Sample Notes: User-Defined Data Types

A2 Level Computer Science – Topic 13.1: User-defined Data Types

1. Purpose of User-defined Data Types

• Built-in types (e.g. Integer, Real, Boolean, Char) are limited for complex real-world modelling.
• User-defined data types allow programmers to define custom formats suited to specific problems.
• Essential for:
  • Abstraction: grouping related data together
  • Modelling complex systems (e.g. banking system, student record system)
  • Readability and maintainability of code

2. Non-composite User-defined Types

A. Enumerated Types (enum)

• Defines a list of named constant values.
• Improves code readability and prevents invalid data.
• Underlying values are integers starting from 0 unless specified.

Example:

TYPE DayOfWeek = (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday)

Usage:

VAR today: DayOfWeek
today := Friday

B. Pointer Types

• A pointer stores the memory address of a variable or object.
• Useful in dynamic data structures (e.g. linked lists, trees).
• In Pascal-like pseudocode:

TYPE NodePointer = ^Node

3. Composite User-defined Types

A. Record

• Groups different types of data under one name.
• Each element in a record is called a field.
• Similar to a struct in C or a class without methods in OOP.

Example:

TYPE Student = RECORD
    ID: INTEGER
    Name: STRING
    Grade: CHAR
END RECORD

Usage:

VAR s: Student
s.ID := 101
s.Name := "Ali"
s.Grade := 'A'

B. Set

• Unordered collection of distinct elements of the same type.
• Supports operations like union, intersection, and membership testing.

Example:

VAR S1, S2: SET OF 1..10
S1 := [1, 2, 3]
S2 := [3, 4, 5]

Operations:

• S1 ∪ S2 = [1, 2, 3, 4, 5]
• S1 ∩ S2 = [3]

C. Class/Object

• Part of Object-Oriented Programming (OOP).
• Combines data (attributes) and functions (methods).
• Supports encapsulation, inheritance, and polymorphism.

Class Example:

CLASS Car
    PRIVATE
        brand: STRING
        speed: INTEGER
    PUBLIC
        PROCEDURE Accelerate()
        BEGIN
            speed := speed + 10
        END PROCEDURE
END CLASS

Object Example:

VAR myCar: Car
myCar.Accelerate()

4. Choosing Appropriate User-defined Types

Problem Example 1: Student Data

• Requirements: Store student name, age, subjects taken.
• Suggested: Use a Record with fields Name: STRING, Age: INTEGER, Subjects: SET OF STRING.

Problem Example 2: Weekday Representation

• Requirements: Allow only valid weekdays as input.
• Suggested: Use an Enumerated Type Day = (Mon, Tue, Wed, Thu, Fri)

Problem Example 3: Traffic Light System

• Requirements: Store current light color (Red, Yellow, Green).
• Suggested: Enumerated type: Light = (Red, Yellow, Green)

Problem Example 4: Dynamic List of Tasks

• Requirements: A list that grows dynamically as tasks are added.
• Suggested: Use a Pointer-based Linked List.

5. Summary Table of User-defined Types