What is a Queue?

A queue is another abstract data type built upon the array data structure.

A queue stores items in a First-In -> First-Out manner, commonly referred to as FIFO.

The ‘head’ is the first element (oldest), the ‘tail’ is the last element (most recent).

Example of a Queue

friend_names = ['Ryan', 'Charlie', 'Sean', 'Kyle']

[0] = 'Ryan' #(head)

[4] = 'Kyle' #(tail)

Methods of a Queue

• enqueue(item) - add an element to the tail of the queue
• dequeue() - remove (return) the head element of the queue

Queue Class Definition in Python

class Queue:
    def __init__(self):
        self.items = []

    def enqueue(self, item):
        self.items.append(item)

    def dequeue(self):
        self.items.pop(0)

    def peek(self):
        return self.items[0]  # view first item (front of queue)

Queue Implementation

q = Queue()  # create a new Stack

q.enqueue('a')
q.enqueue('b')
q.enqueue('c')
q.enqueue('d')

print("Top is:", q.peek(), "\n")  # 'a'
q.dequeue()

print("Top is:", q.peek(), "\n")  # 'b'
q.dequeue()

q.enqueue('e')

# q = ['c','d','e']

Why use a Queue?