How can we improve this? Queues 6. Topological ordering: given a sequence of. should occur prior to b, provide a schedule. Queues 5.

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1 Queues 1 Queues A Queue is a sequential organization of items in which the first element entered is the first removed. They are often referred to as FIFO, which stands for first in first out. Examples: standing in a line, printer queue. The basic operations are: insert(x) (a.k.a. enqueue(x)) places x at the beginning (head) of the queue. remove() (a.k.a. dequeue()) returns and deletes the item at the end (tail) of the queue. Example: Operation Queue Return CreateQueue () insert(7) (7) insert(8) (7,8) insert(5) (7,8,5) remove() (8,5) 7 remove() (5) 8 Queues 4 Queues: A Better Implementation Keep track of both the head and the tail. To remove, increment head. a b c d head = 0 tail = 4 remove a b c d head = 0 tail = 4 update head b c d head = 1 tail = 4 There is still a problem. What is it, and how can we fix it? Queues 2 Queue Applications Operating systems: Queue of jobs or processes ready to run (waiting for CPU): Queues of processes waiting for I/O. Files sent to printer Simulation of real-world queuing systems (queueing theory): Customers in a grocery store, bank, etc. Orders in a factory Hospital emergency room or doctor s office Telephone calls for airline reservations, customer orders, information, etc. Problem applications: Topological ordering: given a sequence of events, and pairs (a, b) indicating that event a should occur prior to b, provide a schedule. Queues 5 Queues: Circular Array Implementation Previous implementation is O(1) per operation, which is great. However, after n inserts (where n is the size of the array), the array is full even if the queue is logically nearly empty. Solution: Use wraparound to reuse the cells at the start of the array. To increment, add one, but if that goes past end, reset to zero. How do you detect a full or empty queue? We will give a simplified implementation for the queue data structure. A better implementation would detect an empty (full) queue before performing a dequeue (enqueue) operation. Queues 3 Queues: Naïve Implementation Using array: Store items in an array. The head is the first element, and the tail is indicated by tail (we ll have tail point to next available element) insert(x): insert element and increment tail remove() is inefficient: all elements have to be shifted Thus, remove is Θ(n). a b c d tail = 4 remove a b c d tail = 4 update tail b c d How can we improve this? tail = 3 Queues 6 Queue C++ Declaration Here is a C++ declaration of an integer queue using a circular array: ; int Full() return (head==(tail+1)%cap); int Empty() return (head == tail ); class Queue private: int *queue; int cap,head,tail; public: Queue(int s=100) cap= s; queue = new int[cap]; head = 0; tail = 0; Queue() delete [] queue; void Enqueue(int v) queue[tail] = v; tail = (tail+1) % cap; int Dequeue() int t = queue[head]; head = (head + 1) % cap; return t;

2 Queues 7 Queue JAVA Declaration public class Queue private int[] queue; private int cap,head,tail; public Queue(int s) cap=s; queue = new int[cap]; head = 0; tail = 0; public void Enqueue(int v) queue[tail] = v; tail = (tail+1) % cap; public int Dequeue() int t = queue[head]; head = (head + 1) % cap; return t; public int Empty() return (head == tail ); public int Full() return (head == ((tail+1)%cap)); ; Queues 8 Example of Queue Using Circular Array Here we use an array E[0..3] to store the elements and the variables h (head) and t (tail) to keep track of the beginning and end of the queue. The value R is the return value of the operation. Operation h t E0 E1 E2 E3 R create 0 0???? insert(55) ??? insert(-7) ?? insert(16) ? remove() ? 55 insert(-8) remove() remove() insert(11) Note that some of the values remain physically in the array, but are logically no longer in the queue. Queues 9 Queues: Linked List Implementation We can maintain head and tail pointers. remove: advance head. insert: add to end of list and adjust tail. head tail A B C D

3 Chapter Objectives Chapter 8 Queues 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 Data Structures Using C++ 1 Data Structures Using C++ 2 Chapter Objectives Discover queue applications Examine the STL class queue Discover priority queues Queues Definition: data structure in which the elements are added at one end, called the rear or tail, and deleted from the other end, called the front or first First In First Out (FIFO) data structure Data Structures Using C++ 3 Data Structures Using C++ 4 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 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 Data Structures Using C++ 5 Data Structures Using C++ 6

4 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 Data Structures Using C++ 7 Data Structures Using C++ 8 Circular Queue Data Structures Using C++ 9 Data Structures Using C++ 10 Data Structures Using C++ 11 Data Structures Using C++ 12

5 Data Structures Using C++ 13 Data Structures Using C++ 14 Data Structures Using C++ 15 Data Structures Using C++ 16 UML Diagram of the class queuetype Data Structures Using C++ 17 Data Structures Using C++ 18

6 Empty Queue and Full Queue Destroy Queue bool queuetype<type>::isemptyqueue() return (count == 0); bool queuetype<type>::isfullqueue() return (count == maxqueuesize); Data Structures Using C++ 19 void queuetype<type>::destroyqueue() queuefront = 0; queuerear = maxqueuesize - 1; count = 0; Data Structures Using C++ 20 front and back Type queuetype<type>::front() assert(!isemptyqueue()); return list[queuefront]; Type queuetype<type>::back() assert(!isemptyqueue()); return list[queuerear]; Data Structures Using C++ 21 Add Queue 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; Data Structures Using C++ 22 Delete Queue Constructor and Destructor 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; 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 Data Structures Using C++ 23 Data Structures Using C++ 24

7 Linked Queue as an ADT 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 Data Structures Using C++ 25 Data Structures Using C++ 26 addqueue Adds a new element to the end of the queue Access the pointer queuerear to implement addqueue 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 Data Structures Using C++ 27 Data Structures Using C++ 28 STL class queue (Queue Container Adapter) 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 Data Structures Using C++ 29 Data Structures Using C++ 30

8 Application of Queues Application of Queues Data Structures Using C++ 31 Data Structures Using C++ 32 Application of Queues Application of Queues: waitingcustomerqueuetype Data Structures Using C++ 33 Data Structures Using C++ 34 Chapter Summary Queue Data Structure Restricted Version of arrays and linked list Basic operations First In First Out (FIFO) Queues Implemented as Arrays Chapter Summary Queues Implemented as Linked Lists STL class queue Priority Queues Application of Queues Data Structures Using C++ 35 Data Structures Using C++ 36

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