1. Csc 2720 Data structure Instructor: Zhuojun Duan
Queue
Csc Data structure
Instructor: Zhuojun Duan

2. Outline Introduction of Queue Implementation Applications
Queue in JAVA Library

3. Preliminary

4. Conceptual View of a Queue
Queue: a collection whose elements are added at one end (the rear or tail of the queue) and removed from the other end (the front or head of the queue)
A queue is a FIFO (first in, first out) data structure
Applications in computer science:
Printer queue (e.g. printer in MC 235)
Keyboard input buffer
GUI event queue (click on buttons, menu items)
To encode messages (more on this later) …

5. Queue operations
boolean isEmpty(); // Determines whether a queue is empty boolean enqueue(newItem); // Adds newItem at the back of a queue. boolean dequeue(); // Retrieves and removes the front of a queue. boolean dequeueAll(); // Removes all items from a queue object peek(); //Retrieves the front of a queue.

6. Conceptual View of a Queue
Adding an element
Front of queue
New element is added to the rear of the queue
Rear of queue

7. Conceptual View of a Queue
Adding an element
Front of queue
Rear of queue

8. Conceptual View of a Queue
Removing an element
Front of queue
Element is removed from the front of the queue
Rear of queue

9. Conceptual View of a Queue
Removing an element
New front element of queue
Element is removed from the front of the queue
Rear of queue

10. Recognizing Palindromes
A palindrome
A string of characters that reads the same from left to right as its does from right to left
Example: Anna, Civic, Kayak, Level, Madam
To recognize a palindrome, a queue can be used in conjunction with a stack
A stack can be used to reverse the order of occurrences
A queue can be used to preserve the order of occurrences

11. Recognizing Palindromes
A recognition algorithm for palindromes
As you traverse the character string from left to right, insert each character into both a queue and a stack
Compare the characters at the front of the queue and the top of the stack
Figure: The results of inserting a string into both a queue and a stack

12. Outline Introduction of Queue Implementation Applications
Queue in JAVA Library

13. Implementations of Queue
A queue can have either
An array-based implementation
Array-based
Circular array- based
A reference-based implementation (linked list )

14. Array Implementation of a Queue
First Approach
First Approach:
Use an array in which index 0 represents one end of the queue (the front) : front is fixed
Integer value rear represents the last element in the array
Discussion: What is the challenge with this approach?

15. An Array Implementation of a Queue
First Approach
front
rear 1 2 3 4 … aq 1 2 3 4
front
A queue aq containing four elements 3
rear

16. Adding an Element in queue
First Approach
front
rear
rear 1 2 3 4 … aq 1 2 3 4 5
front
Add new element for queue aq 5 4 3
rear
Element is added at the array location given by the index of rear+1, and then rear is incremented.

17. Removing an Element from queue
First Approach
front
rear 1 2 3 4 … aq 1 2 3 4 5
front
Step 1: Element is removed from array location 0 4 3
rear
Step 2: remaining elements are shifted forward one position in the array, and then rear is decremented.

18. Removing an Element from queue
First Approach
front
rear 1 2 3 4 … aq 2 3 4 5
front
Step 1: element is removed from array location 0 3 4 3
rear
Step 2: remaining elements are shifted forward one position in the array, and then rear is decremented.

19. Array Implementation of a Queue
First Approach
How to initialize the value of front and rear:
Set front=0, rear=-1;
How to decide the queue is empty?
if rear < front
How to decide the queue is full?
if rear==MAX_QUEUE-1

20. Array Implementation of a Queue
Second Approach
(circular array )
Drawback of first approach:
Shifting cost
Circular Arrays can be used to implement the queue
Use a circular array to insert and remove items from a queue without shifting
The idea of a circular array is that the end of the array “wraps around” to the start of the array 1 3 2 4 5 6 7

21. Array Implementation of a Queue
Second Approach
(circular array )
//Java Code
Queue q = new Queue();
q.enqueue(6);
front =
count = 1 6 1 2 3 4 5
rear = 5
insert item at (rear + 1 ) % queue.length=6%6=0

22. Array Implementation of a Queue
Second Approach
(circular array )
//Java Code
Queue q = new Queue();
q.enqueue(6);
q.enqueue(4);
q.enqueue(7);
q.enqueue(3);
q.enqueue(8);
front =
count = 1 5 2 4 3 6 4 7 3 8 1 2 3 4 5
rear = 4 3 2 1

23. Array Implementation of a Queue
Second Approach
(circular array )
//Java Code
Queue q = new Queue();
q.enqueue(6);
q.enqueue(4);
q.enqueue(7);
q.enqueue(3);
q.enqueue(8);
q.dequeue(); //front = 1
q.dequeue(); //front = 2
q.enqueue(9);
front = 1 2
count = 5 3 4 4 6 4 7 3 8 9 1 2 3 4 5
rear = 5 4
make front = (0 + 1) % 6 = 1
make front = (1 + 1) % 6 = 2

24. Array Implementation of a Queue
Second Approach
(circular array )
//Java Code
Queue q = new Queue();
q.enqueue(6);
q.enqueue(4);
q.enqueue(7);
q.enqueue(3);
q.enqueue(8);
q.dequeue(); //front = 1
q.dequeue(); //front = 2
q.enqueue(9);
q.enqueue(5);
front = 2
count = 5 4 5 7 3 8 9 1 2 3 4 5
rear = 5
insert at (rear + 1) % 6 = (5 + 1) % 6 = 0

25. Array Implementation of a Queue
Second Approach
(circular array )
Illustration by a circle
A circular array eliminates the problem of rightward drift

26. Array Implementation of a Queue
Second Approach
(circular array )
Illustration by a circle

27. Array Implementation of a Queue
Second Approach
(circular array )
Question: how to distinguish between queue-full and queue-empty conditions ?
front and back cannot be used to distinguish between queue-full and queue-empty conditions
Count can be used to do it.

