AQA A-Level Computer Science: Abstract Data Types – Stacks and Queues

Abstract Data Types: Stacks and Queues

This tutorial introduces two fundamental abstract data types (ADTs) – stacks and queues, exploring their operations and applications in data management and process scheduling.

Stacks

A stack is a linear data structure that follows the Last-In, First-Out (LIFO) principle. Imagine a stack of plates: you can only add or remove plates from the top.

Key Operations:

• Push: Adds an element to the top of the stack.
• Pop: Removes and returns the element at the top of the stack.
• Peek: Returns the element at the top of the stack without removing it.
• IsEmpty: Checks if the stack is empty.

Example Implementation:

push(data)
pop()
peek()
isEmpty()

Applications:

• Function Call Stack: When a function is called, its parameters and local variables are pushed onto the stack. When the function returns, these elements are popped off the stack.
• Undo/Redo Functionality: Many applications use stacks to store the history of user actions for undo/redo operations.
• Expression Evaluation: Stacks are used to evaluate arithmetic expressions, especially those involving parentheses.

Queues

A queue is a linear data structure that follows the First-In, First-Out (FIFO) principle. Imagine a queue at a supermarket: the first person in line is served first.

Key Operations:

• Enqueue: Adds an element to the back of the queue.
• Dequeue: Removes and returns the element at the front of the queue.
• Peek: Returns the element at the front of the queue without removing it.
• IsEmpty: Checks if the queue is empty.

Example Implementation:

enqueue(data)
dequeue()
peek()
isEmpty()

Applications:

• Process Scheduling: Queues are used in operating systems to manage processes waiting for resources.
• Buffering: Queues act as buffers to store data temporarily before it's processed, ensuring smooth flow between different parts of a system.
• Print Queues: Print jobs are stored in queues before being sent to the printer.

Summary

Stacks and queues are powerful ADTs that offer efficient data management and processing capabilities. Understanding their operations and applications is crucial for designing and implementing various software solutions, particularly in areas like program logic and algorithm design.