Lesson 11 — Introduction to Lists

Estimated time: 60 minutes

Learning Objectives

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

  1. Create a list and access elements by index
  2. Use negative indexing and slicing on lists
  3. Use len() with lists
  4. Iterate through a list using a for loop

Concepts

What is a List?

A list is an ordered collection of values stored in a single variable. Think of it like a numbered shelf — each item has a position (slot number), and you can access any item by its slot number.

scores = [85, 72, 91, 68, 77]
#          0   1   2   3   4   ← indices

Lists use square brackets [], with items separated by commas.

Lists can hold any type (or even mixed types):

numbers = [1, 2, 3, 4, 5]
names = ["Alice", "Bob", "Charlie"]
mixed = [42, "hello", True, 3.14]    # Possible, though unusual
colours = [(255,0,0), (0,255,0), (0,0,255)]   # List of tuples

Accessing Items — Indexing

Use list[index] to access an item. Indexing starts at 0:

fruits = ["apple", "banana", "mango", "cherry"]
#           0         1        2         3

print(fruits[0])   # "apple"
print(fruits[2])   # "mango"
print(fruits[-1])  # "cherry"  (last item)
print(fruits[-2])  # "mango"   (second-to-last)

Negative indexing counts from the end:

  • -1 = last item
  • -2 = second-to-last
  • etc.

Slicing

Extract a portion of a list with [start:end] (same as string slicing):

nums = [10, 20, 30, 40, 50, 60]

print(nums[1:4])    # [20, 30, 40]  (index 1, 2, 3 — not 4)
print(nums[:3])     # [10, 20, 30]  (from start to index 2)
print(nums[3:])     # [40, 50, 60]  (from index 3 to end)
print(nums[::2])    # [10, 30, 50]  (every other item)
print(nums[::-1])   # [60, 50, 40, 30, 20, 10]  (reversed)

Slicing never raises an IndexError — it just gives you as much as exists.

len()

len() returns the number of items in a list:

fruits = ["apple", "banana", "mango"]
print(len(fruits))   # 3

Last valid index is always len(list) - 1.

Modifying Items

Lists are mutable — you can change individual items:

temperatures = [22, 24, 19, 28, 21]
temperatures[2] = 20    # Change index 2 from 19 to 20
print(temperatures)     # [22, 24, 20, 28, 21]

Iterating — for Loop Over a List

The most natural way to process every item:

# Method 1: iterate values directly
fruits = ["apple", "banana", "mango"]
for fruit in fruits:
    print(fruit)

# Method 2: iterate by index (useful when you need the index)
for i in range(len(fruits)):
    print(f"Item {i}: {fruits[i]}")

# Method 3: enumerate (gives both)
for i, fruit in enumerate(fruits):
    print(f"Item {i}: {fruit}")

Guided Walkthrough

Step 1: Creating and Accessing Lists 

scores = [85, 72, 91, 68, 77]

print("All scores:", scores)
print("First score:", scores[0])        # 85
print("Third score:", scores[2])        # 91
print("Last score:", scores[-1])        # 77
print("Second to last:", scores[-2])    # 68
print("Number of scores:", len(scores)) # 5

Step 2: Slicing 

colours = ["red", "orange", "yellow", "green", "blue", "purple"]

print(colours[1:4])    # ['orange', 'yellow', 'green']
print(colours[:3])     # ['red', 'orange', 'yellow']
print(colours[3:])     # ['green', 'blue', 'purple']
print(colours[::2])    # ['red', 'yellow', 'blue']
print(colours[::-1])   # Reversed

Step 3: Modifying and Iterating 

temps = [22, 24, 19, 28, 21]

# Increase all temperatures by 2 degrees
for i in range(len(temps)):
    temps[i] = temps[i] + 2

print("Updated:", temps)   # [24, 26, 21, 30, 23]

Step 4: LED Colour Palette 

import machine, neopixel, time

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

# A list of (name, RGB) tuples
palette = [
    ("Red",     (255, 0, 0)),
    ("Orange",  (255, 100, 0)),
    ("Yellow",  (255, 200, 0)),
    ("Green",   (0, 255, 0)),
    ("Cyan",    (0, 200, 200)),
    ("Blue",    (0, 0, 255)),
    ("Purple",  (100, 0, 200)),
]

print(f"Palette has {len(palette)} colours")

# Show each colour using indexing
print("First colour:", palette[0][0])
np[0] = palette[0][1]   # Access the RGB tuple inside the outer tuple
np.write()
time.sleep(1)

print("Last colour:", palette[-1][0])
np[0] = palette[-1][1]
np.write()
time.sleep(1)

# Show all colours in order
for name, colour in palette:
    np[0] = colour
    np.write()
    print(f"  {name}")
    time.sleep(0.5)

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

Step 5: Analysing a List 

test_scores = [85, 72, 91, 68, 77, 83, 95, 61, 78, 88]

print(f"Number of scores: {len(test_scores)}")
print(f"Highest score: {max(test_scores)}")
print(f"Lowest score: {min(test_scores)}")
print(f"Total: {sum(test_scores)}")
print(f"Average: {sum(test_scores) / len(test_scores):.1f}")

# Count scores above 80
high_scores = 0
for score in test_scores:
    if score >= 80:
        high_scores += 1

print(f"Scores above 80: {high_scores}")

max(), min(), sum() are built-in functions that work on lists of numbers.

Challenges

⭐ Core

Create a list of 5 of your favourite things (strings). Print each item on its own line with its index number, like “0: pizza”. Then print the list reversed (using slicing). Finally, print the first and last items.

⭐⭐ Extension

Create a list of at least 8 LED colours (tuples). Display each colour for 0.3 seconds, printing the index and colour values. Then display the list in reverse order. Print the index count before you start.

⭐⭐⭐ Stretch

Create two parallel lists — one of colour names (strings) and one of RGB values (tuples). They must have the same length with matching indices (e.g., names[2] and colours[2] are the same colour). Ask the user to type a colour name. If it’s in the names list, find its index and display the matching colour on the LED. If not found, print “not found” and list all available colours.

Common Mistakes & Debugging

IndexError: list index out of range You tried to access an index that doesn’t exist. A 5-item list has indices 0–4. Accessing index 5 raises this error. Check len() and remember the last index is len(list) - 1.

Confusing slicing with indexing list[2] returns one item. list[1:3] returns a list (even if it contains only one item).

TypeError: 'int' object is not subscriptable You wrote scores[0][1] but scores[0] is just an integer, not a list. Use this syntax only when you have a list of lists or list of tuples.

Key Vocabulary

Term Definition
list An ordered, mutable collection of values
index The position of an item in a list (starts at 0)
negative index Counting from the end: -1 = last item
slicing Extracting a portion of a list using [start:end:step]
mutable Can be changed after creation — lists are mutable, strings are not
len() Returns the number of items in a list
max() / min() / sum() Built-in functions for lists of numbers

Copyright © Paul Baumgarten.