CSCD 326 Data Structures I Queues

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1 CSCD 326 Data Structures I Queues 1

6 Linked List Queue Implementation Diagram Front Back A B C D 0 6

7 public interface QueueInterface { Queue Interface public boolean isempty(); // Determines whether a queue is empty. // Precondition: None. // Postcondition: Returns true if the queue is empty; // otherwise returns false. public void enqueue(object newitem) throws QueueException; // Adds an item at the back of a queue. // Precondition: newitem is the item to be inserted. // Postcondition: If the operation was successful, newitem // is at the back of the queue. Some implementations // may throw QueueException if newitem cannot be added to the queue. public Object dequeue() throws QueueException; // Retrieves and removes the front of a queue. // Precondition: None. // Postcondition: If the queue is not empty, the item that was added to the // queue earliest is returned and the item is removed. If the queue is empty, // the operation is impossible and QueueException is thrown. 7

8 Queue Interface (2) public void dequeueall(); // Removes all items of a queue. // Precondition: None. // Postcondition: The queue is empty. public Object peek() throws QueueException; // Retrieves the item at the front of a queue. // Precondition: None. // Postcondition: If the queue is not empty, the item // that was added to the queue earliest is returned. // If the queue is empty, the operation is impossible // and QueueException is thrown. } // end QueueInterface 8

9 Queue Exception public class QueueException extends RuntimeException { public QueueException(String s) { super(s); } // end constructor } // end QueueException 9

10 ListQueue Implementation public class ListQueue implements QueueInterface { private Node lastnode; public ListQueue() { lastnode = null; } // end default constructor // queue operations: public boolean isempty() { return lastnode == null; } // end isempty public boolean isfull() { return false; } // end isfull public void dequeueall() { lastnode = null; } // end dequeueall 10

11 ListQueue Implementation (2) public void enqueue(object newitem) { Node newnode = new Node(newItem); if(isfull()) throw new QueueException("QueueException on enqueue:"+ "queue full"); // insert the new node if (isempty()) { // insertion into empty queue newnode.setnext(newnode); } else { // insertion into nonempty queue newnode.setnext(lastnode.getnext()); lastnode.setnext(newnode); } // end if lastnode = newnode; // new node is at back } // end enqueue 11

12 ListQueue Implementation (3) public Object dequeue() throws QueueException { if (!isempty()) { // queue is not empty; remove front Node firstnode = lastnode.getnext(); if (firstnode == lastnode) { // special case? lastnode = null; // yes, one node in queue } else { lastnode.setnext(firstnode.getnext()); } // end if return firstnode.getitem(); } else { throw new QueueException("QueueException on dequeue:" + "queue empty"); } // end if } // end dequeue 12

13 ListQueue Implementation (4) public Object peek() throws QueueException { if (!isempty()) { // queue is not empty; retrieve front Node firstnode = lastnode.getnext(); return firstnode.getitem(); } else { throw new QueueException("QueueException on peek:" + "queue empty"); } // end if } // end peek } // end ListQueue 13

14 ListQueue Test public class ListQueueTest { public static void main(string[ ] args) { ListQueue aqueue = new ListQueue(); System.out.println("Enqueuing:"); for (int i = 0; i < 9; i++) { System.out.print(" "+i); aqueue.enqueue(new Integer(i)); } // end for System.out.println("\nDequeuing:"); for (int i = 0; i < 9; i++) { System.out.print(" "+aqueue.dequeue()); } // end for System.out.println(); } // end main } // QueueTest 14

15 ListQueue Test Output Enqueuing: Dequeuing:

16 ArrayQueue Implementation public class ArrayQueue implements QueueInterface { private final int MAX_QUEUE = 50; // maximum size of queue private Object[ ] items; private int front, back, count; public ArrayQueue() { items = new Object[MAX_QUEUE]; front = 0; back = MAX_QUEUE-1; count = 0; } // end default constructor // queue operations: public boolean isempty() { return count == 0; } // end isempty public boolean isfull() { return count == MAX_QUEUE; } // end isfull 16

17 ArrayQueue Implementation (2) public void enqueue(object newitem) { if (!isfull()) { back = (back+1) % (MAX_QUEUE); items[back] = newitem; ++count; } else throw new QueueException("QueueException on enqueue: "+ "Queue full"); } // end enqueue public Object dequeue() throws QueueException { if (!isempty()) { // queue is not empty; remove front Object queuefront = items[front]; front = (front+1) % (MAX_QUEUE); --count; return queuefront; } else throw new QueueException("QueueException on dequeue: "+ "Queue empty"); } // end dequeue 17

18 ArrayQueue Implementation (3) public void dequeueall() { items = new Object[MAX_QUEUE]; front = 0; back = MAX_QUEUE-1; count = 0; } // end dequeueall public Object peek() throws QueueException { if (!isempty()) { // queue is not empty; retrieve front return items[front]; } else { throw new QueueException("Queue exception on peek: " + "Queue empty"); } // end if } // end peek } // end ArrayQueue 18

19 ArrayQueue Test public class ArrayQueueTest { public static void main(string[] args) { ArrayQueue aqueue = new ArrayQueue(); System.out.println("Enqueuing:"); for (int i = 0; i < 9; i++) { System.out.print(" "+i); aqueue.enqueue(new Integer(i)); } // end for System.out.println("\nDequeuing:"); for (int i = 0; i < 9; i++) { System.out.print(" "+aqueue.dequeue()); } // end for System.out.println(); } // end main } // QueueTest 19

20 ArrayQueue Test Output Enqueuing: Dequeuing:

23 Full/Empty Determination without Counters (3) [0] A [1] B [2] C [3] D [4] E Now: Back = 4 Front = 0 This suggests that a test for a full Q would be: (Back + 1) % Qsize == Front But this is the same as our test for an empty Q - a new method is needed 23

1/22/13. Queue? CS 200 Algorithms and Data Structures. Implementing Queue Comparison implementations. Photo by David Jump 9. Colorado State University

1/22/13. Queue? CS 200 Algorithms and Data Structures. Implementing Queue Comparison implementations. Photo by David Jump 9. Colorado State University //3 Part Queues CS Algorithms and Data Structures Outline Queue? Implementing Queue Comparison implementations 8 Photo by David Jump 9 //3 "Grill the Buffs" event Queue A queue is like a line of people