Unit 4 · Lesson 1

Lists: Creating & Accessing

Lists let you store multiple values in a single variable — ordered, indexed, and incredibly flexible.

Create lists with square bracket syntax
Access items with positive and negative indexing
Use len() to get list size
Extract portions with slicing

Creating Lists

A list is an ordered collection of items, wrapped in square brackets:

scores = [95, 87, 92, 78, 100] names = ["Alice", "Bob", "Charlie"] mixed = ["hello", 42, 3.14, True] empty = []

Accessing Items by Index

Every item has a position number (index), starting from 0:

colors = ["red", "green", "blue", "yellow"] print(colors[0]) # "red" (first item) print(colors[2]) # "blue" (third item) print(colors[-1]) # "yellow" (last item) print(len(colors)) # 4
Negative indexing: -1 is the last item, -2 is second-to-last, etc. This is incredibly handy when you don't know the list's length!

Slicing

Extract a portion of a list with list[start:stop]:

nums = [10, 20, 30, 40, 50] print(nums[1:4]) # [20, 30, 40] print(nums[:3]) # [10, 20, 30] (first 3) print(nums[2:]) # [30, 40, 50] (from index 2 on) print(nums[::2]) # [10, 30, 50] (every other)
Practice Time

Challenges

🎵

Challenge 1: Playlist Manager

Guided

Create a list of songs and access specific ones by index.

Instructions: Create a list called playlist with 5 song names. Set first_song to the first item, last_song to the last (using negative indexing), and count to the total number of songs.
Hint 1: first_song = playlist[0]
Hint 2: last_song = playlist[-1], count = len(playlist)
🔖

Challenge 2: First and Last

Guided

Print the first and last item of any list using negative indexing.

Instructions: Given any items list, set first and last to the first and last elements. Your code must work for any list length.
Hint: first = items[0] and last = items[-1] — works for any list!
🎯

Challenge 3: Middle Finder

Solo

Find the middle element of a list.

Instructions: Given data, find the middle element and store it in middle. For odd-length lists, there's one exact middle. For even-length, use the lower-middle index (e.g., for 4 items, use index 1).

Example: [10,20,30,40,50] → middle = 30 (index 2)
Hint 1: Middle index = len(data) // 2
Hint 2: middle = data[len(data) // 2]
🔪

Challenge 4: Slice Explorer

Stretch

Use slicing to extract different portions of a list.

Instructions: Given numbers, use slicing to create: first_three (first 3 items), last_three (last 3 items), every_other (every other item starting from index 0).
Hint 1: first_three = numbers[:3]
Hint 2: last_three = numbers[-3:]
Hint 3: every_other = numbers[::2]