2.3 · Data Types & Structures

Goal: pick the right data type for each variable and recognise the simple data structures used in Module D.

Simple data types

Type	Stores	Python equivalent
Integer	Whole numbers	int
Real / Float	Decimal numbers	float
Character	A single character	str of length 1
Boolean	True / False	bool

Choosing the right type

• Use integer if no decimals are needed (age, count). Faster and exact.
• Use float for measurements (height, weight, money — though for money, fixed-point is safer in real systems).
• Use boolean for yes/no, on/off, found/not-found.
• Use character rarely; string is more common in real code.

Simple data structures

The syllabus restricts compulsory data structures to:

• String — sequence of characters
• 1D array — fixed-size ordered collection

(2D arrays, stacks, queues, linked lists are Elective 2C material.)

String basics

name ← "HKDSE"
length ← LENGTH(name)        // 5
first  ← name[1]             // 'H' (1-indexed pseudocode style)
rest   ← SUBSTRING(name, 2, 5) // 'KDSE'

In Python:

name = "HKDSE"
print(len(name))     # 5
print(name[0])       # 'H'   (Python is 0-indexed)
print(name[1:5])     # 'KDSE'

Indexing convention: pseudocode in HKEAA documents is often 1-indexed; Python is 0-indexed. Be careful when converting.

1D array basics

DECLARE marks[1..10]
marks[1] ← 65
marks[2] ← 90
total ← 0
FOR i ← 1 TO 10
    total ← total + marks[i]
END FOR
average ← total / 10

In Python (using a list):

marks = [65, 90, 78, 85, 92, 60, 73, 88, 95, 80]
average = sum(marks) / len(marks)

Boolean logic & truth tables

A	B	A AND B	A OR B	NOT A
F	F	F	F	T
F	T	F	T	T
T	F	F	T	F
T	T	T	T	F

You may be asked to fill in or apply truth tables for compound conditions.

Example · Decide eligibility for a discount

"Members get a discount if they are over 60 OR have spent more than $1,000."

discount ← (age > 60) OR (spent > 1000)

For age = 65, spent = 500 → True (first half true). For age = 30, spent = 2000 → True (second half true). For age = 25, spent = 100 → False.

Common student mistakes

• Storing a phone number as integer — loses leading zeros; use string.
• Confusing integer division with real division. In Python 5/2 = 2.5, 5//2 = 2.
• Off-by-one indexing when translating between pseudocode and Python.

Exam-style question

Q (4 marks): Select the most appropriate data type for each of the following variables and justify your choice: (a) student_id, (b) bmi, (c) is_member, (d) student_name.

Sample answer:

• (a) String — IDs may have leading zeros or letters; numeric arithmetic is never required.
• (b) Real / Float — BMI is a decimal value.
• (c) Boolean — only two states, member or not.
• (d) String — sequence of characters of variable length.

Key takeaways

• Four simple types: integer, real, character, boolean.
• Compulsory data structures: string and 1D array.
• Pick types that match the data's nature (no maths → no number).

➡️ Next: 2.4 Control Structures