What is a data structure?

A data structure is an organised way of holding a collection of related data items together so that a program can store them and reach them systematically.

What is a dynamic data structure?

A dynamic structure can grow and shrink while the program is running, claiming more memory as items are added and releasing it as they are removed.

Is an ordinary array static or dynamic, and what does that mean in practice?

It is static, which means a fixed number of positions is set aside when it is declared, so that space is occupied whether or not it is used and no extra positions can be added later.

Give two advantages of a static structure.

The memory needed is known before the program runs so nothing has to be claimed while it is going, and because the items sit in consecutive locations any one of them can be reached immediately.

WJEC AS-Level Computer Science 601-5345-3

Hardware: Architecture, Memory and Storage

Topic overview Components of a computer system and how they connect Von Neumann architecture and the CPU The fetch-execute cycle Memory and caching Parallel processing Input and output devices and their uses Secondary storage, fragmentation and defragmentation

Networks and Data Transmission

Topic overview Serial and parallel transmission Simplex, half duplex and full duplex Multiplexing and switching Packets and the TCP/IP protocol stack Networks, standards and protocols Collisions and shared media Routing, addressing and the internet

Logic Gates and Boolean Algebra

Topic overview Logic gates and their truth tables Boolean notation and expressions Building and reading truth tables Boolean identities and rules Simplifying Boolean expressions De Morgan's laws Masking and clearing registers

Data Representation and Data Types

Topic overview Bits, bytes and words The binary number system Hexadecimal notation Character sets and storing characters Primitive data types and storage requirements Binary arithmetic and negative numbers Real numbers, truncation and rounding

Data Structures

Topic overview Data structures and data types One-dimensional arrays Two-dimensional arrays Manipulating arrays Records and fields Arrays of records Choosing a data structure

Organisation of Data: Files and Records

Topic overview Files, records and fields Fixed and variable length records Master and transaction files File access and organisation methods Sequential file access and update algorithms File security, backup and archiving Data validation and verification

Database Systems

Topic overview Flat files and the relational model Relational and other contemporary database systems Primary, foreign, composite and candidate keys Indexes and links between tables Entity relationship modelling Normalisation to third normal form The database management system, queries and user views

The Operating System

Topic overview The need for an operating system The kernel and process management Memory protection and the backing store The user interface between user and hardware Directory structures and file attributes Utility software Batch, real time control and transaction processing

Algorithms and Programs

Topic overview Defining and designing algorithms Variables, constants and identifiers Scope, subprograms and parameter passing Sequence, selection and repetition Mathematical and logical operations Standard algorithms: sorting, searching and compression Dry running, trace tables and test data

Principles of Programming

Topic overview Procedural programming Event-driven and visual programming Mark-up languages Object-oriented programming: objects, classes and methods High-level languages Low-level languages Choosing a language for a problem

Systems Analysis

Topic overview The systems life cycle and the feasibility study Methods of investigation Analysis, abstraction and decomposition Design and data flow diagrams Testing and changeover Maintenance Backup, recovery and documentation

Software Engineering and Program Construction

Topic overview Integrated development environments and their purpose Editors, syntax highlighting and code entry tools Debugging tools: breakpoints, stepping and watches Translation, source code and object code Lexical analysis and the symbol table Syntax and semantic analysis Code generation and optimisation

Types of Software System

Topic overview Off-the-shelf, bespoke and custom software Open source and proprietary software Computers in weather forecasting Computer aided design and manufacture Robotics in industry Computer generated graphics and animation Expert systems

Practical Programming

Topic overview Designing data structures: fields, records and data types Key fields and methods of access Validation of input data Designing routines with pseudocode Tracing code and test data Prototyping and evaluating a solution Annotated listings and readable code

Data Security and Integrity

Topic overview Dangers of holding files of personal data Levels of permitted access and access control Passwords and authentication Clerical procedures and write-protection Threats to computer systems Malicious and accidental damage to data Contingency planning and disaster recovery

Computing, Society and the Law

Topic overview Social and economic change from computing Computers and the nature of employment The digital divide and cultural change Environmental cost of computing Data protection and freedom of information law The Computer Misuse Act 1990 and computer crime Copyright, surveillance and professional codes

Data Structures

7 subtopics
21 things to know
AS Unit 1

What Data Structures covers

Data Structures is examined in AS Unit 1, Fundamentals of Computer Science. It covers data structures and data types, one-dimensional arrays, two-dimensional arrays, manipulating arrays, records and fields, arrays of records and choosing a data structure.

Where it is assessed

AS Unit 1

Fundamentals of Computer Science, examined again at A2 at a more demanding level.

Check the WJEC specification ↗

The 7 subtopics

Each one has its own page. WJEC AS Computer Science students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why group related values into a data structure rather than using separate variables?

Key term

Give a suitable record structure for a book in a library system.

Key term

Why is a record better than several parallel arrays for describing one entity?

Key term

Describe how the record with a given membership number is found in an array of records

Key term

In what way does an array of records resemble a file of records?

Key term

Why is a two-dimensional array a poor substitute for an array of records?

Key term

What questions should be asked when choosing a data structure?

Key term

An array of ten elements has a lower bound of 0. What is the last valid index?

Key term

What must be given when a two-dimensional array is declared?

Key term

Describe the algorithm for finding the largest value in an array.

Key term

Distinguish between an elementary data type and a composite one.

Key term

Why does reaching any element of an array take the same amount of work?

Key term

How is a single column of a two-dimensional array totalled?

Key term

How is a new item inserted into the middle of an array while keeping the order?

Key term

When is a single variable all that is needed?

Key term

What is a static data structure?

Key term

State the calculation that gives the address of element i of an array.

Key term

What is row-major order?

Key term

How is an item deleted from the middle of an array while keeping the order?

Key term

How does a record differ from an array?

Key term

Justify a two-dimensional array for a noughts and crosses board.

Take Data Structures further

The same content, four other ways.

Questions from this topic

Each one with the model answer underneath.

1
What is a data structure?
Model answer
A data structure is an organised way of holding a collection of related data items together so that a program can store them and reach them systematically.
2
Distinguish between an elementary data type and a composite one.
Model answer
An elementary type holds a single value of one kind, whereas a composite type is built out of several items grouped together and referred to by one identifier.
3
What is a static data structure?
Model answer
A static structure has its size fixed when it is declared, so the memory is reserved in advance and the structure cannot grow while the program runs.
4
What is a dynamic data structure?
Model answer
A dynamic structure can grow and shrink while the program is running, claiming more memory as items are added and releasing it as they are removed.
5
Is an ordinary array static or dynamic, and what does that mean in practice?
Model answer
It is static, which means a fixed number of positions is set aside when it is declared, so that space is occupied whether or not it is used and no extra positions can be added later.
6
Give two advantages of a static structure.
Model answer
The memory needed is known before the program runs so nothing has to be claimed while it is going, and because the items sit in consecutive locations any one of them can be reached immediately.

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Hardware: Architecture, Memory and Storage

Networks and Data Transmission

Logic Gates and Boolean Algebra

Data Representation and Data Types

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Fetch, , execute.

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Data Structures revision questions

Is Data Structures examined at AS in WJEC Computer Science?: Yes. It sits in AS Unit 1, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does Data Structures cover in WJEC AS-Level Computer Science?: 7 subtopics: data structures and data types, one-dimensional arrays, two-dimensional arrays, manipulating arrays, records and fields, arrays of records and choosing a data structure. Each one has its own page with questions and model answers.
How is Data Structures organised?: By the WJEC 601-5345-3 specification, subtopic by subtopic, so it matches the order you are taught it in.

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