Lesson 8 — While Loops

Estimated time: 75 minutes

• Learning Objectives
• Concepts
  • What is a Loop?
  • The while Loop
  • Counter Variables and +=
  • Infinite Loops with while True:
  • Input Validation Loop
• Guided Walkthrough
  • Step 1: Basic while Loop
  • Step 2: Countdown + LED Flash
  • Step 3: LED Breathing Effect
  • Step 4: While Loop with Button
• Challenges
  • ⭐ Core
  • ⭐⭐ Extension
  • ⭐⭐⭐ Stretch
• Common Mistakes & Debugging
• Key Vocabulary

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Learning Objectives

By the end of this lesson you will be able to:

1. Use a while loop to repeat code while a condition is True
2. Use counter variables and the += operator
3. Write an infinite loop (while True:) and understand why it’s useful
4. Create fading and pulsing LED effects

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Concepts

What is a Loop?

A loop repeats a block of code. Think of a washing machine programme — it keeps running cycles until done. Without loops, you’d have to copy-paste the same code dozens or hundreds of times.

The while Loop

A while loop keeps running as long as its condition is True. When the condition becomes False, the loop ends and the program continues after it.

while condition:
    code to run (loop body)
    ...
(code here runs after the loop ends)

Example — countdown:

count = 5
while count > 0:
    print(count)
    count -= 1    # count = count - 1
print("Blastoff!")

Output: 5 4 3 2 1 Blastoff!

What happens step by step:

1. Check: 5 > 0? Yes → print 5, count becomes 4
2. Check: 4 > 0? Yes → print 4, count becomes 3
3. Check: 3 > 0? Yes → print 3, count becomes 2
4. Check: 2 > 0? Yes → print 2, count becomes 1
5. Check: 1 > 0? Yes → print 1, count becomes 0
6. Check: 0 > 0? No → exit loop
7. Print “Blastoff!”

Counter Variables and +=

A counter variable tracks how many times the loop has run. The += operator is shorthand for “add and assign”:

count = 0
count += 1    # Same as: count = count + 1
count += 5    # Same as: count = count + 5

x = 10
x -= 3        # Same as: x = x - 3  → x = 7
x *= 2        # Same as: x = x * 2  → x = 14

Infinite Loops with while True:

while True: creates an infinite loop — it runs forever because True is always True. This is actually very useful in embedded programming! Your microcontroller should keep running:

import time

while True:
    print("I run forever!")
    time.sleep(1)

Press Ctrl+C in Thonny to stop an infinite loop.

In embedded programming, infinite loops are normal and expected. A program controlling a temperature sensor should loop forever — that’s its job. On a laptop, infinite loops are usually bugs, but on a microcontroller, they’re the design.

Input Validation Loop

A common pattern: keep asking until the user gives a valid answer:

while True:
    age = int(input("Enter your age (1-120): "))
    if 1 <= age <= 120:
        break    # Exit the loop — we have a valid answer
    print("That doesn't look right. Try again.")

print(f"You are {age} years old.")

(break will be covered fully in Lesson 10 — for now, just know it exits the loop.)

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Guided Walkthrough

Step 1: Basic while Loop

count = 1
while count <= 5:
    print(f"Count: {count}")
    count += 1
print("Loop complete!")

Try changing 5 to different values. What happens with count <= 0?

Step 2: Countdown + LED Flash

import machine, neopixel, time

pin = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin, 1)

countdown = 10
while countdown > 0:
    print(f"T minus {countdown}...")

    # Flash colour based on urgency
    if countdown <= 3:
        np[0] = (255, 0, 0)    # Red — urgent
    elif countdown <= 6:
        np[0] = (255, 150, 0)  # Orange — getting close
    else:
        np[0] = (0, 255, 0)    # Green — plenty of time

    np.write()
    time.sleep(0.9)
    np[0] = (0, 0, 0)
    np.write()
    time.sleep(0.1)

    countdown -= 1

print("LAUNCH!")
np[0] = (255, 255, 255)   # White flash
np.write()
time.sleep(0.5)
np[0] = (0, 0, 0)
np.write()

Step 3: LED Breathing Effect

A smooth fade in and out using a while loop with a direction variable:

import machine, neopixel, time

pin = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin, 1)

brightness = 0
direction = 5    # How much to change per step (+5 = getting brighter)

print("Breathing blue LED... Ctrl+C to stop")
while True:
    np[0] = (0, 0, brightness)
    np.write()

    brightness += direction

    # Bounce at the limits
    if brightness >= 255:
        direction = -5
        brightness = 255
    elif brightness <= 0:
        direction = 5
        brightness = 0

    time.sleep(0.02)   # Small delay for smooth animation

Step 4: While Loop with Button

Wait until the button is pressed, then do something:

import machine, neopixel, time
from machine import Pin

pin48 = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin48, 1)
button = Pin(0, Pin.IN, Pin.PULL_UP)

print("Press the button to start the countdown!")

# Wait for button press
while button.value() == 1:   # While button NOT pressed...
    time.sleep(0.05)          # ...keep waiting

print("Button pressed! Starting countdown...")
np[0] = (255, 200, 0)   # Yellow
np.write()
time.sleep(0.5)

# Now do the countdown
count = 5
while count > 0:
    print(f"  {count}...")
    np[0] = (255, int(200 * count/5), 0)   # Orange fading to red
    np.write()
    time.sleep(1)
    count -= 1

np[0] = (0, 255, 0)
np.write()
print("GO!")
time.sleep(2)
np[0] = (0, 0, 0)
np.write()

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Challenges

⭐ Core

Write a while loop that counts from 1 to 20. For each even number, flash the LED green for 0.1 seconds. For odd numbers, just print the number. After the loop, print how many flashes occurred.

⭐⭐ Extension

Create a “patience tester” — the LED slowly pulses green (breathing effect). The program counts how many complete breath cycles occur. After 5 cycles, it turns the LED gold and prints “You are very patient!” If the user presses Ctrl+C before 5 cycles, they see the partial count.

⭐⭐⭐ Stretch

Create a reaction timer in two phases:

1. The LED shows a random colour for a random time (0.5–3 seconds using time.sleep(random.uniform(0.5, 3)))
2. Then the LED turns WHITE — the user presses Enter as fast as possible
3. Use time.ticks_ms() to measure reaction time between the LED going white and Enter being pressed
4. Repeat 5 times and print the average reaction time and best time

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Common Mistakes & Debugging

Infinite loop that never ends You forgot to update the loop variable! while count < 5: with no count += 1 inside will run forever. Check your loop body modifies the variable being tested.

Off-by-one error while count < 5 runs for count = 0, 1, 2, 3, 4 (5 times). while count <= 5 runs for 0, 1, 2, 3, 4, 5 (6 times). Decide carefully which you want.

Loop body not indented All code inside the loop must be indented by the same amount.

Press Ctrl+C if stuck If your loop runs forever and you didn’t intend it to, press Ctrl+C in Thonny to stop it.

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Key Vocabulary

Term	Definition
while loop	Repeats a block of code as long as a condition is True
loop body	The indented block of code that runs on each iteration
iteration	One complete execution of the loop body
counter variable	A variable used to count loop iterations
+=	Shorthand for “add and assign”: x += 1 means x = x + 1
infinite loop	A loop that runs forever — while True: — common in embedded programs
break	Immediately exits the current loop (covered fully in Lesson 10)