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Chapter 7: Queues

Chapter 7: Queues. Queue Implementations. Queue Applications. CS 240. 44. The queue abstract data type is essentially a list using the FIFO (first-in-first-out) policy for adding and removing elements. The principal queue operations:. Create an empty queue. Copy an existing queue.

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Chapter 7: Queues

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  1. Chapter 7: Queues Queue Implementations Queue Applications CS 240 44

  2. The queue abstract data type is essentially a list using the FIFO (first-in-first-out) policy for adding and removing elements. The principal queue operations: • Create an empty queue. • Copy an existing queue. • Destroy a queue. • Determine whether a queue is empty. • Add a new element to a queue. • Remove the least recently added element from a queue. CS 240 45

  3. b b b a a c c a b b c a b c Queue Implementation Alternatives • An Array Implementation • Positives • Avoids pointers (uses front & rear indices) • Negatives • Wraparound is needed to avoid false overflows • Size must be declared in advance • A Linked List Implementation • Positives • Dynamically allocates exactly the right amount of memory • Wraparound is circumvented • Negatives • Our friendly neighborhood pointers CS 240 46

  4. Array Implementation of Queue // Class implementation file: queue.cpp // Array implementation of the queue ADT. #include "Queue.h" #include <assert.h> // Default constructor: Make empty queue // queue:: queue() { front = 0; rear = MAX_QUEUE_SIZE – 1; length = 0; } // Copy constructor: Make copy of queue. // queue:: queue(const queue &q) { intindex; front = q.front; rear = q.rear; length = q.length; for (int i = 0; i < length; i++) { index = (i + q.front) % MAX_QUEUE_SIZE; list[index] = q.list[index]; } } // Class declaration file: Queue.h // Array implementation of the queue ADT. #ifndefQUEUE_H typedefintelementType; const MAX_QUEUE_SIZE = 200; class queue { public: // Class constructors queue(); queue(const queue &q); // Member functions boolisEmpty(); void enqueue(constelementType &item); elementTypedequeue(); protected: // Data members intfront, rear, length; elementType list[MAX_QUEUE_SIZE]; // Member function boolisFull(); }; #define QUEUE_H #endif CS 240 47

  5. // isEmpty function: signals if *this is empty queue. // boolqueue:: isEmpty() { return (length == 0); } // Enqueue function; inserts a new item into the // // rear of queue *this (if there's enough room). // void queue:: enqueue(constelementType &item) { assert(!isFull()); rear = (rear+1) % MAX_QUEUE_SIZE; list[rear] = item; length++; } // Dequeue function; remove the item at the // // front of queue *this (if there's one there). // elementType queue:: dequeue() { elementType item; assert(!isEmpty()); item = list[front]; front = (front+1) % MAX_QUEUE_SIZE; length--; return item; } // isFull function; returns boolean value indicating // // if *this is a full queue (w.r.t. the array). // boolqueue:: isFull() { return (length == MAX_QUEUE_SIZE); } Insert from the rear (with wraparound) Remove from the front (with wraparound) CS 240 48

  6. Linked List Implementation of Queue // Class declaration file: Queue.h // Linked List implementation of queue ADT. #ifndefQUEUE_H #include "LinkedList.h" class queue: protected LinkedList { public: // Class constructors queue(); queue(const queue &q); // Member functions boolisEmpty(); void enqueue(constelementType &item); elementTypedequeue(); protected: // Data members nodePtr tail; }; #define QUEUE_H #endif // Class implementation file: Queue.cpp // Linked List implementation of the queue ADT. #include "Queue.h" #include <assert.h> // Default constructor: Makes an empty queue // queue:: queue(): LinkedList() { tail = NULL; } The queue class “inherits” from the LinkedList class, so all LinkedList members are accessible to any queue. With its “protected” access specifier, the public and protected members of LinkedList are considered protected in the queue class. Let’s assume that the getNode and head members in LinkedList were declared protected, not private! Let’s also assume that the elementType typedef occurred in the LinkedList definition! CS 240 49

  7. // Copy constructor: Makes a deep // // copy of the *this queue. // queue:: queue(constqueue &q) { nodePtrcopyPreviousPtr, copyPtr, origPtr; if (q.head == NULL) tail = head = NULL; else { head = getNode(q.head->item); copyPreviousPtr = head; origPtr = q.head->next; while (origPtr != NULL) { copyPtr = getNode(origPtr->item); copyPreviousPtr->next = copyPtr; copyPreviousPtr = copyPtr; origPtr = origPtr->next; } tail = copyPreviousPtr; } } // isEmpty function; Determines // // if the *this queue is empty. // boolqueue:: isEmpty() { return (head == NULL); } // Enqueue function; Inserts item // // into the back of the *this queue. // void queue:: enqueue(constelementType &elt) { nodePtrnewPtr = getNode(elt); assert (newPtr != NULL); if (head == NULL) head = tail = newPtr; else { tail->next = newPtr; tail = newPtr; } } // Dequeue function; Removes item // // from the front of the *this queue // // (assuming such an item exists). // elementType queue:: dequeue() { elementTypeelt; nodePtroldHead; assert(head != NULL); oldHead = head; elt = head->item; head = head->next; if (head == NULL) tail = NULL; delete oldHead; return elt; } CS 240 50

  8. Example Queue Application // Program file: carwash.cpp // // This program simulates the operation of a // // car wash over 10 hours (600 minutes) of // // operation. The variables timeForWash and // // probOfArrival represent the time it takes // // to run one car through the car wash and // // the probability that a car arrives in any // // given minute. // #include <iostream> #include <iomanip> #include <ctime> #include "Queue.h" using namespace std; float random(); voidinitializeRandomSeed(); // The main function simulates the arrival of // // 600 cars at the car wash, queueing those // // which have to wait, and keeping a running // // tally of how long the cars are queued up. // void main() { inttimeForWash, minute, timeEnteredQueue, carsWashed, totalQueueMin, timeLeftOnCar; float probOfArrival; queue carQueue; cout << "Enter time (in minutes)" << " to wash one car: "; cin >> timeForWash; cout << "Enter probability of " << "arrival in any minute: "; cin >> probOfArrival; carsWashed = 0; totalQueueMin = 0; timeLeftOnCar = 0; for (minute = 1; minute <= 600; minute++) { if (random() < probOfArrival) carQueue.enqueue(minute); if ((timeLeftOnCar == 0) && !carQueue.isEmpty()) { timeEnteredQueue = carQueue.dequeue(); totalQueueMin += (minute – timeEnteredQueue); carsWashed++; timeLeftOnCar = timeForWash; } if (timeLeftOnCar != 0) timeLeftOnCar--; } CS 240 51

  9. cout<< endl << carsWashed << " cars were washed" << endl; cout.setf(ios::fixed); cout << setprecision(2); cout << "Average wait in queue: " << float(totalQueueMin)/carsWashed << " minutes." << endl << endl; } // Function random produces random floating-point number between 0 and 1. // float random() { // Generate random number seed the first time the function is called. static int iteration = 0; iteration++; if (iteration == 1) initializeRandomSeed(); return (float(rand()) / RAND_MAX); } // Function initializeRandomSeed uses the ctimelibrary function // // time() to seed the rand() function via the srand() function. // void initializeRandomSeed() { // Time-elapsed value, used to seed the random number generator. time_trandomNumberSeed; time(&randomNumberSeed); srand(int(randomNumberSeed)); return; } CS 240 52

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