Keeping Order:! Stacks, Queues, & Deques. Travis W. Peters Dartmouth College - CS 10

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1 Keeping Order:! Stacks, Queues, & Deques 1

2 Stacks 2

3 Stacks A stack is a last in, first out (LIFO) data structure Primary Operations: push() add item to top pop() return the top item and remove it peek() return the top item but don t remove it isempty() return True iff the stack is empty java.util.stack is similar to our stack implementation but more fully featured The textbook s version of Stack is similar but has different naming. 3 3

4 Stacks: Why? Reversing strings/lists is easy with stacks! push everything on in order then pop them all off. Depth-first search (DFS) later Maze and graph searching Thank back to lab 1! :) Matching parenthesis, braces, open/close tags for HTML, etc. think about how java expects matching { } and ( ) 4 4

5 Demo: MatchParens.java Find open paren push() Find close paren check top of stack for friend (a.k.a. partner) 5 5

6 Implementing Stacks Q: How do we implement a stack? Simple option: fixed-size array. instance variables (int[] stack and int top) push() top++ stack[top] = pop() peek() top-- Pros: fast (operations are O(1)); space efficient Cons: array can fill up stack[8] top

7 Implementing Stacks: ArrayList Better option: dynamic array. instance variables (ArrayLists<T> stack) push() use add() to add to end pop() use remove() to remove from end Pros: ArrayList never becomes full ; no need to keep track of top since ArrayList does this for us! Cons: push() could be non-constant time op. when the ArrayList has to grow to make more room. 7 7

8 Implementing Stacks: SLL Great option: Singly-Linked List. top instance variables (Element<T> top) top of the stack is the head of the list push() adds item to front of list pop() removes item from front of list Pros: SLL never becomes full ; all operations are O(1) Cons: need extra space for links in list but there is never wasted space for unused spaces in array/arraylist. / 8 8

9 Queues 9

10 Queues A queue is a first in, first out (FIFO) data structure Primary Operations: enqueue() add at rear of the queue dequeue() remove and return the first item in the queue front() return the first item but don t remove it isempty() return True iff the queue is empty Java has a Queue interface but it uses non-conventional names (add()/offer(), element(), peek(), remove()/poll()). The 2 different alternatives for enqueue()/dequeue() are for one that uses exceptions to handle failed operations while the other doesn t 10 10

11 Queues: Why? Useful for simulations of lines (banks, toll booths, etc.) Useful for computer print queues submit a document to be printed enqueue() print the document when it reaches the front dequeue() Round-robin scheduling for threads/processes to run dequeue() process run for fixed period of time or until it blocks enqueue() processes in the order they arrive to ensure fair sharing of CPU processes that finish don t get enqueued again Used when searching (similar to stack) 11 11

12 Implementing Queues See: SLLQueue.java Typical option: Linked Lists. instance variables (Element<T> head, tail) front of the queue is the head of the list; tail is the back. enqueue() adds item to back of list dequeue() removes item from front of list Pros: Never becomes full ; all operations are Θ(1). Cons: need extra space for links in list but there is never wasted space for unused spaces in array/arraylist. tail head / 12 12

13 Implementing Queues Basic setup + front() & isempty() public class SLLQueue<T> implements SimpleQueue<T> { private Element<T> head; // front of the linked list private Element<T> tail; // tail of the linked list!!! }! public SLLQueue() { head = null; tail = null; } //... public T front() throws Exception { if (isempty()) throw new Exception("empty queue"); return head.data; } public boolean isempty() { return head == null; } tail head / 13 13

14 Implementing Queues enqueue()! public void enqueue(t item) { if (isempty()) { // first item head = new Element(item); tail = head; } else { tail.next = new Element(item); tail = tail.next; } } dequeue() public T dequeue() throws Exception { if (isempty()) throw new Exception("empty queue"); T item = head.data; head = head.next; return item; } tail head / 14 14

15 More on Queues 15

16 More on Queues There are many implementations of a Queue in Java ArrayBlockingQueue, ConcurrentLinkedQueue, DelayQueue, LinkedBlockingQueue, and LinkedList LinkedList is probably the most common 16 16

17 Blocking Queues A blocking queue is a bounded buffer used for interprocess communication (IPC). Think back to Producer/Consumer we used a message box with 1 item capacity with a queue, we could hold more items! Producer calls enqueue(); sleep if not enough space Consumer calls dequeue(); sleep if nothing to consume. Many ways to implement blocking queues Producer Consumer enqueue() dequeue() 17 17

18 Blocking Queues Why not an array? Array-based queues might seem an odd choice Why not use an ArrayList? enqueue at end, dequeue from front dequeue then requires us to shift all items left (forward) enqueue at front, dequeue at end enqueue then requires us to shift all items right (back) In array, keep track of indices, f and r (front and rear). For efficiency, when dequeuing at front, leave space empty, and remember that the front of the queue is now actually one space back f r 18 18

19 Blocking Queues a f r 19 19

20 Blocking Queues a b f r 20 20

21 Blocking Queues Skip ahead a bit a b c f r 21 21

22 Blocking Queues Now maybe some dequeue() calls are made a b c d e f g h i f r 22 22

23 Blocking Queues Now maybe some dequeue() calls are made b c d e f g h i f r 23 23

24 Blocking Queues Now maybe some dequeue() calls are made c d e f g h i f r 24 24

25 Blocking Queues Now maybe some dequeue() calls are made d e f g h i f r 25 25

26 Blocking Queues Q: What happens when we reach the end of the array and want to enqueue()?! stuck? no! There is likely free space at the front of the array wrap around! e f g h i f r 26 26

27 Blocking Queues If N is the size of the array, wrap by computing r to be: r = (r+1) % N Same logic works for wrapping f. If r < f this indicates that the queue has wrapped around. (do a couple more enqueue() ops going into next slide) e f g h i j r f 27 27

28 Blocking Queues Book shows more details about how to implement this. Notice that: f holds index of the first item in the queue r contains the index of the space beyond the last item in the queue (i.e., where the next new item should be added) k l e f g h i j r f 28 28

29 Blocking Queues Recall: queue is empty if: r == f empty f r But this is actually the same for a full queue! Therefore we always need to leave one empty space OR keep track of the size full k l m n e f g h i j f r 29 29

30 Deques 30

31 Deques Deque deck (double-ended queue) head Add/delete from either end Deque can be used as stack OR queue Implementation: DLL with head/tail pointers All operations are Θ(1) It is possible to use an array w/ wrapping to implement a deque (similar to how it is done w/ a queue) tail / 31 31

csci 210: Data Structures Stacks and Queues

csci 210: Data Structures Stacks and Queues 1 Summary Topics Stacks and Queues as abstract data types ( ADT) Implementations arrays linked lists Analysis and comparison Applications: searching with stacks