not allowed for protected properties.

3. Step 3: Fix by adding a getter method

   Adding a method in Child to return value allows controlled access without changing access level.

4. Final Answer:

   Add a getter method in Child to access value. -> Option A

5. Quick Check:

   Use getter for protected property access outside class [OK]

Hint: Use getter methods to access protected properties outside class [OK]

Common Mistakes:

• Trying to access protected property directly outside class
• Changing access to public unnecessarily
• Not using inheritance properly

5. You want to create a MATLAB class BankAccount where the balance is private, but child classes can update it. You also want to allow anyone to check the balance. Which access control setup is correct?

hard

A. Declare balance as public; anyone can read and update balance.

B. Declare balance as private and provide a public getter method; child classes cannot update balance directly.

C. Declare balance as protected and provide a public getter method; child classes can update balance, others can only read it.

D. Declare balance as private with no getter; only the class can access balance.

Solution

1. Step 1: Identify access needs for balance

   Balance should be hidden from outside for updates but readable by anyone, and updatable by child classes.

2. Step 2: Choose access modifier and methods

   Protected allows child classes to update balance. A public getter method allows anyone to read balance safely.

3. Final Answer:

   Declare balance as protected and provide a public getter method; child classes can update balance, others can only read it. -> Option C

4. Quick Check:

   Protected + public getter fits requirements [OK]

Hint: Protected for update by children, public getter for read [OK]

Common Mistakes:

• Making balance public allowing unwanted updates
• Using private without getter blocking read access
• Not providing a getter method for balance