Callback functions in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign a callback function to a button.

MATLAB

btn = uicontrol('Style', 'pushbutton', 'String', 'Click me', 'Callback', [1]);

Drag options to blanks, or click blank then click option'

A@myCallback

BmyCallback()

CcallbackFunction

DmyCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using the function name without @

Calling the function with parentheses

Using a string instead of a function handle

The callback property requires a function handle, which is specified with the @ symbol.

2fill in blank

medium

Complete the code to define a callback function that displays 'Button clicked!'.

MATLAB

function [1](src, event)
    disp('Button clicked!')
end

Drag options to blanks, or click blank then click option'

AclickHandler

BcallbackFunction

CbuttonCallback

DmyCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different function name than the callback

Not including input arguments

Misspelling the function name

The function name should match the callback function assigned to the button.

3fill in blank

hard

Fix the error in the callback assignment to use a function handle correctly.

MATLAB

btn.Callback = [1];

Drag options to blanks, or click blank then click option'

AmyCallback()

B@myCallback

CmyCallback

DcallbackFunction

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning the function name as a string

Calling the function instead of passing a handle

Omitting the @ symbol

The callback property must be assigned a function handle using @.

4fill in blank

hard

Fill both blanks to create a button and assign a callback function handle.

MATLAB

btn = uicontrol('Style', 'pushbutton', 'String', 'Press', 'Callback', [1]);
function [2](src, event)
    disp('Pressed!')
end

Drag options to blanks, or click blank then click option'

A@pressCallback

BbuttonCallback

CpressCallback

D@buttonCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatch between function handle and function name

Missing @ in callback assignment

Using a different function name

The callback property needs a function handle (@pressCallback), and the function name must match (pressCallback).

5fill in blank

hard

Fill all three blanks to create a slider with a callback that updates a text label.

MATLAB

fig = figure;
slider = uicontrol('Style', 'slider', 'Min', 0, 'Max', 100, 'Value', 50, 'Callback', [1]);
label = uicontrol('Style', 'text', 'String', '50');

function [2](src, event)
    val = round(src.Value);
    [3].String = num2str(val);
end

Drag options to blanks, or click blank then click option'

A@sliderCallback

BsliderCallback

Clabel

Dfig

Attempts:

3 left

💡 Hint

Common Mistakes

Not using function handle for callback

Mismatched function names

Updating wrong UI element

The slider's callback is a function handle (@sliderCallback). The function name matches (sliderCallback). The label's String property is updated inside the callback.

Practice

1. What is the main purpose of a callback function in MATLAB GUI programming?

easy

A. To speed up the program execution by parallel processing

B. To store data permanently in a file

C. To execute code in response to user actions or events

D. To create new variables automatically

Solution

1. Step 1: Understand the role of callback functions

   Callback functions are designed to respond when a user interacts with a GUI element, like clicking a button.

2. Step 2: Identify the correct purpose

   Among the options, only executing code in response to user actions matches the callback function's role.

3. Final Answer:

   To execute code in response to user actions or events -> Option C

4. Quick Check:

   Callback = respond to events [OK]

Hint: Callbacks run when user interacts with GUI elements [OK]

Common Mistakes:

• Thinking callbacks store data permanently
• Confusing callbacks with parallel processing
• Assuming callbacks create variables automatically

2. Which of the following is the correct way to assign a callback function named myCallback to a push button btn in MATLAB?

easy

A. btn.Callback = myCallback;

B. btn.Callback = &myCallback;

C. btn.Callback = 'myCallback';

D. btn.Callback = @myCallback;

Solution

1. Step 1: Recall MATLAB callback syntax

   In MATLAB, the @ symbol is used to create a function handle for callbacks.

2. Step 2: Identify the correct assignment

   Assigning @myCallback correctly creates a handle to the function for the callback.

3. Final Answer:

   btn.Callback = @myCallback; -> Option D

4. Quick Check:

   Use @ for function handles [OK]

Hint: Use @ before function name for callbacks [OK]

Common Mistakes:

• Omitting @ symbol for function handle
• Using quotes instead of @ for function handle
• Using & which is not valid in MATLAB

3. Consider the following MATLAB code snippet for a button callback:

function myCallback(src, event)
    disp('Button pressed');
end

btn.Callback = @myCallback;

% When the button is clicked, what will be displayed?

medium

A. Button pressed

B. src event

C. Error: Undefined function

D. No output

Solution

1. Step 1: Understand the callback function behavior

   The function myCallback displays the text 'Button pressed' when called.

2. Step 2: Recognize the callback assignment

   The button's callback is set to @myCallback, so clicking the button triggers the function and prints the message.

3. Final Answer:

   Button pressed -> Option A

4. Quick Check:

   Callback prints message on click [OK]

Hint: Callback prints message when triggered [OK]

Common Mistakes:

• Expecting variable names to print
• Thinking callback causes error without GUI
• Assuming no output from disp function

4. What is wrong with this MATLAB callback assignment?

btn.Callback = myCallback;

Assuming myCallback is a function defined elsewhere.

medium

A. Missing @ symbol to create function handle

B. Callback property does not exist for buttons

C. Function name should be a string, not a variable

D. Callback functions cannot be assigned directly

Solution

1. Step 1: Check callback assignment syntax

   In MATLAB, callbacks require a function handle using the @ symbol.

2. Step 2: Identify the missing element

   The code assigns the function name without @, so MATLAB treats it as a variable, causing an error.

3. Final Answer:

   Missing @ symbol to create function handle -> Option A

4. Quick Check:

   Callbacks need @ before function name [OK]

Hint: Always use @ before function name for callbacks [OK]

Common Mistakes:

• Omitting @ symbol
• Confusing property names
• Thinking function name as string is correct

5. You want to create a MATLAB GUI button that changes its own label to 'Clicked!' when pressed. Which callback function correctly achieves this?

hard

A. function changeLabel()
btn.String = 'Clicked!';
end

B. function changeLabel(src, ~)
src.String = 'Clicked!';
end

C. function changeLabel(~, event)
event.String = 'Clicked!';
end

D. function changeLabel(src)
src.Label = 'Clicked!';
end

Solution

1. Step 1: Understand callback inputs

   The first input src is the source object (the button), which can be used to change its properties.

2. Step 2: Identify correct property and syntax

   The button's label text is stored in the String property, so setting src.String = 'Clicked!'; updates the label.

3. Final Answer:

   function changeLabel(src, ~) src.String = 'Clicked!'; end -> Option B

4. Quick Check:

   Use src.String to change button label [OK]

Hint: Use src.String to update button text inside callback [OK]

Common Mistakes:

• Using event instead of src to change properties
• Using wrong property name like Label
• Not including required input arguments