1. Chapter 7 Queues

2. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-2 Chapter Objectives Examine queue processing Define a queue abstract data type Demonstrate how a queue can be used to solve problems Examine various queue implementations Compare queue implementations

3. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-3 Queues A queue is a collection whose elements are added on one end and removed from the other Therefore a queue is processed in a FIFO fashion: first in, first out Elements are removed in the same order they arrive Any waiting line is a queue: – the check out line at a grocery store – the cars at a stop light – an assembly line

4. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-4 Queues A queue is usually depicted horizontally One end of the queue is the rear (or tail), where elements are added The other end is the front (or head), from which elements are removed Unlike a stack, which operates on one end of the collection, a queue operates on both ends

5. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-5 FIGURE 7.1 A conceptual view of a queue

6. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-6 Queue Operations The term enqueue is used to refer to the process of adding an element to a queue Likewise, dequeue is the process of removing an element Like a stack, a pure queue does not allow the user to access the elements in the middle of the queue We include a toString method for convenience

7. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-7 FIGURE 7.2 The operations on a queue

8. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-8 FIGURE 7.3 The QueueADT interface in UML

9. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-9 Listing 7.1

10. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-10 Coded Messages Let's use a queue to help us encode and decode messages A Ceasar cipher encodes a message by shifting each letter in a message by a constant amount k If k is 5, A becomes F, B becomes G, etc. However, this is fairly easy to break An improvement can be made by changing how much a letter is shifted depending on where the letter is in the message

11. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-11 Coded Messages A repeating key is a series of integers that determine how much each character is shifted For example, consider the repeating key 3 1 7 4 2 5 The first character in the message is shifted 3, the next 1, the next 7, and so on When the key is exhausted, we just start over at the beginning of the key

12. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-12 FIGURE 7.4 An encoded message using a repeating key

13. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-13 Coded Messages We'll use a queue to store the values of the key We'll dequeue a value when needed After using a key value, we then enqueue it back onto the end of the queue That way the queue represents the constantly cycling values in the key

14. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-14 Listing 7.2

15. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-15 Listing 7.2 (cont.)

16. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-16 FIGURE 7.5 UML description of the Codes program

17. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-17 Ticket Counter Simulation Now let's use a queue to simulate the waiting line at a movie theatre The goal is to determine how many cashiers are needed to keep the wait time below 7 minutes We'll assume: – customers arrive on average every 15 seconds – processing a request takes two minutes once a customer reaches a cashier

18. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-18 Listing 7.3

19. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-19 Listing 7.3 (cont.)

20. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-20 Listing 7.4

21. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-21 Listing 7.4 (cont.)

22. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-22 Listing 7.4 (cont.)

23. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-23 FIGURE 7.6 UML description of the TicketCounter program

24. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-24 FIGURE 7.7 The results of the ticket counter simulation

25. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-25 Radix Sort Let's look at one more use of queues A radix sort uses queues to order a set of values A queue is created for each possible value of a position (or digit) in the sort key For example, if the sort key is a lowercase alphabetic string, there would be 26 queues If the sort key was a decimal integer, there would be 10 queues corresponding to the digits 0 through 9

26. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-26 Radix Sort Each pass through the sort examines a particular position in the sort value The element is put on the queue corresponding to that position's value Processing starts with the least significant position (1s) to the most significant position The following example uses integers with only the digits 0 through 5

27. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-27 FIGURE 7.8 A radix sort of ten three-digit numbers

28. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-28 Listing 7.5

29. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-29 Listing 7.5 (cont.)

30. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-30 Listing 7.5 (cont.)

31. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-31 FIGURE 7.9 UML description of the RadixSort program

32. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-32 The LinkedQueue Class Like a stack, a queue can be implemented using an underlying array or a linked list A linked version can use the LinearNode class yet again In addition to keeping a reference to the beginning of the list, we will keep a second reference to the end An integer count will keep track of the number of elements in the queue

33. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-33 FIGURE 7.10 A linked implementation of a queue

34. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-34 LinkedQueue - the enqueue Operation

35. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-35 FIGURE 7.11 The queue after adding element E

36. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-36 LinkedQueue - the dequeue Operation

37. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-37 FIGURE 7.12 The queue after a dequeue operation

38. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-38 The ArrayQueue Class A queue can be managed using an array in which index 0 represents one end An integer value rear represents the next open slot in the array and the number of elements currently in the queue The challenge with this approach is that a queue operates on both ends, so the elements in the array must be shifted to keep one end at index 0

39. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-39 FIGURE 7.13 An array implementation of a queue

40. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-40 ArrayQueue - the enqueue Operation

41. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-41 FIGURE 7.14 The queue after adding element E

42. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-42 ArrayQueue - the dequeue Operation

43. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-43 FIGURE 7.15 The queue after removing the first element

44. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-44 The CircularArrayQueue Class If we do not fix one end of the queue at index 0, we will not have to shift the elements A circular queue is an implementation of a queue using an array that conceptually loops around on itself That is, the last index is thought to precede index 0 We keep track of integers that indicate where the front and rear of the queue are at any given time

45. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-45 FIGURE 7.16 A circular array implementation of a queue

46. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-46 FIGURE 7.17 A queue straddling the end of a circular array

47. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-47 FIGURE 7.18 Changes in a circular array implementation of a queue

48. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-48 Circular Queues When an element is enqueued, the value of rear is incremented But it must take into account the need to loop back to 0: rear = (rear+1) % queue.length; Note that this array implementation can also reach capacity and may need enlarging

49. Copyright © 2005 Pearson Addison-Wesley. All rights reserved. 7-49 Queue Implementations The enqueue operation is O(1) for all implementations The dequeue operation is O(1) for linked and circular array implementations, but O(n) for the noncircular array version due to the need to shift the elements in the queue