App Designer basics in MATLAB - Time & Space Complexity

Time Complexity: App Designer basics

O(n^2)

Understanding Time Complexity

When building apps with App Designer, it is helpful to know how the time your app takes to run changes as you add more components or data.

We want to understand how the app's running time grows when it does repeated tasks.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function updateList(app, items)
    app.ListBox.Items = {};
    for i = 1:length(items)
        app.ListBox.Items{end+1} = items{i};
    end
end

This code updates a list box in the app by adding each item from a given list one by one.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the input list to add items to the ListBox.
• How many times: Once for each item in the input list (n times).

How Execution Grows With Input

As the number of items grows, the time to add them grows roughly the same amount.

Input Size (n)	Approx. Operations
10	10
100	100
1000	1000

Pattern observation: The time grows directly with the number of items; doubling items doubles the work.

Final Time Complexity

Time Complexity: O(n^2)

This means the time to update the list grows quadratically with the number of items because appending to a cell array inside a loop causes repeated copying.

Common Mistake

[X] Wrong: "Adding items one by one is instant and does not depend on the number of items."

[OK] Correct: Each item takes time to add, and because the list grows each time, appending causes copying, so the work adds up quadratically.

Interview Connect

Understanding how loops affect app performance helps you write apps that stay fast even with more data or users.

Self-Check

"What if we updated all items at once instead of one by one? How would the time complexity change?"

Practice

1. What is the main purpose of uifigure in MATLAB App Designer?

easy

A. To add a button to the app

B. To create the main window for the app

C. To display text labels

D. To plot graphs automatically

Solution

1. Step 1: Understand the role of uifigure

   uifigure creates the main window or figure where the app components appear.

2. Step 2: Differentiate from other components

   Buttons and labels are added inside this window but are created with uibutton and uilabel.

3. Final Answer:

   To create the main window for the app -> Option B

4. Quick Check:

   Main window = uifigure [OK]

Hint: Remember: uifigure = app window [OK]

Common Mistakes:

• Confusing uifigure with uibutton or uilabel
• Thinking uifigure creates buttons or labels
• Assuming uifigure plots graphs automatically

2. Which of the following is the correct syntax to create a button in App Designer?

easy

A. btn = uibutton(uifigure, 'Text', 'Click Me');

B. btn = uibutton(uifigure, 'Click Me');

C. btn = uibutton('Text', 'Click Me');

D. btn = uibutton('Click Me');

Solution

1. Step 1: Recall uibutton syntax

   In App Designer, uibutton requires the parent figure as the first argument, then property name-value pairs.

2. Step 2: Check each option

   btn = uibutton(uifigure, 'Text', 'Click Me'); correctly uses uibutton(uifigure, 'Text', 'Click Me'). Others miss the parent or property name.

3. Final Answer:

   btn = uibutton(uifigure, 'Text', 'Click Me'); -> Option A

4. Quick Check:

   Button needs parent + 'Text' property [OK]

Hint: uibutton needs parent and 'Text' property [OK]

Common Mistakes:

• Omitting the parent figure argument
• Passing text without 'Text' property name
• Using wrong argument order

3. What will be the output when the following code runs in App Designer?

fig = uifigure;
lbl = uilabel(fig, 'Text', 'Hello');
lbl.Position = [100 100 100 22];

medium

A. No label appears because uilabel needs a callback

B. An error because Position is not a valid property

C. A label appears but text is empty

D. A label with text 'Hello' appears at position (100,100)

Solution

1. Step 1: Understand label creation

   The code creates a figure, then a label with text 'Hello' inside it.

2. Step 2: Position property sets label location

   Setting Position = [100 100 100 22] places the label at x=100, y=100 with width 100 and height 22.

3. Final Answer:

   A label with text 'Hello' appears at position (100,100) -> Option D

4. Quick Check:

   uilabel + Position sets label location [OK]

Hint: Position sets label location and size [OK]

Common Mistakes:

• Thinking Position is invalid property
• Assuming label text is empty by default
• Believing callback is needed to show label

4. Identify the error in this App Designer code snippet:

fig = uifigure;
btn = uibutton('Text', 'Press');
btn.Position = [50 50 100 30];

medium

A. Missing parent figure argument in uibutton

B. Position property cannot be set for buttons

C. Text property should be set after creating button

D. uifigure must be called after uibutton

Solution

1. Step 1: Check uibutton creation syntax

   uibutton requires the parent figure as the first argument to attach the button to the window.

2. Step 2: Identify missing parent

   The code calls uibutton('Text', 'Press') without specifying fig as parent, causing an error.

3. Final Answer:

   Missing parent figure argument in uibutton -> Option A

4. Quick Check:

   uibutton needs parent figure first [OK]

Hint: Always pass parent figure first to uibutton [OK]

Common Mistakes:

• Forgetting to specify parent figure
• Thinking Position can't be set for buttons
• Calling uifigure after uibutton

5. You want to create a button that changes a label's text to 'Clicked!' when pressed. Which code snippet correctly sets up the callback in App Designer?

hard

A. btn = uibutton(fig, 'Text', 'Press'); btn.Callback = @(btn,event) lbl.Text = 'Clicked!';

B. btn = uibutton(fig, 'Text', 'Press', ButtonPushedFcn, @(btn,event) lbl.Text = 'Clicked!');

C. btn = uibutton(fig, 'Text', 'Press'); btn.ButtonPushedFcn = @(btn,event) lbl.Text = 'Clicked!';

D. btn = uibutton(fig, 'Text', 'Press', 'Callback', @(btn,event) lbl.Text = 'Clicked!');

Solution

1. Step 1: Understand callback property name

   In App Designer, the button's callback is set using the ButtonPushedFcn property, usually assigned after creation.

2. Step 2: Check syntax correctness

   btn = uibutton(fig, 'Text', 'Press'); btn.ButtonPushedFcn = @(btn,event) lbl.Text = 'Clicked!'; correctly creates the button then assigns ButtonPushedFcn with an anonymous function to change label text.

3. Final Answer:

   btn = uibutton(fig, 'Text', 'Press'); btn.ButtonPushedFcn = @(btn,event) lbl.Text = 'Clicked!'; -> Option C

4. Quick Check:

   Set ButtonPushedFcn after button creation [OK]

Hint: Assign ButtonPushedFcn after creating button [OK]

Common Mistakes:

• Using 'Callback' instead of 'ButtonPushedFcn'
• Trying to set callback inside uibutton arguments
• Using wrong property names for callback