CSEN 301 Data Structures and Algorithms

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1 CSEN 301 Data Structures and Algorithms Lecture 5: Queues: Implementation and usage Prof. Dr. Slim Abdennadher German University Cairo, Department of Media Engineering and Technology Abdennadher (GUC MET) CSEN / 23

2 Synopsis Synopsis (ADT, stack) Synopsis: Object Stacks and Interfaces How can I construct a generic stack? How can I use a generic stack with primitive data structures? What is an interface? What information does it contain? What is not part of the interface? What is the meaning of the interface in terms of the inside- or outside-view of a class? Abdennadher (GUC MET) CSEN / 23

Queue Queues Queues general idea Queue: First Come First Serve Examples: Toll Station: Car comes, pays, leaves Check-out in Big Market: Customer comes, checks out, and leaves

3 Queue Queues Queues general idea Queue: First Come First Serve Examples: Toll Station: Car comes, pays, leaves Check-out in Big Market: Customer comes, checks out, and leaves Print Server: jobs are printed according to the arrival time More examples: Office hours, Airport security check,... Abdennadher (GUC MET) CSEN / 23

4 Queues Queues general idea What is a Queue? Queue is an abstract data type Adding an entry at the rear (Enqueue). Deleting an entry at the front (DeQueue). A queue is open at two ends. Queue is a first-in first-out (FIFO) sequence of elements. Note that you you can not add/extract entry in the middle of the queue Abdennadher (GUC MET) CSEN / 23

5 Main queue operations Queues Queue basics In a queue, all operations take place at one end or the other. Front original queue Back The dequeue-operation removes an item at the front Front after dequeue() Back The enqueue-operation adds an item to the back Front after enqueue(83) Back Abdennadher (GUC MET) CSEN / 23

6 Queues Queue basics Queue operations synopsis enqueue: add a new item at the rear (back) dequeue: remove an item form the front isempty: check whether the queue is empty or not isfull: check whether the queue is full or not size: return the number of items in the queue peek: return the front item Abdennadher (GUC MET) CSEN / 23

7 Queues Implementing a queue basics Queue implementation ideas A queue can be implemented with an array. We need: The array itself, A front-pointer, A rear-pointer. front rear Front Back Abdennadher (GUC MET) CSEN / 23

8 Queues Queue implementation ideas Implementing a queue problem front rear Front Back Does anybody see a problem? An array has limited size! Once rear is at the end of this array, what do we do? Front front Array implementation rear 42 Back Abdennadher (GUC MET) CSEN / 23

9 front Queues Implementing a queue principle Queue implementation ideas Front front rear 42 Back Solution 1: Shifting the elements Too costly! Solution 2: Wrapping the the array around as a circular store rear Back Front Abdennadher (GUC MET) CSEN / 23

10 Queue class Queues Queue implementation using arrays of integers Instance Variables and Constructor public class Queue { private int maxsize; private int[] quearray; private int front; private int rear; private int nitems; // Maximum Capacity for the queue // The array that holds the items // constructor public Queue(int s) { maxsize = s; quearray = new int[maxsize]; front = 0; rear = -1; nitems = 0; } // end of constructor // set array size // create array // no items yet Abdennadher (GUC MET) CSEN / 23

11 Queue methods I Queues Queue implementation using arrays of integers enqueue public void enqueue(int i) { if (rear == maxsize - 1) rear = -1; quearray[++rear] = i; // increment rear, insert item nitems++; } dequeue public int dequeue() { int result = quearray[front]; front++; if (front == maxsize) front = 0; nitems--; return result; } // get value // increment front // deal with wraparound Abdennadher (GUC MET) CSEN / 23

12 Queue methods II Queues Queue implementation using arrays of integers peek isempty isfull size public int peek() { return quearray[front]; } public boolean isempty() { return (nitems == 0); } public boolean isfull() { return (nitems == maxsize); } public int size() { return nitems; } Abdennadher (GUC MET) CSEN / 23

13 Using a queue Queues Queue usage To use the array implementation: Queue thequeue = new Queue(10); thequeue.enqueue(2); thequeue.enqueue(3); Check isempty before using dequeue or peek while (! thequeue.isempty()) { int value = thequeue.dequeue(); System.out.print(value); System.out.print(" "); } Abdennadher (GUC MET) CSEN / 23

