1. Chapter 8 Queue I CS2006 - Data Structures I COSC2006 April 24, 2017
Chapter 7: Queues

2. Topics
Introduction
Queue Application
Implementation
Linked List

3. COSC2006
April 24, 2017
Queues
A queue differs from a stack in that it follows the first-in-first-out (FIFO) principle.
Inserting an item to the rear is known as "enqueueing".
Removing an item from the front is known as "dequeueing".
cashier
Front of line
Back of line
Chapter 7: Queues

4. Queues Linear, homogeneous structure
Has First-In-First-Out (FIFO) behavior; Items added at one end and removed from the other end
Middle elements are logically inaccessible
Add/
Enqueue
Remove/Dequeue
Back/Rear
Front/Head

5. Queue Applications Real-World Applications
Buy a movie ticket
Cashier lines in any store
 Computer Science Applications
Print lines of a document
Convert digit strings to decimal

6. ADT Queue Specification: Elements Operations Homogeneous Linear
Create an empty queue
Check if a queue is empty / Full
Enqueue (Enq, Enque, Add, Insert):
Adding new element at the back (rear)
Dequeue (Deq, Deque, Remove, Serve):
Deleting an element from the front
Retrieve an element from a queue

7. Queue Applications Converting Digit Strings to Decimal
Enter characters from the keyboard and retain them in order
Assumption:
No typing mistakes
Blank spaces may precede or follow the digits
Formula used:
Initial value
DigitSum = 0

8. Queue Applications Converting Digit Strings to Decimal Pseudocode
// Convert digits in aQueue into decimal integer n
// Get first digit, ignoring any leading blanks
do {
ch=queue.dequeue()
} while ( ch is blank)
// Assertion: ch contains first digit
// Compute n from digits in queue
n = 0;
done = false;
do { n = 10 * n + integer that ch represents
if (! queue.isEmpty( ) )
else
done = true
} while (! done and ch is a digit)
// Assertion: n is result

9. Queue Applications Recognizing Palindromes Uses a queue & a stack
Idea:
Insert the string in both queue & stack
Remove characters from both stack's Top & queue's Front, comparing them
Repeat until:
a) Either the stack or the queue are empty
 String is a palindrome
b) The character from the stack and the corresponding character from the queue are not similar
 String isn't a palindrome

10. Queue Applications Recognizing Palindromes Pseudocode
IsPal(in str:string) : boolean
// Determines whether String is a palindrome
aQueue.createQueue ( ) // Create an empty queue
aStack.createStack ( ) // Create an empty stack
// Insert each character of String into both aQueue and aStack
length = length of str
for ( i = 1 through length)
{ nextChar = ith character of str
aQueue.enqueue ( nextChar )
aStack.push(NextChar)
} // end for
// Compare aQueue with aStack
charactersAreEqual = true;
while ( ! aQueue.isEmpty() && charactersAreEqual)
{ queueFront =aQueue.peek ()
stackTop= aStack.top ()
if ( queueFront equals stackTop)
{ aQueue.dequeue ( )
aStack.pop ( )
} else charactersAreEqual = false
} // end while
return charactersAreEqual

11. ADT Queue Implementation
Possible implementations:
Linked List-based
Linear
Circular
Array-based
ADT List-based

12. Linked List-Based Implementation
More straightforward than array-based
Possible options:
Linear linked list
Two external “pointer” (Front & Back)
Circular linked list
One “pointer” will be enough (Back)

13. Linked List Queue Implementation
Linked List -Based Implementation: Option 1
Insertion to an empty list
front = newNode
back = newNode

14. Linked List Queue Implementation
LL -Based Implementation: Option 1
Insertion to a non-empty list
newNode.setNext ( NULL);
Back.setNext (newNode);
back = newNode;
Deletion
temp = front ;
front = front.getNext()
temp.setNext ( NULL ) 20 15
front 26 84
back

15. Linked List Queue Implementation
LL -Based Implementation: option 1
Deletion
form a one-node (one item) queue
If (front = back&&front!=null){
back = null
front=null }
We will use option 2 to implement our queue

16. Queue Interface public interface QueueInterface {
COSC2006
April 24, 2017
Queue Interface
public interface QueueInterface {
public boolean isEmpty();
public void enqueue(Object newItem) throws QueueException;
public Object dequeue() throws QueueException;
public void dequeueAll();
public Object peek() throws QueueException; }
Chapter 7: Queues

17. Queue Exception public class QueueException extends RuntimeException {
COSC2006
April 24, 2017
Queue Exception
public class QueueException extends RuntimeException {
public QueueException(String s) {
super(s);
} // end constructor
} // end QueueException
Chapter 7: Queues

