Lesson 9 — For Loops and range()

Estimated time: 75 minutes

Learning Objectives

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

  1. Use a for loop to iterate through a sequence
  2. Use range() with one, two, and three arguments
  3. Create nested for loops for complex patterns
  4. Know when to use for vs while

Concepts

The for Loop

A for loop iterates through a sequence, running the loop body once for each item. The loop variable takes each value in turn:

for item in sequence:
    # code using item

Iterating through a list:

fruits = ["apple", "banana", "mango"]
for fruit in fruits:
    print(fruit)
# Output: apple  banana  mango

Using a loop variable that we don’t need — _:

for _ in range(5):    # _ is the convention for "I don't need this value"
    print("Hello!")   # Prints "Hello!" 5 times

range()

range() generates a sequence of numbers. It’s the most common thing to iterate in a for loop:

Call Produces
range(5) 0, 1, 2, 3, 4
range(1, 6) 1, 2, 3, 4, 5
range(0, 10, 2) 0, 2, 4, 6, 8 (step 2)
range(10, 0, -1) 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 (count down)
range(0, 256, 15) 0, 15, 30, 45, … 255 (useful for LED brightness!)

Key rule: range(start, stop, step) — includes start, goes up to but not including stop.

for vs while

Use for when… Use while when…
You know exactly how many times You’re waiting for a condition to change
Iterating through a list/string Waiting for a sensor value
Counting to a fixed number “Keep going until…”

Nested Loops

A loop inside a loop. The inner loop runs completely for each single iteration of the outer loop:

for row in range(3):
    for col in range(4):
        print(f"({row},{col})", end=" ")
    print()   # New line after inner loop completes

Output:

(0,0) (0,1) (0,2) (0,3)
(1,0) (1,1) (1,2) (1,3)
(2,0) (2,1) (2,2) (2,3)

enumerate()

enumerate() gives you both the index and the value when iterating a list:

colours = ["red", "green", "blue"]
for i, colour in enumerate(colours):
    print(f"Colour {i}: {colour}")
# Output:
# Colour 0: red
# Colour 1: green
# Colour 2: blue

Guided Walkthrough

Step 1: Basic for and range() 

# Count 0 to 4
for i in range(5):
    print(i)

print("---")

# Count 1 to 5
for i in range(1, 6):
    print(i)

print("---")

# Even numbers 0 to 10
for i in range(0, 11, 2):
    print(i)

print("---")

# Countdown
for i in range(10, 0, -1):
    print(i)
print("Blastoff!")

Step 2: LED Colour Cycle 

import machine, neopixel, time

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

colours = [
    ("Red",     (255, 0, 0)),
    ("Orange",  (255, 100, 0)),
    ("Yellow",  (255, 200, 0)),
    ("Green",   (0, 255, 0)),
    ("Cyan",    (0, 200, 255)),
    ("Blue",    (0, 0, 255)),
    ("Purple",  (100, 0, 255)),
]

# Run 3 complete cycles
for cycle in range(3):
    print(f"Cycle {cycle + 1}:")
    for i, (name, colour) in enumerate(colours):
        np[0] = colour
        np.write()
        print(f"  {name}")
        time.sleep(0.4)

np[0] = (0, 0, 0)
np.write()
print("Done!")

Step 3: Nested Loop — RGB Channels 

import machine, neopixel, time

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

channel_names = ["Red", "Green", "Blue"]

# For each colour channel, ramp up then down
for channel in range(3):    # 0=Red, 1=Green, 2=Blue
    print(f"Ramping {channel_names[channel]}...")

    # Ramp up: 0 to 255
    for brightness in range(0, 256, 15):
        colour = [0, 0, 0]
        colour[channel] = brightness
        np[0] = tuple(colour)
        np.write()
        time.sleep(0.03)

    # Ramp down: 255 to 0
    for brightness in range(255, -1, -15):
        colour = [0, 0, 0]
        colour[channel] = brightness
        np[0] = tuple(colour)
        np.write()
        time.sleep(0.03)

np[0] = (0, 0, 0)
np.write()
print("Channel sweep complete!")

Explanation: colour is a list [0, 0, 0]. We set colour[channel] to the brightness value (changing only the relevant channel), then use tuple(colour) to convert it to a tuple for the NeoPixel.

Step 4: FizzBuzz with LED Feedback

FizzBuzz is a classic programming challenge: count 1 to 30, print “Fizz” for multiples of 3, “Buzz” for multiples of 5, “FizzBuzz” for multiples of both.

import machine, neopixel, time

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

for i in range(1, 31):
    if i % 3 == 0 and i % 5 == 0:
        print(f"{i}: FizzBuzz!")
        np[0] = (0, 200, 200)    # Cyan — FizzBuzz
    elif i % 3 == 0:
        print(f"{i}: Fizz")
        np[0] = (0, 200, 0)      # Green — Fizz
    elif i % 5 == 0:
        print(f"{i}: Buzz")
        np[0] = (0, 0, 200)      # Blue — Buzz
    else:
        print(f"{i}")
        np[0] = (40, 40, 40)     # Dim white — regular number

    np.write()
    time.sleep(0.3)

np[0] = (0, 0, 0)
np.write()

Step 5: Nested Loop — Multiplication Table 

print("Multiplication table (1–5):")
print()

for row in range(1, 6):
    for col in range(1, 6):
        result = row * col
        print(f"{result:4}", end="")   # :4 = minimum 4 chars wide
    print()   # New line after each row

Challenges

⭐ Core

Write a for loop that counts from 1 to 100. Print only numbers divisible by 7. How many are there? Flash the LED red once for each divisible number found.

⭐⭐ Extension

Create a “beat pattern” using nested loops: 4 quick red flashes, then 1 long green flash, then 4 quick blue flashes, then 1 long green flash. Repeat this whole pattern 4 times. Use for loops throughout.

⭐⭐⭐ Stretch

Generate the first 20 Fibonacci numbers (0, 1, 1, 2, 3, 5, 8, 13…) using a for loop. For each number:

  • Print it
  • If it’s a prime number, flash the LED blue once
  • If it’s divisible by 3, flash it green once You’ll need a helper condition to check for primes: a number n is prime if no number from 2 to n-1 divides it evenly.

Common Mistakes & Debugging

range(5) starts at 0, not 1 If you want 1 to 5, use range(1, 6).

range(10, 0) produces nothing To count down you need a step: range(10, 0, -1).

Modifying the loop variable inside the loop for i in range(10): i = 5 — this doesn’t change what range produces. The loop still runs 10 times with i = 0, 1, 2, …

Nested loop confusion The inner loop completes all its iterations before the outer loop moves to its next iteration.

Key Vocabulary

Term Definition
for loop Iterates through each item in a sequence, once per item
range() Generates a sequence of numbers: range(start, stop, step)
loop variable The variable that takes each value from the sequence (e.g., i in for i in range(5))
nested loop A loop inside another loop
enumerate() Returns both the index and value when iterating a list
_ Conventional variable name when you don’t need the loop variable’s value

Copyright © Paul Baumgarten.