A stack is a last-in-first-out structure in which items are added and removed only at one end, called the top.

How is a stack implemented using an array?

An array holds the items and a separate integer called the stack pointer holds the subscript of the current top, which is increased before a push stores an item and decreased after a pop reads one.

What is stack overflow?

Stack overflow is an attempt to push when the stack pointer already holds the highest subscript of the array, so there is no free position left.

What is stack underflow?

Stack underflow is an attempt to pop when no items are stored, which is detected because the stack pointer still holds the value that marks an empty stack.

WJEC A-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

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Data Representation and Data Types

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Data Structures

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Organisation of Data: Files and Records

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Database Systems

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The Operating System

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Algorithms and Programs

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Principles of Programming

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Systems Analysis

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Software Engineering and Program Construction

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Types of Software System

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Practical Programming

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Data Security and Integrity

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Computing, Society and the Law

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Advanced Data Structures and Logic

Topic overview Arrays in three dimensions Stacks and queues using pointers and arrays Linked lists using pointers and arrays Binary trees using pointers and arrays Hash tables and collision resolution NAND, NOR and Boolean simplification Masking, clearing registers and encryption

Recursion, Sorting and Searching

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Programming Paradigms and Translators

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System Design and Human-Computer Interaction

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Software Engineering Tools and Professional Practice

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Advanced Architecture and Assembly Language

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Communication Networks and File Organisation

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Number Representation and Floating Point

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Databases and Distributed Systems

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Operating System Management

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Software Systems and Applications

Topic overview The range of software system types Safety related and safety critical systems Computer control and automation The scope of automation in practice Expert systems The internet, intranets and search Selecting and evaluating a software system

Data Security, Cryptography and Attack

Topic overview Integrity problems in online updating The need for cryptography Symmetric and asymmetric techniques Classical ciphers and cryptographic algorithms Comparing cryptographic strength Biometric technologies Malicious software and its vectors Attacks, defence and penetration testing

Advanced Data Structures and Logic

Stacks and queues using pointers and arrays

3 things to know
A2 Unit 3

Overview

Stack overflow, stack underflow and priority queue.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is a stack?

Key term

Name the three operations a stack supports and state what each does.

Key term

Write pseudocode for the push operation on an array-based stack.

Questions from this subtopic

Each one with the model answer underneath.

1
What is a stack?
Model answer
A stack is a last-in-first-out structure in which items are added and removed only at one end, called the top.
2
Name the three operations a stack supports and state what each does.
Model answer
Push adds an item to the top, pop removes and returns the top item, and peek reports the top item without removing it.
3
How is a stack implemented using an array?
Model answer
An array holds the items and a separate integer called the stack pointer holds the subscript of the current top, which is increased before a push stores an item and decreased after a pop reads one.
4
What is stack overflow?
Model answer
Stack overflow is an attempt to push when the stack pointer already holds the highest subscript of the array, so there is no free position left.
5
What is stack underflow?
Model answer
Stack underflow is an attempt to pop when no items are stored, which is detected because the stack pointer still holds the value that marks an empty stack.
6
Write pseudocode for the push operation on an array-based stack.
Model answer
The code is IF top = maxIndex THEN OUTPUT overflow ELSE top ← top + 1; stack[top] ← item; ENDIF.

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