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? CSC 212 Data Structures 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 & other stores do not cut 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 extends Collection { public E enqueue() throws EmptyQueueException; public E dequeue() throws EmptyQueueException; public void front(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 Q UEUE while S TACK removes element at the end Stacks vs. Queues 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 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 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 S TACKS are easy for arrays: only 1 end moves Can always find Stacks bottom at index 0 Q UEUES 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 Q UEUE like clock time Circular Access q r f r rrr r r r f r Array-based Queue Two fields track front and rear of Q UEUE f equals index of front element r 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 r q r r f r r r Array-based Queue Operations Based on clock math Uses mod (remainder) Java expressed mod as % How mod works: 0 % 3 = 0 1 % 3 = 1 2 % 3 = 2 3 % 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 then throw FullQueueException else q[r] e r (r + 1) mod q.length q rf Algorithm dequeue() if isEmpty() then throw EmptyQueueException else retVal q[f] f (f + 1) mod q.length return retVal Your Turn Get into your groups and complete activity For Next Lecture Read GT section 5.3 before Friday's class Discusses design of the Deque ADT Array-based implementation of Deque presented Deque implementation of linked-list also shown Week #9 weekly assignment due on Tuesday Midterm #2 Midterm #2 will be in class next Wednesday