Program Inputs (College Board AP® Computer Science Principles): Revision Note
Event-driven programming basics
What is event-driven programming?
Event-driven programming is a programming approach where the flow of the program is determined by events rather than following a fixed sequence of statements
An event is an action that occurs as a result of user interaction or another trigger
When an event occurs, the program runs a specific section of code in response
This allows programs to respond dynamically to user actions instead of executing instructions in a single, predetermined order
Types of events
Events are typically triggered by user interactions with a program's interface
Event | How it occurs | Example |
|---|---|---|
Key press | The user presses a key on the keyboard | Pressing the spacebar to pause a game |
Mouse click | The user clicks on an element in the program | Clicking a "Submit" button on a form |
Mouse movement | The user moves the mouse over an area | Hovering over a menu to reveal options |
Screen tap | The user taps a touchscreen element | Tapping an icon to open an application |
Timer | A set amount of time has passed | A countdown that triggers an action when it reaches zero |
Program start | The program begins running | Initializing variables or playing a welcome animation when a game launches |
Event handlers
An event handler is a section of code (a procedure) that is executed when a specific event is detected
The program listens for events and calls the appropriate handler when one occurs
Multiple event handlers can exist within a single program, each responding to a different event
Event-driven programs spend most of their execution time waiting for events to occur rather than actively running code
Events are one way input enters a program, but programs can receive input from many other sources too
Examiner Tips and Tricks
When the AP exam describes a program that responds to user actions (clicks, key presses, screen taps), it is describing event-driven programming. Look for keywords like "event", "trigger", and "action" in the question to identify this concept.
For the Create Performance Task, event handlers (such as "when button clicked" blocks) are considered built-in abstractions, not student-developed code. If you use one, make sure your student-developed procedure is a separate, named procedure that you have written yourself.
Input processing
How do programs process input?
Input is any data provided to a program from an external source
Input can come in a variety of forms, including tactile (touch), audio, visual, or text, and a program needs to work for a variety of inputs and situations
A computer receives input, processes it using the program's logic, and produces output
The input-output relationship describes how specific inputs are transformed by the program's processing to produce specific outputs
The same program can produce different outputs depending on the data it receives as input
A program's output depends on its input or its prior state (the internal values stored during previous execution)
Input sources
User input: data entered directly by a person (typing text, clicking buttons, speaking into a microphone)
File input: data read from a stored file (loading a saved document or reading a dataset)
Sensor input: data collected automatically from a device (a fitness tracker recording heart rate)
Network input: data received from another system or application over a connection (receiving messages in a chat program)
Examiner Tips and Tricks
The AP exam may ask you to identify what type of input a program uses or how changing the input affects the output. Always trace the path from input through processing to output when answering these questions. Do not confuse the input source with the input itself. The keyboard is the source; the text the user types is the input.
For the AP Create Performance Task, your program must include instructions for input from the user, a device, an online data stream, or a file. On exam day, you will need to explain how your program receives and processes this input to produce output.
Worked Example
A fitness application collects heart rate data from a wearable sensor and displays a message to the user. When the heart rate exceeds 150 beats per minute, the program displays "High intensity". Otherwise, it displays "Normal".
Which of the following best describes the input to this program?
(A) The message displayed on the screen
(B) The heart rate data collected from the sensor
(C) The wearable sensor device
(D) The threshold value of 150 beats per minute
[1]
Answer:
(B) The heart rate data collected from the sensor [1 mark]
The heart rate data is the input — the sensor is merely its source, the message is the output, and the 150 bpm threshold is part of the processing logic
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