Practice Event Handler in CS Thinking

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

Quick Recap

A function that is automatically called when a specific event occurs, such as a button click, key press, or timer tick.

An event handler is like setting a trap — you define what should happen, then wait. When the event fires, your code runs automatically.

Example 1

easy
In a web page, a button has this event handler: WHEN button.clicked: OUTPUT 'Button was clicked!'. Explain what an event handler is and when this code runs.

Example 2

medium
Write pseudocode for a simple calculator with buttons for digits 0-9 and an '=' button. How would you use event handlers to build this?

Example 3

medium
A form has a text input and a submit button. Write pseudocode for event handlers that: (1) validate the input is not empty when submit is clicked, and (2) clear any error message when the user starts typing.

Example 4

hard
Explain the problem of attaching an event handler inside a loop: FOR i = 0 TO 4: WHEN buttons[i].clicked: OUTPUT i. If you click button 3 after the loop finishes, what might be output and why?
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