CSCI2320 Chapter 16 Part2

8/11/2019 CSCI2320 Chapter 16 Part2

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Chapter 16 Stack and Queuespart2

Dr. Bernard Chen Ph.D.

University of Central Arkansas


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Introduction to Queues A queue is a waiting line

Its in daily life: A line of persons waiting to check out at a

supermarket

A line of persons waiting to purchase a ticket for afilm

A line of planes waiting to take off at an airport

A line of vehicles at a toll booth


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Introduction to Queues Difference between Stack and

Queues:

Stack exhibits last-in-first-out (LIFO)

Queue exhibits first-in-first-out (FIFO)


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ADT in Queues Unlike stacks in which elements are popped

and pushed only at the ends of the list,

Collection of data elements:

items are removed from a queue at one end,

called the FRONT of the queue; and elements are added at the other end,

called the BACK


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Queue ADT Basic operations

Construct a queue

Check if empty

Enqueue (add element to back)

Front (retrieve value of element from front)

Dequeue (remove element from front)


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Designing and Building a Queue

Class Array-Based Consider an array in which to store a

queue

Note additional variables neededmyFront, myBack

Picture a queueobject like this


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Queue Operation Empty Queue

Enqueue(70)


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Queue Operation Enqueue(80)

Enqueue(50)


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Queue Operation Dequeue()

Dequeue()


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Queue Operation Enqueue(90)

Enqueue(60)


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Circular Queue Problems

We quickly "walk off the end" of the array

Possible solutions

Shift array elements

Use a circular queue

Note that both emptyand full queue

givesmyBack == myFront


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Circular Queue Using a static array

QUEUE_CAPACITYspecified

Enqueue incrementsmyBackusing mod

operator, checks for full queue

Dequeue incrementsmyFrontusing mod

operator, checks for empty queue


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Circular Example

Both Front and Back wraparound asneeded.

b c d

FrontBack

b c d

FrontBack

e f

e fg


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QUEUE Only tricky part is vector doubling because

the queue items are not necessarily stored inan array starting at location 0, and the

contiguity of wraparound must bemaintained.

Therefore, mostly straightforward; maintain Front Back


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Queue Full Situation If an item were stored in the last position, and an Enqueure()

occurred

myBack would be incremented by 1, giving it the same value as

myFront However, myFront == myBack indicates the queue is empty Thus, we cannot distinguish between emptyand full

We may avoid this situation by maintaining one empty position,

so that myFront will never equal to myBack unless the queue isempty


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Queue Operation

Construct:Create an array, set capacity,

myFront=myBack=0

Empty:test myFront==myBack

Front :if not empty:print array[myFront]


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Algorithm for Enqueue(value)

1. Set newBack ==(myBack+1)%Queue_capacity

2. If newBack == myFrontSignal Queue Full

otherwise:Set Array[myBack] == value

Set myBack == newBack


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Algorithm for Dequeue()

If queue is empty

signal Queue Empty

Otherwise

Set myFront=(myFront+1)%Queue_capacity


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Linked Queues

We could also use linked list to store queue

elements Can grow and shrink to fit the situation

No need for upper bound (myCapacity)


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Linked Queues

Constructor initializesmyFront,myBack

Empty myFront == Null

Front return myFront->data

Dequeue Delete first node (watch for empty queue)

Enqueue Insert node at end of list


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Enqueue

newptr= new Node(value)

if (empty())

myFront=myBack=newptr;

else

{

myBack->next=newptr;

myBack=newwptr;

}


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Dequeue

(if not empty)

ptr=myFront

myFront=myFront->next

delete ptr;


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Queue ADT implement byVectors

Basic operations

Construct a queue

Check if empty

Enqueue (add element to back)

Front (retrieve value of element from front)



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Dequeue


L.erase( L.begin() );

L.begin() is an iterator, in erase(), youcannot just gives the location


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Functions related to Queue

Constructor: vector L;

Empty(): L.size() == 0?

Enqueue(): L.push_back(value); Front(): L.begin();

Dequeue(): L.erase(L.begin());


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QUEUE

Write a queue program


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Vector Functions


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Queue + Stack

SCROLL: a queue-stack hybrid model

Thus, you may have functions in both

Push(value)=Enqueue(value)Pop(); Top() --- top of stack

Dequeue(); Front()

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CSCI2320 Chapter 16 Part2