14 Using a queue Queues Queue usage Check isfull before using enqueue if (! thequeue.isfull()) thequeue.enqueue(item); else System.out.println("Can t insert, queue is full"); } Abdennadher (GUC MET) CSEN / 23

15 Queues Excursion Infering the item-count Question Can we implement the queue without keeping track of the item count nitems? Why? Item Count may influence performance if dealing with huge number of insertions and deletions. Not necessary? Without an item count we can still rely on front and rear to figure out whether the queue is empty or full to figure out the number of items in general Does anybody see a problem? How do you distinguish and empty queue from a full queue? Abdennadher (GUC MET) CSEN / 23

16 Queues Excursion Infering the item-count rear rear rear front front front Start with an empty queue Enqueue 125, 8, 22, 17, 83, , 64, 92. The queue is full, but the relative position of front and rear is the same as empty! Abdennadher (GUC MET) CSEN / 23

17 Queues Excursion Infering the item-count rear rear rear front front front The problem only occurs when the queue becomes completely full! One solution is to make the array one position bigger than the maximum of items. An implementation can be found in the textbook, pp.141f Abdennadher (GUC MET) CSEN / 23

18 Queues Queues of objects Question Queue is an abstract data structure. Item can be Integer, Double, String, also can be any data type, such as Employee, Faculty,... How to implement a generic (general) for all those types? Same as Stack, we use Object data type instead of int or double or String or other data type. Use an array in Queue, which stores all items come in Object[] Queue; Other implementations are possible. Abdennadher (GUC MET) CSEN / 23

19 Queues Queue of objects class Queues of objects Instance Variables and Constructor public class ObjectQueue { private int maxsize; // Maximum Capacity for the queue private Object[] quearray; // The array that holds the items private int front; private int rear; private int nitems; // constructor public ObjectQueue(int s) { maxsize = s; // set array size quearray = new Object[maxSize]; // create array front = 0; rear = -1; nitems = 0; // no items yet } // end of constructor Abdennadher (GUC MET) CSEN / 23

20 Queues Queues of objects Queue of objects some methods enqueue public void enqueue(object o) { if (rear == maxsize - 1) rear = -1; quearray[++rear] = o; // increment rear, insert item nitems++; } dequeue public Object dequeue() { Object result = quearray[front];// get value front++; // increment front if (front == maxsize) // deal with wraparound front = 0; nitems--; return result; } Abdennadher (GUC MET) CSEN / 23

21 Queues Queues of objects How to use the Generic Queue Class Create two queues with 10 elements Queue charqueue = new Queue(10); Queue empqueue = new Queue(10); enqueue and dequeue characters in Queue charqueue charqueue.enqueue(new Character( a )); char ch = (Character) charqueue.dequeue().charvalue(); enqueue and dequeue Employee in Queue empqueue empqueue.enqueue( new Employee("Bush", "2233")); Employee emp = (Employee) empqueue.dequeue(); Abdennadher (GUC MET) CSEN / 23

22 Example Queues Queue examples Print job management: Three documents are ready to print. Output the printer status Enqueue the documents public void printstatus() { Queue que = new Queue( ); //Create a new queue // print the printer status System.out.println("Printing job: Null "); // new printing jobs come, a.doc, b.doc, c.doc System.out.println("New printing job: a.doc"); que.enqueue( "a.doc" ); System.out.println("New printing job: que.enqueue( "b.doc" ); System.out.println("New printing job: que.enqueue( "c.doc" ); b.doc"); c.doc"); Abdennadher (GUC MET) CSEN / 23

23 Example Queues Queue examples Print job management: Three documents are ready to print. Which document is finished? } // print the printer status System.out.println("\nPrinting job: there are " + que.size() + "jobs in the queue"); System.out.println(" " + que.dequeue() + " is Finished"); System.out.println(" " + que.dequeue() + " is Finished"); System.out.println(" " + que.dequeue() + " is Finished"); Abdennadher (GUC MET) CSEN / 23

Stacks and Queues III

EE Data Structures and lgorithms http://www.ecs.umass.edu/~polizzi/teaching/ee/ Stacks and Queues III Lecture 8 Prof. Eric Polizzi Queues overview First In First Out (FIFO) Input D Output Queues allow