Button press detection in Raspberry Pi - Time & Space Complexity

Time Complexity: Button press detection

O(n)

Understanding Time Complexity

When detecting button presses on a Raspberry Pi, the program often waits and checks repeatedly for input.

We want to understand how the time the program takes grows as it keeps checking for button presses.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.IN, pull_up_down=GPIO.PUD_UP)

while True:
    if GPIO.input(18) == GPIO.LOW:
        print("Button pressed!")
        time.sleep(0.2)  # debounce delay

This code continuously checks if a button connected to pin 18 is pressed and prints a message when it is.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The while loop that repeatedly reads the button state.
• How many times: It runs indefinitely, checking the button many times per second.

How Execution Grows With Input

As time passes, the program keeps checking the button state over and over.

Input Size (n)	Approx. Operations
10 (seconds)	Many repeated checks, about 5 per second, so ~50 checks
100 (seconds)	About 500 checks
1000 (seconds)	About 5000 checks

Pattern observation: The number of checks grows directly with time passed, increasing steadily.

Final Time Complexity

Time Complexity: O(n)

This means the program's work grows linearly with how long it runs, checking the button repeatedly.

Common Mistake

[X] Wrong: "The button check only happens once, so time doesn't grow with input."

[OK] Correct: The code uses a loop that runs forever, so it keeps checking many times, making the work grow with time.

Interview Connect

Understanding how repeated checks affect program time helps you explain how real devices respond to inputs over time.

Self-Check

What if we changed the code to use an interrupt instead of a loop? How would the time complexity change?