# Differences Between Stack and Queue Data Structures

09 May 2024
Beginner
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## Stack Vs. Queue: An Overview

Stack and Queue are the types of non-primitive linear data structures. In a stack, you can add or remove elements only from one end called the top. In the case of a queue, there are two ends, REAR and FRONT for insertion and deletion respectively.

In this DSA tutorial, we'll analyze the differences between stacks and queues in data structures. To further enhance your understanding of data structures, enroll in our best Data Structures and Algorithms Course.

## What is Stack?

A stack is an ordered list or we can say a container in which insertion and deletion can be done from the one end known as the top of the stack. The last inserted element is available first and is the first one to be deleted. Hence, it is known as Last In, First Out LIFO, or First In, Last Out FILO.

In a stack, the element at the top is known as the top element. When a new element is added to the stack, it is placed on top of the existing elements. Stacks are dynamic; i.e they do not have a fixed size and their size can be increased or decreased depending on the number of elements

### Operations of a Stack

1. push(): The push() operation is one of the fundamental operations in a stack data structure. It means inserting an element at the top of the stack. If the stack is full, it is said to be an Overflow condition.
2. pop(): The pop() operation removes the topmost element of the stack and returns its value. This operation modifies the state of the stack by removing the topmost element. The items are popped in the reversed order in which they are pushed. If the Stack is empty, it is an Underflow condition.
3. peek(): The peek() operation returns the value of the topmost element of the stack without modifying the stack.
4. isFull(): The isFull() operation determines if the stack is full. A stack is said to be full if it has reached its maximum capacity and there is no more space to add new elements to the stack.
5. isEmpty(): The isEmpty() operation is used to check if the stack is empty or not. It returns a boolean value, true when the stack is empty, otherwise false.

## What is Queue?

A queue is an ordered list in which insertion is done at one end called REAR and deletion at another end called FRONT. The first inserted element is available first for the operations to be performed and is the first one to be deleted. Hence, it is known as First In First Out, FIFO, or Last In Last Out, LILO.

### Operations of a Queue

1. enqueue(): The enqueue() operation is used to insert an element at the back of a queue, to the end of a queue, or the rear end of the queue.
2. dequeue(): The dequeue() operation removes and returns the element from the front of a queue.
3. peek(): The peek() operation returns the value at the front end of the queue without removing it.
4. isFull(): The isFull() operation determines if the queue is full. A queue is said to be full if it has reached its maximum capacity and there is no more space to add new elements to it.
5. isEmpty(): The isEmpty() operation is used to check if the queue is empty or not. It returns a boolean value, true when the queue is empty, otherwise false.

Read More: Queue in Data Structures

## How Stacks and Queues are Similar?

• Linear Data Structure: Both stacks and queues come in the category of linear data structure i.e. data in these data structures is arranged in a sequence, one after the other i.e. each element appears to be connected linearly.
• Implementation: Both of them can be implemented using arrays and linked lists.
• Flexible in Size: Both stack and queue are flexible in size i.e. they can grow and shrink as per the requirements at the run-time.
• Concurrency: Stacks and queues are used in concurrent programming and parallel computing to manage tasks and data access.

## Stacks Vs. Queues: A Thorough Comparison

 Parameters Stack Queue Working Principle It follows the `LIFO` (Last In First Out) order to store the elements, which means the element that is inserted last will come out first. It follows the `FIFO` (First In First Out) order to store the elements, which means the element that is inserted first will come out first. Pointers It has only one end, known as the `top`, at which both insertion and deletion take place. It has two ends,`rear` and `front`, which are used for insertion and deletion. The `rear` end is used to insert the elements, whereas the `front` end is used to delete the elements from the queue. Operations The insertion operation is known as `push` and the deletion operation is known as `pop`. The insertion operation is known as `enqueue` and the deletion operation is known as `dequeue`. Empty Condition The condition for checking whether the stack is empty is `top ==-1` as `-1` refers to no element in the stack. The condition for checking whether the queue is empty is `front == -1` Full Condition The condition for checking if the stack is full is `top==max-1` as `max` refers to the maximum number of elements that can be in the stack. The condition for checking if the queue is full is `rear==max-1` as `max` refers to the maximum number of elements that can be in the queue. Variants There are no other variants or types of the stack. There are three types of queues: `circular queue`, `double-ended queue`, and `priority queue`. Implementation It has a simple implementation compared to queues as no two pointers are involved. It has a complex implementation compared to stacks as two pointers `front` and `rear` are involved. Data Representation Often implemented with `arrays` or `linked lists`. Can also be implemented with `arrays` or doubly linked lists. Example the Undo/Redo operation in Word or Excel. operating system process scheduling queues. Application It is used to solve `recursion`-based problems. It is used to solve sequential processing-based problems. Visualization A stack can be visualized as a vertical collection. Queue can be visualized as a horizontal collection.
##### Summary

I hope that you are now clear on the differentiating factors between the two most popular data structures; stacks and queues. We know the wide applications of both data structures in real-time applications. So, you must be thorough with all the underlying concepts of these two data structures.

## FAQs

### Q1. What is the difference between a queue and a stack?

A queue follows the FIFO (First-In-First-Out) principle, where the first element added is the first to be removed, while a stack adheres to the LIFO (Last-In-First-Out) principle, where the last element added is the first to be removed.

### Q2. Why queue is preferred over stack?

Queues are preferred over stacks in scenarios where elements need to be processed in the order they were added (FIFO), such as task scheduling, breadth-first search, and handling requests in networking protocols, etc.

### Q3. When should I use a stack, and when should I use a queue?

You should use a stack when you need to implement operations that follow the LIFO principle, such as function calls, expression evaluation, and backtracking algorithms. and queue when you need to implement operations that follow the FIFO principle, such as task scheduling, breadth-first search, and handling requests in networking protocols.

### Q4. What are the typical operations performed on a stack and a queue?

Operations on Stack:
1. push()
2. pop()
3. peek()
4. isFull()
5. isEmpty()
Operations on Queue:
1. enqueue()
2. dequeue()
3. peek()
4. isFull()
5. isEmpty()

### Q5. Are there any real-world analogies for stacks and queues?

• Real world analogies for stack are: stack of pates, books, call stack, etc.
• Real world analogies for queue are: students standing in a line for prayer, print queue, etc.
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