Queue

Queue

A queue is a list where you can only insert new items at the back and remove items from the front. This ensures that the first item you enqueue is also the first item you dequeue. First come, first serve!

Why would you need this? Well, in many algorithms you want to add objects to a temporary list at some point and then pull them off this list again at a later time. Often the order in which you add and remove these objects matters.

A queue gives you a FIFO or first-in, first-out order. The element you inserted first is also the first one to come out again. It's only fair! (A very similar data structure, the stack, is LIFO or last-in first-out.)

For example, let's enqueue a number:

queue.enqueue(10)

The queue is now [ 10 ]. Add the next number to the queue:

queue.enqueue(3)

The queue is now [ 10, 3 ]. Add one more number:

queue.enqueue(57)

The queue is now [ 10, 3, 57 ]. Let's dequeue to pull the first element off the front of the queue:

queue.dequeue()

This returns 10 because that was the first number we inserted. The queue is now [ 3, 57 ]. Everyone moved up by one place.

queue.dequeue()

This returns 3, the next dequeue returns 57, and so on. If the queue is empty, dequeuing returns nil or in some implementations it gives an error message.

Note: A queue is not always the best choice. If the order in which the items are added and removed from the list isn't important, you might as well use a stack instead of a queue. Stacks are simpler and faster.

The code

Here is a very simplistic implementation of a queue in Swift. It's just a wrapper around an array that lets you enqueue, dequeue, and peek at the front-most item:

public struct Queue<T> {
  fileprivate var array = [T]()

  public var isEmpty: Bool {
    return array.isEmpty
  }
  
  public var count: Int {
    return array.count
  }

  public mutating func enqueue(_ element: T) {
    array.append(element)
  }
  
  public mutating func dequeue() -> T? {
    if isEmpty {
      return nil
    } else {
      return array.removeFirst()
    }
  }
  
  public var front: T? {
    return array.first
  }
}

This queue works just fine but it is not optimal.

Enqueuing is an O(1) operation because adding to the end of an array always takes the same amount of time, regardless of the size of the array. So that's great.

You might be wondering why appending items to an array is O(1), or a constant-time operation. That is so because an array in Swift always has some empty space at the end. If we do the following:

var queue = Queue<String>()
queue.enqueue("Ada")
queue.enqueue("Steve")
queue.enqueue("Tim")

then the array might actually look like this:

[ "Ada", "Steve", "Tim", xxx, xxx, xxx ]

where xxx is memory that is reserved but not filled in yet. Adding a new element to the array overwrites the next unused spot:

[ "Ada", "Steve", "Tim", "Grace", xxx, xxx ]

This is simply matter of copying memory from one place to another, a constant-time operation.

Of course, there are only a limited number of such unused spots at the end of the array. When the last xxx gets used and you want to add another item, the array needs to resize to make more room.

Resizing includes allocating new memory and copying all the existing data over to the new array. This is an O(n) process, so it's relatively slow. But since it happens only every so often, the time for appending a new element to the end of the array is still O(1) on average, or O(1) "amortized".

The story for dequeueing is slightly different. To dequeue we remove the element from the beginning of the array, not the end. This is always an O(n) operation because it requires all remaining array elements to be shifted in memory.

In our example, dequeuing the first element "Ada" copies "Steve" in the place of "Ada", "Tim" in the place of "Steve", and "Grace" in the place of "Tim":

before   [ "Ada", "Steve", "Tim", "Grace", xxx, xxx ]
                   /       /      /
                  /       /      /
                 /       /      /
                /       /      /
 after   [ "Steve", "Tim", "Grace", xxx, xxx, xxx ]

Moving all these elements in memory is always an O(n) operation. So with our simple implementation of a queue, enqueuing is efficient but dequeueing leaves something to be desired...

A more efficient queue

To make dequeuing more efficient, we can use the same trick of reserving some extra free space, but this time do it at the front of the array. We're going to have to write this code ourselves as the built-in Swift array doesn't support this out of the box.

