4.2.1 Arrays, Records & Abstract Data Types — AQA 7517

Abstract Data Types (ADTs)

An Abstract Data Type (ADT) is a logical description of how data is organised and the operations that can be performed on it, without specifying how those operations are implemented. ADTs define the interface, not the implementation.

Examples of ADTs: Stack, Queue, Graph, Tree, Dictionary, Set. Each has defined operations but can be implemented in different ways (e.g. using arrays or linked lists).

Arrays

An array is a static data structure that stores an ordered, fixed-size collection of elements of the same data type in contiguous memory locations. Arrays are the most fundamental built-in data structure.

1D Arrays

// AQA pseudocode — declare a 1D array (0-indexed)
DECLARE scores : ARRAY[0:9] OF INTEGER   // 10 elements, index 0-9
scores[0] ← 85
scores[1] ← 92
OUTPUT scores[0]   // Output: 85

2D Arrays

A 2D array stores data in rows and columns (a grid/matrix).

DECLARE grid : ARRAY[0:2, 0:2] OF INTEGER   // 3x3 grid
grid[0,0] ← 1
grid[1,2] ← 7
OUTPUT grid[1,2]   // Output: 7

Array Properties

Property	Value
Size	Fixed at declaration (static)
Data type	All elements must be the same type (homogeneous)
Access	Direct (random) access via index in O(1)
Memory	Contiguous — elements stored next to each other
Indexing	0-based in AQA pseudocode (ARRAY[0:n-1])

Records

A record is a data structure that stores a collection of related fields that may be of different data types, grouped together to represent a single entity. Like a row in a database table.

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

DECLARE s1 : StudentRecord
s1.name ← "Alice"
s1.age ← 17
s1.grade ← 'A'
s1.score ← 94.5
OUTPUT s1.name   // Output: Alice

Arrays of Records

DECLARE students : ARRAY[0:29] OF StudentRecord   // 30 students
students[0].name ← "Bob"
students[0].score ← 88.0

Key Differences: Arrays vs Records

Feature	Array	Record
Data types	All elements same type (homogeneous)	Fields can be different types (heterogeneous)
Access	By numerical index	By field name (dot notation)
Purpose	Collection of similar items	Single entity with multiple attributes
Size	Fixed at declaration	Fixed structure

Exam tip: AQA questions may ask you to declare arrays and records in pseudocode, access specific elements, and explain the difference between them. Know that arrays are homogeneous (same type) and accessed by index, while records are heterogeneous (different types) and accessed by field name. ADT questions often ask you to distinguish between the logical description (ADT) and the actual implementation.

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Worksheet — 4.2.1 Arrays, Records & ADTs

8 questions · instantly marked · AQA 7517 standard

Q1Define an Abstract Data Type (ADT). Explain how it differs from a concrete data structure.[3]

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An ADT is a logical description of how data is organised and the operations that can be performed on it [1]; without specifying how those operations are implemented [1]; a concrete data structure specifies an actual implementation (e.g. using arrays or pointers) whereas an ADT only defines the interface [1].

Q2Write AQA pseudocode to declare a 1D array called temps that stores 7 real numbers (0-indexed), then assign 23.5 to the third element.[3]

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DECLARE temps : ARRAY[0:6] OF REAL [1]; temps[2] ← 23.5 [1] (index 2 = third element in 0-indexed array) [1 for correct understanding of 0-based indexing].

Q3State two differences between an array and a record.[2]

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Any two: arrays store elements of the same type (homogeneous); records can store fields of different types (heterogeneous) [1]; arrays are accessed by numerical index; records are accessed by field name [1]; arrays store a collection of similar items; records represent a single entity with multiple attributes [1].

Q4A school wants to store a register of 30 students, each with a name (STRING), form number (INTEGER), and whether they are present (BOOLEAN). Define an appropriate record type in AQA pseudocode.[4]

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TYPE StudentReg [1]; DECLARE name : STRING [1]; DECLARE formNumber : INTEGER; DECLARE present : BOOLEAN [1]; ENDTYPE [1].

Q5Using the record type from Q4, write code to declare an array of 30 StudentReg records and assign "Alice" and TRUE to the first student's name and present fields.[3]

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DECLARE register : ARRAY[0:29] OF StudentReg [1]; register[0].name ← "Alice" [1]; register[0].present ← TRUE [1].

Q6Give two properties of an array that make it different from other data structures.[2]

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Any two: fixed size (static) [1]; homogeneous — all elements same type [1]; direct (random) access by index in O(1) [1]; contiguous memory storage [1].

Q7Write AQA pseudocode to declare a 3×4 2D array called matrix of integers, and output the element at row 2, column 3 (0-indexed).[2]

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DECLARE matrix : ARRAY[0:2, 0:3] OF INTEGER [1]; OUTPUT matrix[2,3] [1].

Q8Name four examples of ADTs and, for each, state one operation associated with it.[4]

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Stack — push [1]; Queue — enqueue [1]; Graph — add edge [1]; Tree — insert node [1]; Dictionary — look up key [1]; Set — union/intersection [1]. Award one mark per correct ADT+operation pair (any four).

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Term	Definition

Abstract Data Types (ADTs)

Arrays

1D Arrays

2D Arrays

Array Properties

Records

Arrays of Records

Key Differences: Arrays vs Records

Worksheet — 4.2.1 Arrays, Records & ADTs

Mini Test — Arrays, Records & ADTs