Bab-005_Queue

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Semester Genap 2010/2011

Beni Suranto, [email protected]

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8 A-2

The Abstract Data Type Queue

A queue

New items enter at the back, or rear, of the queue

Items leave from the front of the queue

First-in, first-out (FIFO) property

The first item inserted into a queue is the firstitem to leave

This is the same as waiting in a line. Firstperson in line gets served first.

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8 A-3


ADT queue operations

Create an empty queue

Determine whether a queue is empty

Add a new item to the queue Remove from the queue the item that was added

earliest

Remove all the items from the queue

Retrieve from the queue the item that was addedearliest

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8 A-4


Queues

Are appropriate for many real-worldsituations

Example: A line to buy a movie ticket Have applications in computer science

Example: A request to print a document

A simulation

A study to see how to reduce the wait involved inan application

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8 A-5


Pseudocode for the ADT queue operationscreateQueue()

// Creates an empty queue.

isEmpty()

// Determines whether a queue is empty

enqueue(newItem) throws QueueException

// Adds newItem at the back of a queue. Throws

// QueueException if the operation is not

// successful

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8 A-6


Pseudocode for the ADT queue operations(Continued)dequeue() throws QueueException

// Retrieves and removes the front of a queue.

// Throws QueueException if the operation is

// not successful.

dequeueAll()

// Removes all items from a queue

peek() throws QueueException

// Retrieves the front of a queue. Throws

// QueueException if the retrieval is not

// successful

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8 A-7

Recognizing Palindromes

A palindrome

A string of characters that reads the same from leftto right as its does from right to left

To recognize a palindrome, a queue can beused in conjunction with a stack

A stack can be used to reverse the order ofoccurrences

A queue can be used to preserve the order ofoccurrences

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8 A-8

Recognizing Palindromes

A nonrecursiverecognition algorithm forpalindromes

As you traverse thecharacter string from leftto right, insert eachcharacter into both aqueue and a stack

Compare the charactersat the front of the queueand the top of the stack

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Implementations of the ADT

Queue

A queue can have either

An array-based implementation

A reference-based implementation

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8 A-10

A Reference-Based

Implementation

Possible implementations of a queue

A linear linked list with two external references

A reference to the front

A reference to the back

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8 A-11

A Reference-Based

Implementation

Possible implementations of a queue(Continued)

A circular linked list with one external reference

A reference to the back

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8 A-12

A Reference-Based

ImplementationInserting an item into a nonempty queue

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8 A-13

A Reference-Based

Implementation

Inserting an item into an empty queue: a) before insertion; b) after insertion

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8 A-14

A Reference-Based

ImplementationDeleting an item from a queue of more than one item

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8 A-15

An Array-Based Implementation

a) A naive array-based implementation of a queue; b) rightward drift can causethe queue to appear full

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16

Array implementation of queues

Notice how the array contents crawl to the right as elements are insertedand deleted

This will be a problem after a while!

17 23 97 44 333After insertion:

23 97 44 333After deletion:

rear = 4front = 1

17 23 97 44Initial queue:

rear = 3front = 0

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Circular arrays

We can treat the array holding the queue elements as circular (joined

at the ends)

44 55 11 22 33

0 1 2 3 4 5 6 7

myQueue:

rear = 1 front = 5

Elements were added to this queue in the order 11, 22, 33, 44, 55,

and will be removed in the same order

Use: front = (front + 1) % myQueue.length;

and: rear = (rear + 1) % myQueue.length;

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18

Full and empty queues

If the queue were to become completely full, it would look like this:

If we were then to remove all eight elements, making the queue completelyempty, it would look like this:

44 55 66 77 88 11 22 33

0 1 2 3 4 5 6 7

myQueue:

rear = 4 front = 5

0 1 2 3 4 5 6 7

myQueue:

rear = 4 front = 5

This is a problem!

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19

Full and empty queues: solutions

Solution #1: Keep an additional variable

Solution #2: (Slightly more efficient) Keep a gap between elements: consider

the queue full when it has n-1 elements

44 55 66 77 88 11 22 33

0 1 2 3 4 5 6 7

myQueue:

rear = 4 front = 5count = 8

44 55 66 77 11 22 33

0 1 2 3 4 5 6 7

myQueue:

rear = 3 front = 5

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Enqueueing a node

17

Node to be

enqueued

To enqueue (add) a node:

Find the current last node

Change it to point to the new last node

Change the last pointer in the list header

2344

lastfirst

97

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Dequeueing a node

To dequeue (remove) a node:

Copy the pointer from the first node into the header

44 97 23 17

lastfirst

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Queue implementation details

With an array implementation:

you can have both overflow and underflow

you should set deleted elements to null

With a linked-list implementation:

you can have underflow

overflow is a global out-of-memory condition

there is no reason to set deleted elements to null

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8 A-23


A circular arrayeliminates theproblem of

rightward drift

A circular implementation of a queue

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8 A-24


The effect of some operations of the queue in Figure 8-8

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8 A-25


A problem with the circular arrayimplementation

front and back cannot be used to distinguish

between queue-full and queue-empty conditions

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8 A-26


a) front passes back when the queue becomes empty

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8 A-27


b) back catches up to front when the queue becomes full

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8 A-28


To detect queue-full and queue-emptyconditions

Keep a count of the queue items

To initialize the queue, set

front to 0

back to MAX_QUEUE 1

count to 0

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8 A-29

So, how do I enqueue and

dequeue from a circular array-based queue?

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8 A-30


Inserting into a queueback = (back+1) % MAX_QUEUE;

items[back] = newItem;

++count;

Deleting from a queuefront = (front+1) % MAX_QUEUE;

--count;

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8 A-31

Using a List (Vector or ArrayList)

How do I implement enqueueand dequeue?

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8 A-32

An Implementation That Uses the ADT List

If the item in position 1 of a list list

represents the front of the queue, the

following implementations can be used dequeue()

list.remove(1)

peek()

list.get(1)

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The Java Collections Framework

Interface Queue JCF has a queue interface called Queue

Derived from interface Collection

Adds methods: element: retrieves, but does not remove head

offer: inserts element into queue

peek: retrieves, but does not remove head

poll: retrieves and removes head

remove: retrieves and removes head

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Bab-005_Queue

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