1. COSC 1P03 Data Structures and Abstraction 9.1 The Queue Whenever you are asked if you can do a job, tell 'em, "Certainly, I can!" Then get busy and find out how to do it. Theodore Roosevelt

2. COSC 1P03 Data Structures and Abstraction 9.2 Queue  a list (initially empty) of items (of some type) to which items may be added at one end (called the rear ) and from which items may be removed at the other end (called the front )  examples  waiting lines  print queues  process queue  behaviour  FIFO ordering  error conditions:  underflow  overflow

3. COSC 1P03 Data Structures and Abstraction 9.3 Queue Interface  generic ( Stores objects of type E )  no requirements  operations:  enter (enqueue, add, insert)  leave (dequeue, remove, delete)  front (head, first)  length (count, size)  empty  exceptions  NoItemException  NoSpaceException

4. COSC 1P03 Data Structures and Abstraction 9.4 Queue ADT Contiguous Implementation  based on variable-sized array  two indices: front & rear  add at rear, remove at front  queue moves towards rear  repositioning on delete: O(n)  circular array  at end of array reuse front  index modulo array size

5. COSC 1P03 Data Structures and Abstraction 9.5 Queue ADT.  implementation  instance variables  count  constructors  empty state  methods  enter  overflow  increment  leave, front  underflow  length, empty  compute?  empty vs full

6. COSC 1P03 Data Structures and Abstraction 9.6 Queue ADT Linked Implementation  sequentially-linked structure of items  deletion from front  insertion at end  keep pointer to rear O(1)  length?  keep count else O(n)  comparison with contiguous  all operations O(1)  space tradeoffs

7. COSC 1P03 Data Structures and Abstraction 9.7

10. COSC 1P03 Data Structures and Abstraction 9.10 Queue Example Input: 1 2 3 4 5Output: 1 2 3 4 5 12345

11. COSC 1P03 Data Structures and Abstraction 9.11 Process Queue

12. COSC 1P03 Data Structures and Abstraction 9.12 Circular Queue 123...N123...N Front Rear N-1 0 1

13. COSC 1P03 Data Structures and Abstraction 9.13 Queue ADT Interface package Collections; public interface Queue { public void enter ( E item ); public E leave ( ); public E front ( ); public boolean empty ( ); public int length ( ); }// Queue Part of the Collections Package Formal type definitionAccept an item of type E Both return an item of type E, much like Pop and Top do for a Stack.

14. COSC 1P03 Data Structures and Abstraction 9.14 Array Queue package Collections; public class ConQueue implements Queue, Serializable { private E[]items;// the items of the queue private intfront;// index of the front item private intrear;// index of next available element private intcount;// number of items in queue public ConQueue ( ) { this(100); };// constructor public ConQueue ( int size ) { items = (E[]) new Object[size]; front = 0; rear = 0; count = 0; };// constructor public void enter ( E item ) { if ( count >= items.length ) { throw new NoSpaceException(); } else { items[rear] = item; rear = (rear + 1) % items.length; count = count + 1; }; };// enter Constructor creates the array and initializes front, rear and count Check for overflow, is count >= to the capacity of the array Add the item into the array indexed by rear Perform modular addition to increment rear index. We have added 1 item, so increment count. Instance variables, items is an array of E data types ConQueue implements an interface call Queue of the same type

15. COSC 1P03 Data Structures and Abstraction 9.15 Array Queue. public E leave ( ) { Ei;// item removed if ( count <= 0 ) { throw new NoItemException(); } else { i = items[front]; items[front] = null; front = (front + 1) % items.length; count = count - 1; return i; }; };// leave public E front ( ) { if ( count <= 0 ) { throw new NoItemException(); } else { return items[front]; }; };// front public boolean empty ( ) { return count <= 0; };// empty public int length ( ) { return count; };// length }// ConQueue Check for underflow, is count 0 Extract the item Set pointer to null so garbage collection will reclaim the memory. Modular addition to increment front. Removed a item so reduce count. Return the extracted item Return the front item if it exists. Leave it in the queue. If count is 0 then it is empty. How many items are in the queue

16. COSC 1P03 Data Structures and Abstraction 9.16 The End