Difference between "enqueue" and "dequeue"

Question

Can somebody please explain the main differences? I don't have a clear knowledge about these functions in programming for any language.

Sinful

Answer 1

Some of the basic data structures in programming languages such as C and C++ are stacks and queues.

The stack data structure follows the "First In Last Out" policy (FILO) where the first element inserted or "pushed" into a stack is the last element that is removed or "popped" from the stack.

Similarly, a queue data structure follows a "First In First Out" policy (as in the case of a normal queue when we stand in line at the counter), where the first element is pushed into the queue or "Enqueued" and the same element when it has to be removed from the queue is "Dequeued".

This is quite similar to push and pop in a stack, but the terms enqueue and dequeue avoid confusion as to whether the data structure in use is a stack or a queue.

Class coders has a simple program to demonstrate the enqueue and dequeue process. You could check it out for reference.

http://classcoders.blogspot.in/2012/01/enque-and-deque-in-c.html

Faculty

I used to interpret enqueue as insert in the end of the queue, dequeue to remove from the queue and not using the element at all, and unqueue to remove the element from the head to use it. Is my thinking correct? – Aguilar

UPDATE: after reading en.wikipedia.org/wiki/Queue_%28abstract_data_type%29 , I got that Dequeue is to extract the element from the head to make actual use of it. – Aguilar

I think is is needed to clarify it as: Enqueue: Add element to tail of queue; Dequeue: Extract and use element at head of queue; Unqueue: Remove and not use at all element from queue; Requeue: Add again to the end of the queue an element previously extracted. – Aguilar

Never heard of an "unqueue" or "requeue". I believe the principle of the data structure is simply to remove an object from the head of a queue or add an object to the tail of a queue. What you choose to do with those objects has no specific naming convention. – Freethinker

Have never in my long career heard the acronym FILO (except in relation to certain pastry products). It's always been LIFO which, while meaning the same thing, is probably more common :-) – Fauch

This is confusing af. Thankfully, Python deque simply uses "popleft" and "pop". Easy to understand for anyone familiar with lists. – Colotomy

Answer 2

Enqueue and Dequeue tend to be operations on a queue, a data structure that does exactly what it sounds like it does.

You enqueue items at one end and dequeue at the other, just like a line of people queuing up for tickets to the latest Taylor Swift concert (I was originally going to say Billy Joel but that would date me severely).

There are variations of queues such as double-ended ones where you can enqueue and dequeue at either end but the vast majority would be the simpler form:

           +---+---+---+
enqueue -> | 3 | 2 | 1 | -> dequeue
           +---+---+---+

That diagram shows a queue where you've enqueued the numbers 1, 2 and 3 in that order, without yet dequeuing any.

---

By way of example, here's some Python code that shows a simplistic queue in action, with enqueue and dequeue functions. Were it more serious code, it would be implemented as a class but it should be enough to illustrate the workings:

import random

def enqueue(lst, itm):
    lst.append(itm)        # Just add item to end of list.
    return lst             # And return list (for consistency with dequeue).

def dequeue(lst):
    itm = lst[0]           # Grab the first item in list.
    lst = lst[1:]          # Change list to remove first item.
    return (itm, lst)      # Then return item and new list.

# Test harness. Start with empty queue.

myList = []

# Enqueue or dequeue a bit, with latter having probability of 10%.

for _ in range(15):
    if random.randint(0, 9) == 0 and len(myList) > 0:
        (itm, myList) = dequeue(myList)
        print(f"Dequeued {itm} to give {myList}")
    else:
        itm = 10 * random.randint(1, 9)
        myList = enqueue(myList, itm)
        print(f"Enqueued {itm} to give {myList}")

# Now dequeue remainder of list.

print("========")
while len(myList) > 0:
    (itm, myList) = dequeue(myList)
    print(f"Dequeued {itm} to give {myList}")

A sample run of that shows it in operation:

Enqueued 70 to give [70]
Enqueued 20 to give [70, 20]
Enqueued 40 to give [70, 20, 40]
Enqueued 50 to give [70, 20, 40, 50]
Dequeued 70 to give [20, 40, 50]
Enqueued 20 to give [20, 40, 50, 20]
Enqueued 30 to give [20, 40, 50, 20, 30]
Enqueued 20 to give [20, 40, 50, 20, 30, 20]
Enqueued 70 to give [20, 40, 50, 20, 30, 20, 70]
Enqueued 20 to give [20, 40, 50, 20, 30, 20, 70, 20]
Enqueued 20 to give [20, 40, 50, 20, 30, 20, 70, 20, 20]
Dequeued 20 to give [40, 50, 20, 30, 20, 70, 20, 20]
Enqueued 80 to give [40, 50, 20, 30, 20, 70, 20, 20, 80]
Dequeued 40 to give [50, 20, 30, 20, 70, 20, 20, 80]
Enqueued 90 to give [50, 20, 30, 20, 70, 20, 20, 80, 90]
========
Dequeued 50 to give [20, 30, 20, 70, 20, 20, 80, 90]
Dequeued 20 to give [30, 20, 70, 20, 20, 80, 90]
Dequeued 30 to give [20, 70, 20, 20, 80, 90]
Dequeued 20 to give [70, 20, 20, 80, 90]
Dequeued 70 to give [20, 20, 80, 90]
Dequeued 20 to give [20, 80, 90]
Dequeued 20 to give [80, 90]
Dequeued 80 to give [90]
Dequeued 90 to give []

Fauch

Picture speaks a thousand words. – Anlace

Answer 3

These are terms usually used when describing a "FIFO" queue, that is "first in, first out". This works like a line. You decide to go to the movies. There is a long line to buy tickets, you decide to get into the queue to buy tickets, that is "Enqueue". at some point you are at the front of the line, and you get to buy a ticket, at which point you leave the line, that is "Dequeue".

Claman

Answer 4

A queue is a certain 2-sided data structure. You can add new elements on one side, and remove elements from the other side (as opposed to a stack that has only one side). Enqueue means to add an element, dequeue to remove an element. Please have a look here.

Maziar

Answer 5

Enqueue means to add an element, dequeue to remove an element.

var stackInput= []; // First stack
var stackOutput= []; // Second stack

// For enqueue, just push the item into the first stack
function enqueue(stackInput, item) {
  return stackInput.push(item);
}

function dequeue(stackInput, stackOutput) {
  // Reverse the stack such that the first element of the output stack is the
  // last element of the input stack. After that, pop the top of the output to
  // get the first element that was ever pushed into the input stack
  if (stackOutput.length <= 0) {
    while(stackInput.length > 0) {
      var elementToOutput = stackInput.pop();
      stackOutput.push(elementToOutput);
    }
  }

  return stackOutput.pop();
}

Verbosity

Answer 6

In my opinion one of the worst chosen word's to describe the process, as it is not related to anything in real-life or similar. In general the word "queue" is very bad as if pronounced, it sounds like the English character "q". See the inefficiency here?

enqueue: to place something into a queue; to add an element to the tail of a queue;

dequeue to take something out of a queue; to remove the first available element from the head of a queue

source: https://www.thefreedictionary.com

Abutter

Answer 7

As other's have said,

'Enqueue' is to add to a queue.

'Dequeue' is to remove from a queue

However, 'Dequeue' should not be confused with 'Deque' (which means a 'Double-Ended Queue' and usually pronounced as 'deck')

Redeeming