Mastering data structures in Ruby — Queues

Queues are a special kind of linked lists that allows you to handle a variable-sized collection of elements in first in first out fashion (FIFO). As well as all of the data structures we have seen so far; queues are just sequences of connected nodes.

The differentiating factor for queues is that the order of retrievals is guaranteed to be first in first out.

Contrary to what happens with the other lists we have seen; queues won’t allow us to insert/remove elements at random locations. All elements are inserted at the head of the queue and removed from its tail.

This is how a queue that holds two elements would look like:

[ 1 | next ] -> [ 2 | next ] -> X (nil)

Now let’s say we want to store a new element:

[ 1 | next ] -> [ 2 | next ] -> [ 3 | next ] -> X (nil)

And now we want to remove one:

[ 2 | next ] -> [ 3 | next ] -> X (nil)

Notice how queues grow to their tail and shrink from their head.

Queues interface

In comparison with other list alike data structures, the interface for queues is quite simple because they had few operations.

While you can add helper methods like find_first or cat, I think that those methods dim the semantics of this data structure and that is why I ‘ve decided to leave them out.

Implementation details

Each element on the queue is represented by a node that contains two attributes:

  • The value that the current node holds.
  • A pointer to the next node in the queue.

The first node of the queue is called head and the last one tail. To mark the end of the queue we must set the next pointer of the last element to nil.

What follows is a brief description of each method, its complexity and source code.


This method is the queue constructor. It creates an empty queue, sets the head node to nil and the queue’s length to 0.

The complexity of this method is O(1).

def initialize
self.head = nil
self.length = 0


Creates a new node to insert a value into the queue. If the queue has no nodes, the new node becomes the head of the queue; otherwise, it’s appended at the tail of the queue.

Since the queue keeps pointers to its head and its tail, the complexity of this method is O(1).

def enqueue data
node = data
unless head
self.head = node
else = node
self.tail = node
self.length += 1


Removes an element from the queue. In contrast to what happens on regular linked lists, removals on queues are straightforward because they always happen at the head of the queue.

To remove an element from the queue, we point the head to the node that it’s next to it and set tail to nil if the queue contains just one element.

def dequeue
return nil unless self.length > 0
self.head =
self.tail = nil if self.length == 1
self.length -= 1


Returns the node that’s at the head of the queue without removing it or nil if the queue is empty.

Since dequeue doesn’t return the element that’s being removed, peek is the way to go if you have to do something with that element.

The complexity of this method is O(1).

def peek


Dequeues all elements from the queue.

def clear
while self.peek


This method walks the queue yielding one at a time until it reaches the last element.

Because of the semantics of queues, I’m not a big fan of having methods like this one on them, but I’ve to admit that from time to time they might be handy. (i.e., while running debugging sessions.)

The complexity to yield the next item in the queue is O(1). The complexity to yield the whole queue is O(n).

def each
return nil unless block_given?
current = self.head
while current
yield current
current =


Print the contents of the queue by walking its items.

The complexity of this method is O(n).

def print
if self.length == 0
puts "empty"
self.each { |node| puts }

When to use queues

Queues work well in those cases where you need to handle elements in first come first served basis.

Conference registrations, online selling systems, and event dispatchers are all good fits for queues.

As it happens with other data structures we have seen so far; queues work great for a relatively small variable-sized collection of data. If you have to handle millions of elements, you will have to wait until we get to “advance data structures” in this series.

So, that’s it for queues. I hope you enjoyed it!

You can get the source code from here.

Next post, Stacks.

Thanks for reading! Also, don’t forget to clap if you like this post :)

PS: This post is part of a series and as we move forward subjects that were thoroughly discussed on previous posts are shallowly covered. If you want to get the most of this series, I recommend you to start from the first post on singly linked list.

PS2: If you are a member of the Kindle Unlimited program, you can read a bit polished version of this series for free on your Kindle device.

Data Structures From Scratch — Ruby Edition.