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Question of the Day How can you change the position of 1 toothpick and leave the giraffe in exactly the same form, but possibly mirror-imaged or oriented differently, as before?
Question of the Day
How can you change the position of 1 toothpick and leave the giraffe in exactly the same form, but possibly mirror-imaged or oriented differently, as before?
Question of the Day
How can you change the position of 1 toothpick and leave the giraffe in exactly the same form, but possibly mirror-imaged or oriented differently, as before?
LECTURE 23:QUEUES
CSC 212 – Data Structures
Last-In, First-Out principle used to access data Also called LIFO ordering
Top of stack is where data added & removed Only useful location; cannot access
anything else
Using Stack
Stack Limitations
Great for Pez dispensers, JVMs,& methods All of these use most recent item added
only Do not complain when later additions
served first Many situations use items in order
added Checker at Wegmans & others prevent
cutting in line Use first-come, first-served getting food at
dining hall
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 front element in structure
Also includes method to peek in at first element front() returns element at front without removing
Queue ADT
Queue Interface
public interface Queue<E> extends Collection {public E front() throws EmptyQueueException;public E dequeue() throws EmptyQueueException;public void enqueue(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 start of this QUEUE… …while STACK removes element at the end
Stacks vs. Queues
Access data with Stack in LIFO order
Last In-First Out Completely unfair (unless you are always
late) Data accessed in Queue using FIFO
order
First In-First Out Lines at bank, airports represented fairly
with these
“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 No additional exceptions needed, 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
head rear
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
head rear
elem
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
head rear
elem
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
head rear
elem
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
head rear
elem
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
head rear
retVal
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
head rear
retVal
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
head rear
retVal
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 calls will remove element at front Add element to back with calls to enqueue
Ends of a array-based QUEUE like clock time
Circular Access
q
rf
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 calls will remove element at front Add element to back with calls to enqueue
Ends of a array-based QUEUE like clock time
Circular Access
qrf
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 calls will remove element at front Add element to back with calls to enqueue
Ends of a array-based QUEUE like clock time
Circular Access
qrf
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 calls will remove element at front Add element to back with calls to enqueue
Ends of a array-based QUEUE like clock time
Circular Access
qrf
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 calls will remove element at front Add element to back with calls to enqueue
Ends of a array-based QUEUE like clock time
Circular Access
qrf
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 calls will remove element at front Add element to back with calls to enqueue
Ends of a array-based QUEUE like clock time
Circular Access
qrf
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 calls will remove element at front Add element to back with calls to enqueue
Ends of a array-based QUEUE like clock time
Circular Access
qrf
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 calls will remove element at front Add element to back with calls to enqueue
Ends of a array-based QUEUE like clock time
Circular Access
qr f
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 calls will remove element at front Add element to back with calls to enqueue
Ends of a array-based QUEUE like clock time
Circular Access
qr f
Two fields track front and rear of QUEUEf equals index of front elementr holds index immediately after rear element
Add & remove elements from opposite ends Uses circular access to the array Works like clock: when end (12) reached,
loop to start
Array must be empty at index in rf
Array-based Queue
q
r
Two fields track front and rear of QUEUEf equals index of front elementr holds index immediately after rear element
Add & remove elements from opposite ends Uses circular access to the array Works like clock: when end (12) reached,
loop to start
Array must be empty at index in rf
Array-based Queue
q
r
Two fields track front and rear of QUEUEf equals index of front elementr holds index immediately after rear element
Add & remove elements from opposite ends Uses circular access to the array Works like clock: when end (12) reached,
loop to start
Array must be empty at index in rf
Array-based Queue
q
r
Two fields track front and rear of QUEUEf equals index of front elementr holds index immediately after rear element
Add & remove elements from opposite ends Uses circular access to the array Works like clock: when end (12) reached,
loop to start
Array must be empty at index in rf
Array-based Queue
q
r
Two fields track front and rear of QUEUEf equals index of front elementr holds index immediately after rear element
Add & remove elements from opposite ends Uses circular access to the array Works like clock: when end (12) reached,
loop to start
Array must be empty at index in rf
Array-based Queue
q
r
Two fields track front and rear of QUEUEf equals index of front elementr holds index immediately after rear element
Add & remove elements from opposite ends Uses circular access to the array Works like clock: when end (12) reached,
loop to start
Array must be empty at index in rf
Array-based Queue
q
r
Two fields track front and rear of QUEUEf equals index of front elementr holds index immediately after rear element
Add & remove elements from opposite ends Uses circular access to the array Works like clock: when end (12) reached,
loop to start
Array must be empty at index in rf
Array-based Queue
q
r
Two fields track front and rear of QUEUEf equals index of front elementr holds index immediately after rear element
Add & remove elements from opposite ends Uses circular access to the array Works like clock: when end (12) reached,
loop to start
Array must be empty at index in rff
Array-based Queue
q
r
q
r
Array-based Queue Operations
Based on clock math Uses mod
(remainder) Java expressed mod
as %
How mod works:0 % 3 = 01 % 3 = 12 % 3 = 23 % 3 = 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.lengthq
rf
Algorithm dequeue()if isEmpty() then
throw EmptyQueueException
elseretVal q[f]f (f + 1) mod
q.lengthreturn retVal
Your Turn
Get into your groups and complete activity
For Next Lecture
Read GT section 5.3 before Wednesday's class Discusses design of the Deque ADT Array-based implementation of Deque
presented Deque implementation of linked-list also
shown
Week #8 weekly assignment due on Tuesday
Midterm #2 will be in class next Monday