The idea is as follows: whenever we dequeue an item, we don't shift the contents of the array to the front (slow) but mark the item's position in the array as empty (fast). After dequeuing "Ada", the array is:

[ xxx, "Steve", "Tim", "Grace", xxx, xxx ]

After dequeuing "Steve", the array is:

[ xxx, xxx, "Tim", "Grace", xxx, xxx ]

These empty spots at the front never get reused for anything, so they're wasting space. Every so often you can trim the array by moving the remaining elements to the front again:

[ "Tim", "Grace", xxx, xxx, xxx, xxx ]

This trimming procedure involves shifting memory so it's an O(n) operation. But because it only happens once in a while, dequeuing is now O(1) on average.

Here is how you could implement this version of Queue:

public struct Queue<T> {
  fileprivate var array = [T?]()
  fileprivate var head = 0
  
  public var isEmpty: Bool {
    return count == 0
  }

  public var count: Int {
    return array.count - head
  }
  
  public mutating func enqueue(_ element: T) {
    array.append(element)
  }
  
  public mutating func dequeue() -> T? {
    guard head < array.count, let element = array[head] else { return nil }

    array[head] = nil
    head += 1

    let percentage = Double(head)/Double(array.count)
    if array.count > 50 && percentage > 0.25 {
      array.removeFirst(head)
      head = 0
    }
    
    return element
  }
  
  public var front: T? {
    if isEmpty {
      return nil
    } else {
      return array[head]
    }
  }
}

The array now stores objects of type T? instead of just T because we need some way to mark array elements as being empty. The head variable is the index in the array of the front-most object.

Most of the new functionality sits in dequeue(). When we dequeue an item, we first set array[head] to nil to remove the object from the array. Then we increment head because now the next item has become the front one.

We go from this:

[ "Ada", "Steve", "Tim", "Grace", xxx, xxx ]
  head

to this:

[ xxx, "Steve", "Tim", "Grace", xxx, xxx ]
        head

It's like some weird supermarket where the people in the checkout lane don't shuffle forward towards the cash register, but the cash register moves up the queue.

Of course, if we never remove those empty spots at the front then the array will keep growing as we enqueue and dequeue elements. To periodically trim down the array, we do the following:

    let percentage = Double(head)/Double(array.count)
    if array.count > 50 && percentage > 0.25 {
      array.removeFirst(head)
      head = 0
    }

This calculates the percentage of empty spots at the beginning as a ratio of the total array size. If more than 25% of the array is unused, we chop off that wasted space. However, if the array is small we don't want to resize it all the time, so there must be at least 50 elements in the array before we try to trim it.

Note: I just pulled these numbers out of thin air -- you may need to tweak them based on the behavior of your app in a production environment.

To test this in a playground, do:

var q = Queue<String>()
q.array                   // [] empty array

q.enqueue("Ada")
q.enqueue("Steve")
q.enqueue("Tim")
q.array             // [{Some "Ada"}, {Some "Steve"}, {Some "Tim"}]
q.count             // 3

q.dequeue()         // "Ada"
q.array             // [nil, {Some "Steve"}, {Some "Tim"}]
q.count             // 2

q.dequeue()         // "Steve"
q.array             // [nil, nil, {Some "Tim"}]
q.count             // 1

q.enqueue("Grace")
q.array             // [nil, nil, {Some "Tim"}, {Some "Grace"}]
q.count             // 2

To test the trimming behavior, replace the line,

    if array.count > 50 && percentage > 0.25 {

with:

    if head > 2 {

Now if you dequeue another object, the array will look as follows:

q.dequeue()         // "Tim"
q.array             // [{Some "Grace"}]
q.count             // 1

The nil objects at the front have been removed and the array is no longer wasting space. This new version of Queue isn't much more complicated than the first one but dequeuing is now also an O(1) operation, just because we were a bit smarter about how we used the array.

See also

There are many other ways to create a queue. Alternative implementations use a [linked list](../Linked List/), a [circular buffer](../Ring Buffer/), or a heap.

Variations on this theme are deque, a double-ended queue where you can enqueue and dequeue at both ends, and [priority queue](../Priority Queue/), a sorted queue where the "most important" item is always at the front.

Written for Swift Algorithm Club by Matthijs Hollemans