CAIE 9618 · Paper 3 · Topic 3.1.1

User-Defined
Data Types

Enumerated · Pointer · Composite Types · Set · Abstract Data Types

CSZone Cambridge International AS & A Level Computer Science 9618

Why User-Defined Types?

Beyond the Built-in Primitives

Primitive types (INTEGER, REAL, BOOLEAN, CHAR, STRING) are limited. User-defined types let programmers create types that precisely model the real-world problem. Cambridge 9618 Paper 3 requires four user-defined type categories.

ENUMERATED

List of named constants. E.g. days of week, seasons, card suits. Improves readability over using integer codes.

POINTER

Stores a memory address. Points to another variable or node. Used in dynamic data structures (linked lists, trees).

COMPOSITE (RECORD)

Groups multiple fields of different types under one name. E.g. a Student record with name, age, grade fields.

SET

A collection of unique values of the same type. Supports set operations: union, intersection, difference.

Enumerated Types

Named Constants in a List

TYPE Season = (Spring, Summer, Autumn, Winter)
ENDTYPE

TYPE Direction = (North, South, East, West)
ENDTYPE

DECLARE currentSeason : Season
currentSeason <- Summer

IF currentSeason = Winter THEN
OUTPUT "Wear a coat"
ENDIF

BENEFITS

Self-documenting: code reads naturally

Prevents invalid values (can't assign "Monsoon" to a Season)

Better than magic numbers (0=Spring, 1=Summer...)

Compiler can check for completeness in CASE statements

EXAM TIP

Values in enumerated types are ordered — you can compare them (Spring < Winter). The first value has implicit index 0 in many languages.

Pointer Types

Storing Memory Addresses

// Define a record type
TYPE Node
DECLARE data : INTEGER
DECLARE next : NodePtr
ENDTYPE

// Define a pointer type
TYPE NodePtr = ^Node
ENDTYPE

DECLARE p : NodePtr
NEW(p) // allocate memory
p^.data <- 42 // dereference with ^
DISPOSE(p) // free memory

KEY OPERATIONS

NEW(p) — allocate heap memory, p now points to it

DISPOSE(p) — deallocate heap memory, free it back

p^ — dereference: access the value p points to

NIL — null pointer (points to nothing); used as end marker

^ SYNTAX

TYPE NodePtr = ^Node means "pointer to a Node". p^.data means "the data field of the node that p points to".

Composite (Record) Types

Grouping Related Fields

TYPE Student
  DECLARE name : STRING
  DECLARE age : INTEGER
  DECLARE grade : CHAR
  DECLARE score : REAL
ENDTYPE

DECLARE s : Student
s.name <- "Amara"
s.age <- 17
s.grade <- 'A'
s.score <- 94.5

// Array of records
DECLARE class : ARRAY[1:30] OF Student
class[1].name <- "Ben"

KEY POINTS

TYPE ... ENDTYPE defines the structure (blueprint)

DECLARE varname : TypeName creates an instance

Access fields using dot notation: s.name

Can have arrays of records: class[i].grade

Fields can be any type — including other user-defined types

Can include pointer fields for linked structures

Exam Practice

Cambridge-style questions

Question 1

A programmer wants to store the suit of a playing card (Hearts, Diamonds, Clubs, Spades). Write the pseudocode to define a suitable user-defined type and declare a variable to store a card suit. [3]

TYPE Suit = (Hearts, Diamonds, Clubs, Spades)
ENDTYPE

DECLARE cardSuit : Suit

1

Correct use of TYPE ... = (...) ENDTYPE syntax

1

All four values listed inside parentheses, comma-separated

1

DECLARE variable with correct type name

Common Mistakes

Don't lose easy marks

1

Writing TYPE Season = Spring, Summer, Autumn, Winter — missing the parentheses. The CAIE pseudocode standard requires parentheses around the list: TYPE Season = (Spring, Summer, Autumn, Winter). Also missing ENDTYPE.

2

Forgetting ^ for dereference — if p is a pointer to a Node, p^.data accesses the data field. Writing p.data is wrong — that implies p is the record itself, not a pointer to one.

3

Using = instead of <- for assignment — in CAIE pseudocode, assignment is always <- not =. Writing s.name = "Amara" will lose marks. The equals sign = is used only for comparison.

Topic Summary — 3.1.1

What You Need to Know

ENUMERATED

TYPE Name = (val1, val2, val3) ENDTYPE
Use where finite set of named values needed

POINTER

TYPE PtrName = ^TypeName ENDTYPE
NEW(p) → allocate · DISPOSE(p) → free
p^.field → dereference · NIL → null

COMPOSITE / RECORD

TYPE Name
  DECLARE field : TYPE
ENDTYPE
Access: varname.fieldname
Arrays of records: arr[i].field

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3.1.2

File Organisation & Access

Serial · Sequential · Random · Index Files

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