Difference between Stack and Queue

This article compares stacks and queues in programming. Stacks use LIFO for undo mechanisms and function calls, while queues use FIFO for managing processes, such as printing jobs or CPU scheduling. The article provides examples and real-world analogies for easy understanding, making it a valuable resource for computer science and programming professionals.

Table of contents

• Difference Between Stack and Queue: Stack vs Queue
• What is Stack?
  • How Does Stack Work?
  • Applications of Stacks
• What is Queue?
  • How Does Queue Work?
  • Application of Queue
• When to use Stack and Queue?

What is the Difference Between Stack and Queue?

Parameter	Stack	Queue
Order of Operation	Last In First Out (LIFO)	First In First Out (FIFO)
Primary Operations	Push (add to top), Pop (remove from top)	Enqueue (add to rear), Dequeue (remove from front)
Access to Elements	Only the top element is accessible	Access to both front and rear elements
Use Cases	Undo mechanisms, function calls, backtracking algorithms, and expression evaluation.	Order processing, scheduling tasks, BFS in graphs, handling real-time data
Nature of Work	Adding and removing occur at the same end (top)	Adding occurs at one end (rear), and removing occurs at the other end (front)
Efficiency	Fast operations as it involves dealing with only one end	Slightly less efficient for arrays due to shifting elements but efficient with linked lists
Example	A pile of plates, browser history (back button)	A line of people waiting for a ticket, a printer queue
Real-world Analogy	Stack of books: can only access the top book	Queue at a grocery store: the first person in line is the first to be served
Data Access Pattern	Suitable for LIFO access pattern: accessing most recently added data first	Suitable for FIFO access pattern: processing data in the order it arrives

What is a Stack?

A Stack is a linear data structure operating on the "Last In, First Out" (LIFO) principle. This means the last element added to the stack is the first to be removed. It's akin to a stack of plates or books; you can only take the top item off first.

How Does a Stack Work?

A stack primarily has two operations:

• Push: Adds an item to the top of the stack.
• Pop: Removes the top item from the stack.

Additionally, stacks often support other operations like:

• Peek/Top: Returns the top element without removing it.
• IsEmpty: Checks if the stack is empty.

Example of Stack

• Undo mechanism in software application
• Web browsing history
• Function Calls in Programming
• Expression evaluation and syntax parsing
• Backtracking algorithm

Some other applications of the queue in real life are:

• People on an escalator
• Cashier line in a store
• A car wash line
• One way exits

Application of Stack

• Stack is used for memory management.
• Used by the Java Virtual Machine.
• The Stack is used for expression conversion. For example, infix to postfix or prefix to postfix.
• Used for string parsing or string reversal.
• Stacks are used for matching HTML tags in web development.
• Used to solve the backtracking problem 
• The Stack is also used in function calls for recursive functions.

What is a Queue?

A Queue is a linear data structure that operates on the "First In, First Out" (FIFO) principle. This means the first element added to the queue will be the first to be removed. It's similar to how a line or queue of people at a checkout operates: the first person in line is the first to be served and leave the line.

 

How Does a Queue Work?

A queue mainly involves two operations:

• Enqueue: Adds an item to the end of the queue.
• Dequeue: Removes an item from the front of the queue.

Other auxiliary operations often associated with queues include:

• Front: Retrieves the front item from the queue without removing it.
• Rear: Retrieves the last item from the queue without removing it.
• IsEmpty: Checks if the queue is empty.

Example of Queue

• In a call centre, incoming calls are placed in a queue. The next customer service representative attends to the first call in the queue.
• Vehicles at traffic signals or toll booths queue up and move forward as the light turns green or as they take the toll.
• In graph theory, the Breadth-First Search algorithm uses a queue to explore a graph level by level or layer by layer.
• A restaurant may use queuing to manage its tables and chairs to serve its customers efficiently.

Application of Queue

• Spooling on the printer
• Buffers for devices such as keyboards
• Applied to WhatsApp, if you send a message to a friend and you don’t have an internet connection, these messages will be queued on WhatsApp’s servers.
• It is used as a queue for a single shared resource such as CPU, disk, or printer.
• It is used as a buffer for MP3 players and portable CD players.
• It applies to the operating system to handle interrupts. 
• It applies to adding songs to the end or playing from the beginning.

When to Use Stack and Queue?

When to use Stack

• You're developing a text editor and need to implement an undo feature.
• In a maze-solving algorithm, you must track the path to backtrack if a dead end is encountered easily.
• In programming, when functions call other functions, the order of operations needs to be managed.
• When writing a calculator program that evaluates expressions like "3 + 4 * 2".

When to use Queue

• In an e-commerce platform, you need to process customer orders in the order they were placed.
• Multiple users send print jobs to a shared printer.
• Implementing an algorithm to traverse or search through a graph structure level by level.
• Managing which process gets the next CPU time or IO resource in an operating system.

Note:

The choice between using a stack or a queue largely hinges on the nature of the task:

• Use a Stack when you need to work with data in reverse order or need to backtrack.
• Use a Queue when you need to maintain the original order of data or process things as they come in.

Conclusion

Stacks and queues are two essential data structures frequently used in computer programming. Having a clear understanding of how these structures work is crucial for developing efficient software. In a queue, items are added to the end and retrieved from the top. The items enter the queue at the bottom and exit from the top. As an item leaves the top, another item can be added to the bottom without interfering with the previous item. On the other hand, in a stack, items are added to the top and retrieved from the bottom. The last item added in a stack is the first to be retrieved.

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