Difference between "enqueue" and "dequeue"

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

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.

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.

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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 []

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".