C++ Programming: Program Design Including Data Structures, Fourth Edition

1. C++ Programming: Program Design Including Data Structures, Fourth Edition Chapter 18: Stacks and Queues (part 3)

2. Queues • Queue: group of homogeneous elements • Elements are: • Added at one end (the back or rear) • Deleted from the other end (the front) • First In First Out (FIFO) data structure • Middle elements are inaccessible • Example: • Waiting line in a bank C++ Programming: Program Design Including Data Structures, Fourth Edition

3. Queue Operations • Some of the queue operations are: • initializeQueue • isEmptyQueue • isFullQueue • front • back • addQueue • deleteQueue • Abstractclass queueADT defines these operations C++ Programming: Program Design Including Data Structures, Fourth Edition

4. Implementation of Queues as Arrays • You need at least four (member) variables: • An array to store the queue elements • queueFront and queueRear • To keep track of first and last elements • maxQueueSize • To specify the maximum size of the queue C++ Programming: Program Design Including Data Structures, Fourth Edition

5. Implementation of Queues as Arrays (continued) • To add an element to the queue: • Advance queueRear to next array position • Add element to position pointed by queueRear • Example: array size is 100; originally empty C++ Programming: Program Design Including Data Structures, Fourth Edition

6. Implementation of Queues as Arrays (continued) • To delete an element from the queue: • Retrieve element pointed to by queueFront • Advance queueFront to next queue element C++ Programming: Program Design Including Data Structures, Fourth Edition

7. Implementation of Queues as Arrays (continued) • Will this queue design work? • Suppose A stands for adding an element to the queue • And D stands for deleting an element from the queue • Consider the following sequence of operations: • AAADADADADADADADA... C++ Programming: Program Design Including Data Structures, Fourth Edition

8. Implementation of Queues as Arrays (continued) • The sequence AAADADADADADADADA... would eventually set queueRear to point to the last array position • Giving the impression that the queue is full C++ Programming: Program Design Including Data Structures, Fourth Edition

9. Implementation of Queues as Arrays (continued) • Solution 1: • When the queue overflows to the rear (i.e., queueRear points to the last array position): • Check value of queueFront • If value of queueFront indicates that there is room in the front of the array, slide all of the queue elements toward the first array position • Problem: too slow for large queues • Solution 2: assume that the array is circular C++ Programming: Program Design Including Data Structures, Fourth Edition

10. Implementation of Queues as Arrays (continued) • To advance the index in a (logically) circular array: C++ Programming: Program Design Including Data Structures, Fourth Edition

11. Implementation of Queues as Arrays (continued) • Case 1: C++ Programming: Program Design Including Data Structures, Fourth Edition

12. Implementation of Queues as Arrays (continued) • Case 2: C++ Programming: Program Design Including Data Structures, Fourth Edition

13. Implementation of Queues as Arrays (continued) • Problem: • Figures 18-47 and 18-49 have identical values for queueFront and queueRear • However, the former represents an empty queue, whereas the latter shows a full queue • Solution? C++ Programming: Program Design Including Data Structures, Fourth Edition

14. Implementation of Queues as Arrays (continued) • Solution 1: keep a count • Incremented when a new element is added to the queue • Decremented when an element is removed • Initially, set to 0 • Very useful if user (of queue) frequently needs to know the number of elements in the queue • We will implement this solution C++ Programming: Program Design Including Data Structures, Fourth Edition

15. Implementation of Queues as Arrays (continued) • Solution 2: let queueFront indicate index of the array position preceding the first element • queueRear still indicates index of last one • Queue empty if: • queueFront == queueRear • Slot indicated by queueFront is reserved • Queue can hold 99 (not 100) elements • Queue full if the next available space is the reserved slot indicated by queueFront C++ Programming: Program Design Including Data Structures, Fourth Edition

18. Empty Queue and Full Queue C++ Programming: Program Design Including Data Structures, Fourth Edition

19. Initialize Queue C++ Programming: Program Design Including Data Structures, Fourth Edition

20. Front • Returns the first element of the queue C++ Programming: Program Design Including Data Structures, Fourth Edition

21. Back • Returns the last element of the queue C++ Programming: Program Design Including Data Structures, Fourth Edition

22. addQueue C++ Programming: Program Design Including Data Structures, Fourth Edition

23. deleteQueue C++ Programming: Program Design Including Data Structures, Fourth Edition

24. Constructors and Destructors C++ Programming: Program Design Including Data Structures, Fourth Edition

25. Constructors and Destructors (continued) • The array to store the queue elements is created dynamically • When the queue object goes out of scope, the destructor simply deallocates the memory occupied by the array C++ Programming: Program Design Including Data Structures, Fourth Edition

26. Linked Implementation of Queues • Array size is fixed:only a finite number of queue elements can be stored in it • The array implementation of the queue requires array to be treated in a special way • Together with queueFront and queueRear • The linked implementation of a queue simplifies many of the special cases of the array implementation • In addition, the queue is never full C++ Programming: Program Design Including Data Structures, Fourth Edition

27. Linked Implementation of Queues (continued) • Elements are added at one end and removed from the other • We need to know the front of the queue and the rear of the queue • Two pointers: queueFront and queueRear C++ Programming: Program Design Including Data Structures, Fourth Edition

30. Empty and Full Queue • The queue is empty if queueFront is NULL • The queue is never full C++ Programming: Program Design Including Data Structures, Fourth Edition

31. Initialize Queue • Initializes queue to an empty state • Must remove all the elements, if any C++ Programming: Program Design Including Data Structures, Fourth Edition

32. addQueue C++ Programming: Program Design Including Data Structures, Fourth Edition

33. front and back Operations C++ Programming: Program Design Including Data Structures, Fourth Edition

34. deleteQueue C++ Programming: Program Design Including Data Structures, Fourth Edition

35. Default Constructor C++ Programming: Program Design Including Data Structures, Fourth Edition

36. Summary • Stack: items are added/deleted from one end • Last In First Out (LIFO) data structure • Operations: push, pop, initialize, destroy, check for empty/full stack • Can be implemented as array or linked list • Middle elements should not be accessed • Postfix notation: operators are written after the operands (no parentheses needed) C++ Programming: Program Design Including Data Structures, Fourth Edition

37. Summary (continued) • Queue: items are added at one end and removed from the other end • First In First Out (FIFO) data structure • Operations: add, remove, initialize, destroy, check if queue is empty/full • Can be implemented as array or linked list • Middle elements should not be accessed • Restricted versions of arrays and linked lists C++ Programming: Program Design Including Data Structures, Fourth Edition