18. LListQueue Implementation
COSC2006
April 24, 2017
LListQueue Implementation
public class LListQueue implements QueueInterface {
private Node lastNode; // we use option 2
public LListQueue() {
lastNode = null;
} // end default constructor
// queue operations:
public boolean isEmpty() {
return lastNode == null;
} // end isEmpty
public boolean isFull() {
return false;
} // end isFull
public void dequeueAll() {
} // end dequeueAll
Chapter 7: Queues

19. LListQueue Implementation
COSC2006
April 24, 2017
LListQueue Implementation
public class LListQueue implements QueueInterface {
private Node lastNode;
public LListQueue() {
lastNode = null;
} // end default constructor
// queue operations:
public boolean isEmpty() {
return lastNode == null;
} // end isEmpty
public boolean isFull() {
return false;
} // end isFull
public void dequeueAll() {
} // end dequeueAll
Chapter 7: Queues

20. LListQueue Implementation (2)
COSC2006
April 24, 2017
LListQueue Implementation (2)
public void enqueue(Object newItem) {
Node newNode = new Node(newItem);
if(isFull())
throw new QueueException("QueueException on enqueue:"+ "queue full");
// insert the new node
if (isEmpty()) {
// insertion into empty queue
newNode.setNext(newNode); }
else {
// insertion into nonempty queue
newNode.setNext(lastNode.getNext());
lastNode.setNext(newNode);
} // end if
lastNode = newNode; // new node is at back
} // end enqueue
Chapter 7: Queues

21. LListQueue Implementation (3)
COSC2006
April 24, 2017
LListQueue Implementation (3)
public Object dequeue() throws QueueException {
if (!isEmpty()) {
// queue is not empty; remove front
Node firstNode = lastNode.getNext();
if (firstNode == lastNode) { // special case?
lastNode = null; // yes, one node in queue }
else {
lastNode.setNext(firstNode.getNext());
} // end if
return firstNode.getItem();
throw new QueueException("QueueException on dequeue:"
+ "queue empty");
} // end dequeue
Chapter 7: Queues

22. LListQueue Implementation (4)
COSC2006
April 24, 2017
LListQueue Implementation (4)
public Object peek() throws QueueException {
if (!isEmpty()) {
// queue is not empty; retrieve front
Node firstNode = lastNode.getNext();
return firstNode.getItem(); }
else {
throw new QueueException("QueueException on peek:"
+ "queue empty");
} // end if
} // end peek
} // end ListQueue
Chapter 7: Queues

23. LListQueue Implementation (4)
COSC2006
April 24, 2017
LListQueue Implementation (4)
public Object peek() throws QueueException {
if (!isEmpty()) {
// queue is not empty; retrieve front
Node firstNode = lastNode.getNext();
return firstNode.getItem(); }
else {
throw new QueueException("QueueException on peek:"
+ "queue empty");
} // end if
} // end peek
} // end ListQueue
Chapter 7: Queues

24. LListQueue Test public class LListQueueTest {
COSC2006
April 24, 2017
LListQueue Test
public class LListQueueTest {
public static void main(String[ ] args) {
LListQueue aQueue = new LListQueue();
System.out.println("Enqueuing:");
for (int i = 0; i < 9; i++) {
System.out.print(" "+i);
aQueue.enqueue(new Integer(i));
} // end for
System.out.println("\nDequeuing:");
System.out.print(" "+aQueue.dequeue());
System.out.println();
} // end main
} // QueueTest
Chapter 7: Queues

25. Review In a queue, items can be added ______.
only at the front of the queue
only at the back of the queue
either at the front or at the back of the queue
at any position in the queue

26. Review Operations on a queue can be carried out at ______.
its front only
its back only
both its front and back
any position in the queue

27. Review Which of the following is NOT an ADT queue operation? enqueue
isEmpty
Pop
peek

28. Review
The ______ operation retrieves and removes the front of a queue.
isEmpty
enqueue
dequeue
peek

29. Review
The ______ operation retrieves the item that was added earliest to a queue, but does not remove that item.
enqueue
dequeue
dequeueAll
peek

30. Review
A reference-based implementation of a queue that uses a linear linked list would need at least ______ external references.
one
two
three
four

31. Review
A reference-based implementation of a queue that uses a circular linked list would need at least ______ external references.
one
two
three
four

32. Review Which of the following operations leaves a queue unchanged?
enqueue
dequeue
dequeueAll
peek