User-Defined Types (Cambridge (CIE) A Level Computer Science): Revision Note

What are user-defined data types?

• A user-defined data type is a custom data structure created by the programmer to match the specific needs of a program

  • It is built using primitive data types such as INTEGER, STRING, BOOLEAN, etc

  • It helps represent real-world entities more accurately than built-in types alone

Non-composite

What is an enumerated data type?

• An enumerated data type is a type of non-composite, user-defined data type that consists of a list of named, fixed values

• It allows the programmer to define a variable that can only take one of a limited set of values

• Making code clearer, more readable, and easier to validate

• An enumerated data type should be used:

  • When a variable should only hold predefined values

  • To improve code readability and reduce errors

  • To restrict input to a known, fixed list

Example code

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

DECLARE Today : DayType

Today ← Monday

IF Today = Saturday OR Today = Sunday THEN
    OUTPUT "It's the weekend!"
ELSE
    OUTPUT "It's a weekday."
ENDIF

• DayType is the enumerated type

• Today can only be one of the seven values listed in DayType

• This makes it easy to check against valid options and avoids invalid assignments

What is a pointer data type?

• A pointer is a special type of variable that stores the memory address of another variable or data item, rather than the data itself

• Pointers are used to demonstrate dynamic data structures, such as linked lists

• The ^ indicates the data type is a pointer

• <data type> indicates the type of data stored in the memory location

Example code

TYPE Node
    DECLARE Data : INTEGER
    DECLARE Next : ^Node
ENDTYPE

DECLARE StartPtr : ^Node
DECLARE NewNode : ^Node

// Create a new node
NEW(NewNode)
NewNode^.Data ← 10
NewNode^.Next ← NULL

// Initialise the list
StartPtr ← NewNode

Use of pointers in dynamic structures

• Linked lists: Each node uses a pointer to link to the next

• Stacks and queues: Implemented using pointers to track top/front positions

• Memory efficiency: Allows storage to be reused and allocated at runtime

Composite

What are records?

• A record is a composite user-defined data type that allows you to group together multiple related fields, each of which can have different data types

• Records are used to represent real-world entities, such as a student, product, or customer, in a single structure

Example code

TYPE Student
    DECLARE Name : STRING
    DECLARE Age : INTEGER
    DECLARE Enrolled : BOOLEAN
ENDTYPE

DECLARE S1 : Student

S1.Name ← "Ali"
S1.Age ← 17
S1.Enrolled ← TRUE

OUTPUT S1.Name, " is ", S1.Age, " years old."

• Groups related data together for easy access

• Makes programs easier to manage and read

• Ideal for arrays of records (e.g. a list of students)

What are sets?

• A set is an unordered collection of unique elements

• Sets are useful for situations where duplication is not allowed, and where membership testing is common

• Features of a set include:

  • No duplicate values

  • Order is not important

  • Can perform operations like union, intersection, and difference

Example code

TYPE ColourSet = SET OF STRING
DEFINE Colours ("Red", "Green", "Blue") : ColourSet

IF "Green" ISIN Colours THEN
    OUTPUT "Green is in the set"
ENDIF

• This creates a set named Colours containing "Red", "Green", and "Blue" using the user-defined set type ColourSet