Access control in MATLAB - Time & Space Complexity

Time Complexity: Access control

O(n)

Understanding Time Complexity

Access control often involves checking permissions before allowing actions. Understanding how long these checks take helps us write efficient programs.

We want to know how the time to check access grows as the number of users or permissions increases.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function allowed = checkAccess(user, resource, permissions)
    allowed = false;
    for i = 1:length(permissions)
        if strcmp(permissions(i).user, user) && strcmp(permissions(i).resource, resource)
            allowed = true;
            break;
        end
    end
end

This code checks if a user has permission to access a resource by scanning a list of permissions.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Loop through the permissions list to find a matching user and resource.
• How many times: Up to the total number of permissions (length of permissions array).

How Execution Grows With Input

As the number of permissions grows, the time to check access grows roughly the same way.

Input Size (n)	Approx. Operations
10	Up to 10 comparisons
100	Up to 100 comparisons
1000	Up to 1000 comparisons

Pattern observation: The time grows directly with the number of permissions checked.

Final Time Complexity

Time Complexity: O(n)

This means the time to check access grows linearly with the number of permissions.

Common Mistake

[X] Wrong: "Checking access is always instant, no matter how many permissions there are."

[OK] Correct: The code checks each permission one by one, so more permissions mean more checks and longer time.

Interview Connect

Understanding how access checks scale helps you design systems that stay fast as they grow. This skill shows you can think about real-world program efficiency.

Self-Check

"What if we used a hash map to store permissions instead of a list? How would the time complexity change?"

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