Queue Data Structure, GeeksForGeeks.org

A Queue is a linear data structure characterised by its first-in-first-out (FIFO) behaviour. Queues pop (remove, 'dequeue') the head (front, first element) of the Queue (i.e 1 is 'dequeued' from [1,2,3,4,5]) and append (add, put, 'enqueue') an element at the tail (back, rear, last element) of the Queue. Queues are fixed sized, meaning an object cannot be enqueued if the Queue is full.

q = Queue([1,2,3,4,5])
print(q)

[1,2,3,4,5]
Size: 6 (one more than the length of the Queue)

q.enqueue(6)
print(q)

[1,2,3,4,5,6]

q.dequeue() # no args since it will always remove the head of the Queue
print(q)

[2,3,4,5,6]
Removed: 1 (previous head)
New head: 2

q.enqueue(7)

[2,3,4,5,6,7]

q.enqueue(8)

raise QueueFullError("Cannot enqueue at full size: 6")

• Queue(items: object = None, size: int = None) - Optional parameters: items (the items in the queue) and size (the size allocated for the queue)

• items - the queue itself, more specifically its items, constructed as a list
• size - the size of the queue
• head - the first element of the queue, i.e self.items[0]
• tail - the last element of the queue, i.e self.items[-1]

• enqueue(item: object) - appends an object to the end of the queue; extends the queue if object is of type list
• dequeue() - removes the head of the queue (self.items.pop(0))
• peek() - equivalent to head attribute
• clear() - empties the queue
• merge(queue: 'Queue') - merges two queues together via enqueue() method
• frequency(item: object) - returns the amount of times (frequency) an item is present in a queue
• copy() - creates a copy of current queue and returns it
• is_full() - returns a True if queue is full
• is_empty() - returns True if queue is empty
• available_size() - subtracts the length of the queue by the size attribute to calculate the available size remaining in the queue
• sort() - sorts the queue in ascending order

• QueueFullError() - Raised when attempting to enqueue a full Queue, exceeding its fixed size
• QueueEmptyError() - Raised when attempting to dequeue an empty Queue
• QueueError() - Raised when ran into esoteric errors, such as trying to enqueue 2D arrays