Chapter 7: Lists

How Does It Work?

List Syntax

A list is created with square brackets, and each item is separated by a comma:

my_list = ["item1", "item2", "item3"]

Lists can hold strings, integers, floats, booleans — even a mixture of types — although in practice you will usually store items of the same type in a single list.

# A list with mixed types (valid, but uncommon)
mixed = ["Alice", 16, True, 9.5]

Indexing: It Starts at 0

Every item in a list has a position number called its index. The crucial thing to remember is that indexing starts at 0, not 1.

You can also use negative indexing to count backwards from the end of the list. -1 is the last item, -2 is the second-to-last, and so on.

fruits = ["apple", "banana", "cherry"]
print(fruits[-1])   # cherry  (last item)
print(fruits[-2])   # banana  (second-to-last)

Common List Methods

Python gives you many built-in methods to work with lists. Here are the ones you will use most often:

Looping Through a List

Using a for loop with a list is one of the most common patterns in Python. The loop variable takes the value of each item in turn:

for item in my_list:
    print(item)

If you also need the index number alongside the item, you can use enumerate():

for index, item in enumerate(my_list):
    print(index, item)

This links directly to what you learnt in Chapter 5: Loops. The for loop is the natural partner of a list.

Slicing in Detail

List slicing uses the syntax list[start:stop:step]. All three values are optional:

Slicing always creates a new list — it never changes the original.

Arrays vs Lists — What the GCSE Exam Expects

In your GCSE Computer Science exam, the term “array” is used rather than “list”. It is important to understand that in Python, a list is the same concept as an array. Python does not have a separate built-in “array” data type in the way that languages like C or Java do — instead, Python uses lists to do the same job.

1D Arrays (One-Dimensional)

A 1D array is a single row of items — what we have been calling a list so far:

# A 1D array of names
names = ["Alice", "Bob", "Charlie", "Diana"]

This stores one sequence of items in a straight line. You access items with a single index: names[2] returns "Charlie".

2D Arrays (Two-Dimensional)

A 2D array stores data in rows and columns, like a table or spreadsheet. In Python, you create this as a list of lists — each inner list is one row:

# A 2D array: 3 rows, 4 columns
# Each row = [Name, Maths, English, Science]
marks = [
    ["Alice", 85, 92, 78],
    ["Bob", 91, 67, 83],
    ["Charlie", 76, 88, 95]
]

You access items using double indexing: marks[row][column]. For example:

• marks[0][0] → "Alice" (row 0, column 0)
• marks[1][2] → 67 (row 1, column 2 — Bob’s English mark)
• marks[2][3] → 95 (row 2, column 3 — Charlie’s Science mark)

When the exam asks you to “use a 2D array”, this is how you do it in Python.

Key Vocabulary

These are the key terms you need to know for this topic. Make sure you can define each one:

Common Mistakes

Worked Example: Building a Class Register

Let’s work through a complete example step by step. We will build a class register program that demonstrates many of the list operations you have learnt.

Scenario: You are a form tutor. You need to create a register of student names, handle a late arrival, remove a student who has left the school, sort the register alphabetically, check if a specific student is in your class, and count the total number of students.

# WORKED EXAMPLE: Class Register
# =================================

# Step 1: Create the initial register
register = ["Emily", "James", "Priya", "Oliver", "Fatima"]
print("Initial register:", register)
print("Number of students:", len(register))

# Step 2: A late student arrives - add them to the end
register.append("Liam")
print("\nAfter Liam arrives late:", register)

# Step 3: Oliver has left the school - remove him
register.remove("Oliver")
print("After Oliver leaves:", register)

# Step 4: Sort the register alphabetically
register.sort()
print("\nSorted alphabetically:", register)

# Step 5: Check if a specific student is in the class
search_name = "Priya"
if search_name in register:
    position = register.index(search_name)
    print(f"\n{search_name} is in the register at position {position}")
else:
    print(f"\n{search_name} is NOT in the register")

# Step 6: Count total students
print(f"\nTotal students in class: {len(register)}")

# Bonus: Print a numbered register
print("\n--- Final Register ---")
for i in range(len(register)):
    print(f"  {i + 1}. {register[i]}")

Key points from this example:

• append() adds the late student to the end of the list.
• remove() finds and deletes a student by their name (value).
• sort() puts the names in alphabetical (A-Z) order.
• The in keyword checks whether a name exists in the list.
• index() tells us the position of a name in the sorted list.
• len() gives the total count at any point.
• We use range(len(register)) to loop with a counter, so we can print numbered lines.