Python Lists: Everything You Need to Know

What is a List?

A list is an ordered collection of items. In HKDSE ICT, lists are the most tested data structure after integers and strings.

scores = [85, 72, 91, 68, 79]
names = ["Chan", "Lee", "Wong"]
mixed = [1, "hello", 3.14, True]   # Lists can hold anything

Indexing

Each item has a position (index) starting from 0:

scores = [85, 72, 91, 68, 79]
#         0   1   2   3   4     (indexes)
#        -5  -4  -3  -2  -1     (negative indexes)

print(scores[0])    # 85 (first)
print(scores[-1])   # 79 (last)
print(scores[2])    # 91

Slicing

scores = [85, 72, 91, 68, 79]

print(scores[1:4])    # [72, 91, 68]
print(scores[:3])     # [85, 72, 91]
print(scores[2:])     # [91, 68, 79]
print(scores[::-1])   # [79, 68, 91, 72, 85] (reversed)

Modifying Lists

scores = [85, 72, 91]

# Change an item
scores[0] = 90
# scores is now [90, 72, 91]

# Add to the end
scores.append(68)
# [90, 72, 91, 68]

# Insert at position
scores.insert(1, 100)
# [90, 100, 72, 91, 68]

# Remove by value
scores.remove(72)
# [90, 100, 91, 68]

# Remove by index
removed = scores.pop(0)
# removed = 90, scores = [100, 91, 68]

# Clear everything
scores.clear()
# []

Useful Functions

nums = [3, 1, 4, 1, 5, 9, 2, 6]

len(nums)   # 8 (length)
sum(nums)   # 31
max(nums)   # 9
min(nums)   # 1
sorted(nums)  # [1, 1, 2, 3, 4, 5, 6, 9] (new list)

nums.sort()   # Sort in place
nums.reverse()  # Reverse in place
nums.count(1)   # 2 (how many 1s)
nums.index(5)   # position of first 5

Iterating Over Lists

scores = [85, 72, 91, 68, 79]

# Simple iteration
for score in scores:
    print(score)

# With index
for i, score in enumerate(scores):
    print(f"Student {i+1}: {score}")

# Calculate total
total = 0
for score in scores:
    total += score

List Comprehension: The Pythonic Way

A concise way to create new lists from existing ones:

# Long way
squares = []
for n in range(10):
    squares.append(n * n)

# Pythonic way (same result)
squares = [n * n for n in range(10)]

# With condition
evens = [n for n in range(20) if n % 2 == 0]

# Transform strings
names = ["chan", "lee", "wong"]
upper = [name.upper() for name in names]
# ["CHAN", "LEE", "WONG"]

2D Lists (Lists of Lists)

Essential for matrices and grids:

grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

print(grid[1][2])   # 6 (row 1, column 2)

# Iterate all cells
for row in grid:
    for val in row:
        print(val, end=" ")
    print()

Common Pitfalls

Copying Lists

a = [1, 2, 3]
b = a           # Both variables point to SAME list
b.append(4)
print(a)        # [1, 2, 3, 4] — a changed too!

# To copy:
b = a.copy()    # or a[:] or list(a)

Modifying While Iterating

lst = [1, 2, 3, 4, 5]
for x in lst:
    if x % 2 == 0:
        lst.remove(x)   # ✗ Dangerous, skips items

# Better: iterate over a copy
for x in lst[:]:
    if x % 2 == 0:
        lst.remove(x)

Real-World Example

# Class score analysis
scores = [85, 72, 91, 68, 79, 45, 82, 90]

print(f"Students: {len(scores)}")
print(f"Average: {sum(scores)/len(scores):.1f}")
print(f"Highest: {max(scores)}")
print(f"Lowest: {min(scores)}")
print(f"Passes: {len([s for s in scores if s >= 50])}")
print(f"Top 3: {sorted(scores, reverse=True)[:3]}")