DATA STRUCTURE &
ALGORITHMS
CHAPTER 4: QUEUE
Queues
• Queue: list 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)
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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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C++ Programming: Program
Design Including Data
Structures, Fourth Edition
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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Implementation of Queues
as Arrays (continued)
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Exercise
Consider the following queue where circular QUEUE is allocated
6 memory cells:
FRONT = 2, REAR = 5
QUEUE: _______, London, Berlin, Rome, Paris, _______
Describe the queue, including FRONT and REAR, as the
following operations take place:
1. Athens is added
2. Two cities are deleted
3. Madrid is added
4. Moscow is added
5. Three cities are deleted
6. Oslo is added
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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addQueue
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deleteQueue
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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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addQueue
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front and back Operations
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deleteQueue
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Default Constructor
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Queue Derived from the class
unorderedLinkedListType
• The linked implementation of a queue is
similar to the implementation of a linked list
created in a forward manner
–
–
–
–
–
–
addQueue is similar to insertFirst
initializeQueue is like initializeList
isEmptyQueue is similar to isEmptyList
deleteQueue can be implemented as before
queueFront is the same as first
queueRear is the same as last
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Application of Queues:
Simulation
• Simulation: a technique in which one
system models the behavior of another
system
• Computer simulations using queues as
the data structure are called queuing
systems
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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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