28. Array Implementation of a Queue
Second Approach
(circular array )
front and back cannot be used to distinguish between queue-full and queue-empty conditions
Figure:
front passes back when the queue becomes empty

29. Array Implementation of a Queue
Second Approach
(circular array )
front and back cannot be used to distinguish between queue-full and queue-empty conditions
Figure:
back catches up to front when the queue becomes full

30. Array Implementation of a Queue
Second Approach
(circular array )
Notes for circular array based queue:
To detect queue-full and queue-empty conditions
Keep a count of the queue items
To initialize the queue, set
front to 0
rear(back) to MAX_QUEUE – 1
count to 0
Inserting into a queue
rear = (rear+1) % MAX_QUEUE;
items[rear] = newItem;
++count;

31. Array Implementation of a Queue
Second Approach
(circular array )
Notes for circular array based queue:
Deleting from a queue
front = (front+1) % MAX_QUEUE;
--count;

32. Implementations of Queue
A queue can have either
An array-based implementation
Array-based
Circular array- based
A reference-based implementation (linked list )

33. Queue: Linked List Implementation
Removing items from the front of the queue is straightforward
But we need to insert items at the back of the queue in constant time
So cannot traverse through the list
By using an additional reference (and a little administration) we can keep track of the node at the back of the queue

34. List Queue Example singly linked list
//Java Code
Queue q = new Queue();
q.enqueue(6); 6
front
rear

35. List Queue Example 6 front 4 rear //Java Code Queue q = new Queue();
singly linked list
//Java Code
Queue q = new Queue();
q.enqueue(6);
q.enqueue(4); 6
front 4
rear

36. List Queue Example 6 front 4 rear 7 //Java Code Queue q = new Queue();
singly linked list
//Java Code
Queue q = new Queue();
q.enqueue(6);
q.enqueue(4);
q.enqueue(7); 6
front 4
rear 7

37. List Queue Example 6 front 4 rear 7 3 //Java Code
singly linked list
//Java Code
Queue q = new Queue();
q.enqueue(6);
q.enqueue(4);
q.enqueue(7);
q.enqueue(3); 6
front 4
rear 7 3

38. List Queue Example 6 front 4 rear 7 3 //Java Code
singly linked list
//Java Code
Queue q = new Queue();
q.enqueue(6);
q.enqueue(4);
q.enqueue(7);
q.enqueue(3);
q.dequeue(); 6
front 4
rear 7 3

39. Queue: Circular Linked List Implementation
Possible implementations of a queue
A circular linked list with one external reference
A reference to the back
Figure:
A reference-based implementation of a queue: b) a circular linear linked list with one external reference

40. Queue: Circular Linked List Implementation
Figure :
Inserting an item into a nonempty queue

41. Queue: Circular Linked List Implementation
Figure :
Inserting an item into an empty queue: a) before insertion; b) after insertion

42. Queue: Circular Linked List Implementation
Figure :
Deleting an item from a queue of more than one item

43. Queue: Linked List Implementation
How to decide the queue is empty if the queue is implemented by linked list?
Singly linked list implementation :
rear==null;
Circular Linked List Implementation
lastNode==null;

44. Outline Introduction of Queue Implementation Applications
Queue in JAVA Library

45. Applications

46. Application Simulation
A technique for modeling the behavior of both natural and human-made systems
Goal
Generate statistics that summarize the performance of an existing system
Predict the performance of a proposed system
Example
A simulation of the behavior of a bank

47. Application: Simulation
Figure :
A blank line at at time a) 0; b) 12

48. Application: Simulation
Figure:
A blank line at at time c) 20; d) 38

49. Outline Introduction of Queue Implementation Applications
Queue in JAVA Library

50. Interface Queue in Java
Java has a queue interface called Queue
Derived from interface Collection

51. Interface Queue in Java
Java has a queue interface called Queue
Derived from interface Collection
Known Implementing Classes:
AbstractQueue, ArrayBlockingQueue, ArrayDeque, ConcurrentLinkedDeque, C oncurrentLinkedQueue, DelayQueue, LinkedBlockingDeque, LinkedBlockingQ ueue, LinkedList, LinkedTransferQueue, PriorityBlockingQueue, PriorityQueue,  SynchronousQueue
Please reference:

52. Interface Deque in Java
Deque = double-ended queue
(pronounced “deck”)
Allows us to insert and delete from either end
Useful methods: addFirst, addLast, peekFirst, peekLast, getFirst, getLast, removeFirst, removeLast,…
Thus, may function as both a stack and a queue
Known Implementing Classes:
ArrayDeque, ConcurrentLinkedDeque, LinkedBlockingDeque, LinkedList
Please reference:

53. Comparing Implementations
All of the implementations of the ADT queue mentioned are ultimately either
Array based
Reference based
Fixed size versus dynamic size
A statically allocated array
Prevents the enqueue operation from adding an item to the queue if the array is full
A resizable array or a reference-based implementation
Does not impose this restriction on the enqueue operation

54. A Summary of Position-Oriented ADTs
List
Stack
Queue
Stacks and queues
Only the end positions can be accessed
Lists
All positions can be accessed