3.6 · Lists (1D Arrays)

Goal: create, read, update, search and process Python lists. (Lists are Python's 1D arrays.)

Creating a list

empty = []
fruits = ["apple", "banana", "cherry"]
zeros = [0] * 10        # ten zeros
counts = list(range(1, 6))  # [1, 2, 3, 4, 5]

Indexing (zero-based!)

fruits = ["apple", "banana", "cherry"]
print(fruits[0])    # apple
print(fruits[2])    # cherry
print(fruits[-1])   # cherry (last)

Slicing

nums = [10, 20, 30, 40, 50]
print(nums[1:4])    # [20, 30, 40]
print(nums[:2])     # [10, 20]
print(nums[2:])     # [30, 40, 50]
print(nums[::-1])   # [50, 40, 30, 20, 10]  (reverse)

Updating

fruits[1] = "blueberry"
fruits.append("date")
fruits.insert(0, "apricot")
fruits.remove("apple")
last = fruits.pop()

Length & membership

print(len(fruits))           # 4
print("banana" in fruits)    # True or False

Iterating

for f in fruits:
    print(f)

for i, f in enumerate(fruits):
    print(i, f)

Common operations

Need	Code
Sum	sum(nums)
Min / max	min(nums), max(nums)
Count occurrences	nums.count(5)
Sort (in place)	nums.sort()
Sorted copy	sorted(nums)
Reverse (in place)	nums.reverse()

Worked example · Class average

marks = [78, 65, 90, 55, 82, 73]
print("Average:", sum(marks) / len(marks))
print("Top:", max(marks))
print("Pass count:", sum(1 for m in marks if m >= 50))

Worked example · Read 5 marks then print stats

marks = []
for _ in range(5):
    marks.append(int(input("Mark: ")))

print("Min:", min(marks))
print("Max:", max(marks))
print("Avg:", sum(marks) / len(marks))

List vs array (in other languages)

In Pascal/C++ you usually declare an array with a fixed size and one type:

DECLARE marks[1..30] OF INTEGER

Python's list is more flexible: dynamic size, can hold mixed types. For HKDSE syllabus purposes, treat list as the Python equivalent of a 1D array.

Common student mistakes

• Index out of range: fruits[5] when the list has 3 items.
• Forgetting Python is 0-indexed when translating from pseudocode (1-indexed).
• Confusing nums.sort() (in place, returns None) with sorted(nums) (returns new list).
• Modifying a list while iterating over it.

Exam-style question

Q (5 marks): Write a Python program that reads 10 numbers into a list, then prints the maximum, minimum, and the index of the maximum.

Sample answer:

nums = [int(input(f"Number {i+1}: ")) for i in range(10)]
mx = max(nums)
mn = min(nums)
idx = nums.index(mx)
print(f"Max = {mx} at index {idx}")
print(f"Min = {mn}")

Key takeaways

• Python list = the 1D array of HKDSE.
• 0-indexed, mutable, dynamic size.
• Standard tools: len, sum, min, max, sort, sorted, in, count.

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