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Chapter 8

Queues

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Chapter Objectives

• Learn about queues

• Examine various queue operations

• Learn how to implement a queue as an array

• Learn how to implement a queue as a linked list

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Chapter Objectives

• Discover queue applications

• Examine the STL class queue

• Discover priority queues

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Queues

• Definition: data structure in which the elements are added at one end, called the rear, and deleted from the other end, called the front or first

• First In First Out (LIFO) data structure

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Basic Operations on a Queue

• initializeQueue: Initializes the queue to an empty state

• destroyQueue: Removes all the elements from the queue, leaving the queue empty

• isEmptyQueue: Checks whether the queue is empty. If the queue is empty, it returns the value true; otherwise, it returns the value false

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Basic Operations on a queue

• isFullQueue: Checks whether the queue is full. If the queue is full, it returns the value true; otherwise, it returns the value false

• front: Returns the front (first) element of the queue; the queue must exist

• back: Returns the front (first) element of the queue; the queue must exist

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Basic Operations on a queue

• addQueue: Adds a new element to the rear of the queue; the queue must exist and must not be full

• deleteQueue: Removes the front element of the queue; the queue must exist and must not be empty

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Queues as Arrays

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Queues as Arrays

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Circle Queue

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Queues as Arrays

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Queues as Arrays

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Queues as Arrays

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Queues as Arrays

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Queues as Arrays

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Queues as Arrays

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Queues as Arrays

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UML Diagram of theclass queueType

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Initialize Queue

Definition of the function initializeQueue:

template<class Type>void queueType<Type>::initializeQueue(){ queueFront = 0; queueRear = maxQueueSize - 1; count = 0;}

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Empty Queue and Full Queue

template<class Type>

bool queueType<Type>::isEmptyQueue()

{

return (count == 0);

}

template<class Type>

bool queueType<Type>::isFullQueue()

{

return (count == maxQueueSize);

}

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Destroy Queue

template<class Type>

void queueType<Type>::destroyQueue()

{

queueFront = 0;

queueRear = maxQueueSize - 1;

count = 0;

}

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front and backtemplate<class Type>

Type queueType<Type>::front()

{

assert(!isEmptyQueue());

return list[queueFront];

}

template<class Type>Type queueType<Type>::back(){ assert(!isEmptyQueue()); return list[queueRear];}

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Add Queue

template<class Type>void queueType<Type>::addQueue(const Type& newElement){ if(!isFullQueue()) { queueRear = (queueRear + 1) % maxQueueSize; //use the mod //operator to advance queueRear //because the array is circular count++; list[queueRear] = newElement; } else cerr<<"Cannot add to a full queue."<<endl;}

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Delete Queue

template<class Type>void queueType<Type>::deleteQueue(){ if(!isEmptyQueue()) { count--; queueFront = (queueFront + 1) % maxQueueSize; //use the mod //operator to advance queueFront //because the array is circular } else cerr<<"Cannot remove from an empty queue."<<endl;}

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Constructor and Destructor

• Constructor – Creates an array of the size specified by the user

– Default value is 100

– Initializes queueFront queueRear to indicate that the queue is empty

• Destructor– When queue object goes out of scope, destructor reallocates

memory occupied by the array that stores the queue elements

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Linked Queue as an ADT

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Empty, Full, and Destroy Queue

• Queue is empty if queueFront is NULL

• Queue is full only if we run out of memory

• Destroy queue removes all elements of the queue

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addQueue

• Adds a new element to the end of the queue

• Access the pointer queueRear to implement addQueue

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Front, Back, and Delete Queue

• If queue is nonempty:– operation front returns the first element of the queue

– operation back returns the last element of the queue

– operation deleteQueue removes the first element of the queue

• If queue is empty:– function front terminates the program

– function back terminates the program

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STL class queue (Queue Container Adapter)

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Priority Queue

• FIFO rules of a queue are relaxed

• Customers or jobs with higher priority are pushed to front of queue

• To implement:– use an ordinary linked list, which keeps the

items in order from the highest to lowest priority

– use a treelike structure

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Application of Queues

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Application of Queues

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Application of Queues

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Application of Queues: waitingCustomerQueueType

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Poisson Distribution

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Chapter Summary

• Queue Data Structure– Restricted Version of arrays and linked

list

– Basic operations

• First In First Out (FIFO)

• Queues Implemented as Arrays

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Chapter Summary

• Queues Implemented as Linked Lists

• STL class queue

• Priority Queues

• Application of Queues