LECTURE 26:QUEUESCSC 212 – Data Structures

Using Stack

Great for Pez dispensers, calculators,& methods All of these use only the last item added Do not complain when later additions

served first Many situations use 1 item, but in

different order Take reservations in order they are

received Cards (usually) dealt from top of the deck

Stack Limitations

Collection’s operations are part of Queue As in Stack, declares size() & isEmpty()

Add & remove elements using 2 methods Element gets added to end with enqueue(elem)

dequeue() removes element at front of structure

Also includes method to peek in at first element front() returns element at front without

removing

Queue ADT

public interface Queue<E> extends Collection {public E front() throws EmptyQueueException;public E dequeue() throws EmptyQueueException;public void dequeue(E element);

}

Very similar to Stack interface Defines specific methods to add, remove, &

view data Holds many elements, but can access only

one Stack & Queue always add to the end

Remove element at the start of a QUEUE… …while STACK removes element at the end

Queue Interface

Stacks vs. Queues

Access data with Stack in LIFO order LLast IIn-FFirst OOut Completely unfair (unless you are always

late) Data accessed in Queue using FIFO

order FFirst IIn-FFirst OOut Better resembles lines at bank, Sabres

game, airports

Among the most fundamental data types Selecting application to run in modern OS Handling and processing network traffic Message checking in IM chat servers Accepting ticket orders for Sabres game

Other data structures also rely upon a Queue

Key feature in many algorithms

Queue Applications

“Obvious” implementation uses an array Must consume a constant amount of space enqueue() throws exception when it lacks

space Instead write linked list-based

implementation Singly-, doubly-, or circular-linked list could

work Size of the Queue grows & shrinks as

needed More straightforward, but is it slower?

Queue Implementation

Class defines fields aliased to first & last nodes head & rear often used as fields’ names

(creative!) enqueue element by adding new Node after rear

Set head to next Node in list to dequeue element

Linked-list based Queue

Ø

head rear

STACKS are easy for arrays: only 1 end “moves” Can always find Stack’s bottom at index 0

QUEUES are harder, because both ends move dequeue removes element from the front Add element to back with calls to enqueue

Ends of a array-based QUEUE like clock time

Circular Access

q

rf r r r r rr r

fr

Array-based Queue

Two fields track front and rear of QUEUEf equals index of front elementr holds index immediately after rear element

Add & remove elements at opposite ends of array Uses circular access to the array Like a clock –when end (12) reached we

loop to start

Array must be empty at index in rq

r rf rrr

Array-based Queue Operations Based on clock

math Uses mod

(remainder) Java expressed as %

How mod works:4 % 3 = 13 % 4 = 36 % 6 = 012 % 6 = 0

Algorithm size()N q.length return (N f + r) mod N

Array-based Queue Operations

Algorithm enqueue(e)if size() = q.length 1

thenthrow

FullQueueExceptionelse

q[r] er (r + 1) mod

q.length

q

rf

Algorithm dequeue()if isEmpty() then

throw EmptyQueueException

elseretVal q[f]f (f + 1) mod

q.lengthreturn retVal

Finish week #9 assignment Due by 5PM tomorrow

Continue programming assignment #3 Messages are not always sent to everyone!

Read section 5.3 in book before class Discusses merger of Stack & Queue ADT Implement this Deque ADT with array Could instead implement it with a linked-list Make also sorts of bad jokes about porches

Before Next Lecture…