Chapter 18: Stacks and Queues
(part 3)

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
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Queue Operations
• Some of the queue operations are:
− initializeQueue
− isEmptyQueue
− isFullQueue
− front
− back
− addQueue
− deleteQueue
• Abstract class queueADT defines these operations
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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
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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
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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
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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...
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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
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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
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Implementation of Queues as
Arrays (continued)
• To advance the index in a (logically) circular array:
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Implementation of Queues as
Arrays (continued)
• Case 1:
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Implementation of Queues as
Arrays (continued)
• Case 2:
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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?
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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
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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
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Empty Queue and Full Queue
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Initialize Queue
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Front
• Returns the first element of the queue
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Back
• Returns the last element of the queue
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Constructors and Destructors
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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
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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
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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
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Empty and Full Queue
• The queue is empty if queueFront is NULL
• The queue is never full
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Initialize Queue
• Initializes queue to an empty state
− Must remove all the elements, if any
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and
Operations
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Default Constructor
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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)
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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
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