Queue

Task: Add three values to the queue

Why we need a Queue

Imagine you are standing in a line at a ticket counter. Riya comes first. Then Arjun. Then Sara.
Riya gets her ticket first and leaves. Arjun is next. Sara waits behind Arjun.
A person who joins later cannot leave before someone who was already waiting.
That is the basic idea of a queue: first in, first out.

What is a queue?

A queue is a data structure that follows the FIFO rule: First In, First Out.

There are two ends. The front is where elements leave, and the rear is where new elements join.

So enqueue goes to the rear, and dequeue takes from the front.

The four basic operations

Operation	What it does
enqueue(x)	Adds x at the rear
dequeue()	Removes the element from the front
peek()	Shows the front element
isEmpty()	Checks if the queue is empty

Try it yourself

First in, first out

outin12front275rearempty

Add a value to begin

3 waiting

Try dequeue a few times. The front leaves and everyone behind moves up a place, which is the one thing a queue does that a stack never has to.

Queue in one picture

New elements join at the rear. Old elements leave from the front.

That is why the first element added is the first one removed.

A queue using an array

A queue can be stored inside an array.

But a normal array creates a small problem. Suppose the queue reaches the end of the array and some elements at the beginning have already been removed. Those empty spaces are still free, but the rear has reached the end.

We do not want to waste that space.

Circular queue

The solution is a circular queue. Instead of stopping at the last index, the queue goes back to the beginning.

So after index 5, the next position is index 0. This lets us reuse the empty spaces at the beginning.

When the rear reaches the end, it wraps around to the beginning. That is why it is called a circular queue.

Queue vs stack

The easiest way to remember the difference:

Stack: the last person leaves first.
Queue: the first person leaves first.

	Queue	Stack
Rule	FIFO	LIFO
Example	Line of people	Pile of plates
Add	enqueue	push
Remove	dequeue	pop
Main end	Front and rear	Top
Common use	Scheduling, BFS	Undo, DFS

Ask yourself: do I need the newest item first? That is a stack. Do I need the oldest item first? That is a queue.

Where are queues used?

Queues are useful whenever things need to be handled one after another in order.

Printer jobs: print jobs run in the order they arrive
CPU scheduling: processes wait for their turn
BFS: visits nodes level by level
Customer service: customers are served in order
Network requests: requests wait in a queue
Message systems: messages wait to be processed

The common idea is simple: something arrives, waits, then gets processed.

Queue in code

You do not need much code to understand a queue. In Python, deque can be used:

Python

from collections import deque queue = deque() queue.append(10)   # enqueuequeue.append(20) queue.popleft()    # dequeue -> 10queue[0]           # peek -> 20

After adding 10 and 20, the queue holds 10, 20 with 10 at the front. The first item added is the first one removed.

Common mistakes

Adding at the front. A normal queue adds new elements at the rear.
Removing from the rear. A normal queue removes elements from the front.
Forgetting the empty case. You cannot dequeue when the queue is empty.

Watch out

In Python, list.pop(0) works, but removing the first element from a normal list takes O(n) time.

Use deque when you need an efficient queue.

Quick recap

A queue is a line of people. FIFO, first in first out.

Enqueue: add at the rear
Dequeue: remove from the front
Peek: see the front element
isEmpty: check whether the queue is empty
Circular queue: reuse space by wrapping around

New things join at the back. Old things leave from the front.