Chapter 16 Stack and Queues part2 Dr. Bernard Chen Ph.D. University of Central Arkansas

Introduction to Queues A queue is a waiting line It’s in daily life: A line of persons waiting to check out at a supermarket A line of persons waiting to purchase a ticket for a film A line of planes waiting to take off at an airport A line of vehicles at a toll booth

Introduction to Queues Difference between Stack and Queues: Stack exhibits last-in-first-out (LIFO) Queue exhibits first-in-first-out (FIFO)

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

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)

Designing and Building a Queue Class Array-Based Consider an array in which to store a queue Note additional variables needed myFront, myBack Picture a queue object like this

Queue Operation Empty Queue Enqueue(70)

Queue Operation Enqueue(80) Enqueue(50)

Queue Operation Dequeue()

Queue Operation Enqueue(90) Enqueue(60)

Circular Queue Problems Possible solutions We quickly "walk off the end" of the array Possible solutions Shift array elements Use a circular queue Note that both empty and full queue gives myBack == myFront

Circular Queue Using a static array QUEUE_CAPACITY specified Enqueue increments myBack using mod operator, checks for full queue Dequeue increments myFront using mod operator, checks for empty queue

Circular Example Both Front and Back wraparound as needed. b c d Front g

QUEUE Only tricky part is vector doubling because the queue items are not necessarily stored in an array starting at location 0, and the contiguity of wraparound must be maintained. Therefore, mostly straightforward; maintain Front Back

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 empty and full We may avoid this situation by maintaining one empty position, so that myFront will never equal to myBack unless the queue is empty

Queue Operation Construct: Empty: Front : Create an array, set capacity, myFront=myBack=0 Empty: test myFront==myBack Front : if not empty: print array[myFront]

Algorithm for Enqueue(value) 1. Set newBack == (myBack+1)%Queue_capacity 2. If newBack == myFront Signal “Queue Full” otherwise: Set Array[myBack] == value Set myBack == newBack

Algorithm for Dequeue() If queue is empty signal “Queue Empty” Otherwise Set myFront=(myFront+1)%Queue_capacity

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)

Linked Queues Constructor initializes myFront, myBack Empty myFront == Null Front return myFront->data Dequeue Delete first node (watch for empty queue) Enqueue Insert node at end of list

Enqueue newptr= new Node(value) if (empty()) else { myFront=myBack=newptr; else { myBack->next=newptr; myBack=newwptr; }

Dequeue (if not empty) ptr=myFront myFront=myFront->next delete ptr;

Queue ADT implement by Vectors 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)

Enqueue and Front Enqueue (add element to back) This is the same with push(), therefore: L.push_back(value); Front (retrieve value of element from front) L.begin();

Dequeue Dequeue (remove element from front) L.erase( L.begin() ); L.begin() is an iterator, in erase(), you cannot just gives the location

Functions related to Queue Constructor: vector<int> L; Empty(): L.size() == 0? Enqueue(): L.push_back(value); Front(): L.begin(); Dequeue(): L.erase(L.begin());

QUEUE Write a queue program

Vector Functions

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()