Queues 4/11/18. Many of these slides based on ones by Cynthia Lee

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1 Queues 4/11/18 Many of these slides based on ones by Cynthia Lee

2 Administrivia HW 4 due tomorrow night (list implementations) Reading: For Friday: Beginning of Chapter 7 (pp ) For Monday: Rest of Chapter 7 Midterm is imminent No class next Wednesday (4/18) Multi-day takehome, same idea as HW 4 Open book, closed web and friends

3 Queue ADT Stores collection of items in the order they were added to the queue Supports: Queue() void enqueue(item i) Item dequeue() boolean isempty() int size()

4 When an item is removed from a queue, it is...? A. The one that has been in the queue for the longest time B. The one that has been in the queue for the shortest time C. The smallest one in the queue D. The largest one in the queue E. Not exactly one of the above

5 When an item is removed from a queue, it is...? A. The one that has been in the queue for the longest time B. The one that has been in the queue for the shortest time C. The smallest one in the queue D. The largest one in the queue E. Not exactly one of the above

6 When using enqueue() to place the following items into a queue: enqueue(32) enqueue(65) enqueue(0) enqueue(23) enqueue(-1) the output when dequeueing from the queue is: A. -1, 23, 0, 65, 32 B. -1, 0, 23, 32, 65 C. 65, 32, 23, 0, -1 D. 32, 65, 0, 23, -1 E. None of the above

7 When using enqueue() to place the following items into a queue: enqueue(32) enqueue(65) enqueue(0) enqueue(23) enqueue(-1) the output when dequeueing from the queue is: A. -1, 23, 0, 65, 32 B. -1, 0, 23, 32, 65 C. 65, 32, 23, 0, -1 D. 32, 65, 0, 23, -1 E. None of the above

8 Queue implementation 1 Array with front element of queue at index 0 Attribute rear is index of first unoccupied cell Empty queue denoted by rear being 0

9 Implement enqueue into an array-based queue whose first value is always stored in cell 0. (array is the array of values and rear holds the index of the first unoccupied cell). public void enqueue(t val) { //YOUR CODE HERE

10 public E dequeue(){ // potential issue if empty, for now, assume not empty E e = array[front]; <YOUR CODE HERE> return e; Select the correct code to insert from below: A. front++; B. rear = rear-1; C. for(int i= 0; i<rear-1; i++) array[i] = array[i+1]; rear = rear -1; D. None of these are correct

11 public E dequeue(){ // potential issue if empty, for now, assume not empty E e = array[front]; <YOUR CODE HERE> return e; Select the correct code to insert from below: A. front++; B. rear = rear-1; C. for(int i= 0; i<rear-1; i++) array[i] = array[i+1]; rear = rear -1; D. None of these are correct

12 Queue implementation 2 Array with rear element of queue at index 0 Attribute front is index of the first unoccupied cell after the queue Empty queue denoted by front being 0

13 public void enqueue( E element){ // potential issue if full, for now, assume room <YOUR CODE HERE> front++; Select the correct code to insert from below: A array[0] = element; C for(int i= 0; i<front; i++) { array[i+1] = array[i]; array[rear] = element; B array[front] = element; D None of these are correct

14 public void enqueue( E element){ // potential issue if full, for now, assume room <YOUR CODE HERE> front++; Select the correct code to insert from below: A array[0] = element; C for(int i= 0; i<front; i++) { array[i+1] = array[i]; array[rear] = element; B array[front] = element; D None of these are correct

15 Summary Queue ADT: enqueue adds an element at the back dequeueremoves one from the front Array-based queue implementations put the front in cell 0: bad for dequeue put the back in cell 0: bad for enqueue

17 front = index of first occupied cell in queue rear = index of last occupied cell in queue

18 public E dequeue(){ // potential issue if empty, // for now, assume not empty size--; E e = array[front]; <YOUR CODE HERE> return e; Select the correct code to insert from below: A front++; if(front == array.length) front = 0; B rear = rear-1; if(rear <0) rear = array.length-1; C for(int i= 0; i<rear; i++) { array[i] = array[i+1]; rear = rear -1; if(rear < 0) rear = array.length-1; D Not exactly one of the above

19 public E dequeue(){ // potential issue if empty, // for now, assume not empty size--; E e = array[front]; <YOUR CODE HERE> return e; Select the correct code to insert from below: A front++; if(front == array.length) front = 0; B rear = rear-1; if(rear <0) rear = array.length-1; C for(int i= 0; i<rear; i++) { array[i] = array[i+1]; rear = rear -1; if(rear < 0) rear = array.length-1; D Not exactly one of the above

20 public void enqueue(e e){ // potential issue if full, // for now, assume not full <YOUR CODE HERE> size++; Select the correct code to insert from below: A rear++; if(rear == array.length) rear = 0; array[rear] = e; C for(int i= front; i<rear; i++) { array[i] = array[i+1]; array[rear] = e; front--; B rear++ array[rear] =e; D None of these are correct

21 public void enqueue(e e){ // potential issue if full, // for now, assume not full <YOUR CODE HERE> size++; Select the correct code to insert from below: A rear++; if(rear == array.length) rear = 0; array[rear] = e; C for(int i= front; i<rear; i++) { array[i] = array[i+1]; array[rear] = e; front--; B rear++ array[rear] =e; D None of these are correct

22 Suppose we have a Queue implemented with a circular array. The capacity is 10 and the size is 5. Which are not legal values for front and rear? front rear A 0 4 B 5 9 C 7 2 D 9 3 E Notexactly one of the above

23 Suppose we have a Queue implemented with a circular array. The capacity is 10 and the size is 5. Which are not legal values for front and rear? front rear A 0 4 B 5 9 C 7 2 D 9 3 E Notexactly one of the above

24 Array-based queue Possible using a circular array A bit complicated by need to wrap indices Modular arithmetic: Increment: i = (i + 1) % vals.length; Decrement: i = (i 1 + vals.length) % vals.length; Easier with linked lists because you can make local changes

25 Queue running times using a circular array linked list (enqueue to last) enqueue dequeue

26 Queue running times using a circular array linked list (enqueue to last) enqueue Linear (constant amortized) Linear dequeue Constant Constant For linked list, whichever operation changes the end of list takes linear time

correctly adds to the end? A. tail = new Node(value, null); B. tail = new Node(value, tail); C. tail.next = new Node(value, null); D.

27 Recall: Adding a tail pointer LinkedList head next Node next Node next Node next Node tail data data data data Tail pointer (tail reference) refers to last Node in the list Which of the following correctly adds to the end? A. tail = new Node(value, null); B. tail = new Node(value, tail); C. tail.next = new Node(value, null); D. tail.next = new Node(value, tail.next); E. Not exactly one of the above

28 Queue running times using a circular array linked list w/ tail pointer (enqueue to last) enqueue Linear (constant amortized) Constant dequeue Constant Constant

CSE 373: Data Structures and Algorithms

CSE 373: Data Structures and Algorithms Lecture 2: Wrap up Queues, Asymptotic Analysis, Proof by Induction Instructor: Lilian de Greef Quarter: Summer 2017 Today: Announcements Wrap up Queues Begin Asymptotic