Queues

1
Queues

• Lecture 30
• Mon, Apr 10, 2006

2
Topics

• Queues
• Queue ADT
• Queue implementation

3
Queues

• A queue is a List that operates under the
  principle first in, first out (FIFO).
• New elements are enqueued into the queue.
• Old elements are dequeued from the queue.
• To enforce the FIFO principle, we enqueue and
  dequeue at opposite ends.

4
Implementation of Queues

• Implement a Queue as a subclass of a List class.
• Use pushFront() and popBack(), or
• Use pushBack() and popFront().
• Choose a List class for which enqueuing and
  dequeuing will be efficient.

5
Private Inheritance

• Private inheritance prevents violations of the
  FIFO principle.
• The Queue class has access to all public and
  protected List functions.
• The Queue class user has access only to the queue
  functions.

6
Queue Constructors

• Construct an empty queue.
• Construct a copy of the specified queue.

Queue()
Queue(const Queue q)
7
Inspectors

• Get a copy of the element at the head of the
  queue.
• Get the number of elements in the queue.
• Determine whether the queue is empty.

T head() const
int size() const
bool isEmpty() const
8
Mutators

• Enqueue the specified value at the tail of the
  queue.
• Dequeue and return the element at the head of the
  queue.
• Make the queue empty.

void enqueue(const T value)
T dequeue()
void makeEmpty()
9
Facilitators

• Read a queue from the specified input stream.
• Write a queue to the specified output stream.

void Input(istream in)
void Output(ostream out) const
10
Other Member Functions

• Determine whether the queue has a valid structure.

void isValid() const
11
Non-Member Functions

• Read a queue from the specified input stream.
• Write a queue to the specified output stream.

istream operatorgtgt(istream in, Queue q)
ostream operatorltlt(ostream out, const Queue q)
12
Queue Implementation

• Choose an appropriate List class as a base class.
  
• Good choices
• CircArrayList
• LinkedListwTail
• DoublyLinkedList
• CircLinkedList

13
Queue Implementation

• Bad choices
• ArrayList
• LinkedList
• Use private inheritance to enforce the Queue
  structure on the List.