Stack as an ADT. COMP 182: Data Structures and Program Design Lecture Slides 8. Stack as an Idea

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1 Stack as an ADT COMP 182: Data Structures and Program Design Lecture Slides 8 John Noga CSU Northridge April 9, 200 public interface Stack { public boolean push(ourthing ot); public OurThing (); public boolean isempty(); public OurThing peek(); Given an implementation of this interface you can use it without knowing how it is implemented. Allows division of labor while programming. No problems caused by changing implementation. Stack as an Idea Stack Implemented with Vector (Arrays) Idea: bottom of stack pos 0, top size-1 push push push 3 push 2 push 12 3 Can t take from middle/bottom Can t put to middle/bottom Restrictive, but intentional public class StkUsingArray implements Stack { OurThing[] data; int size, capacity; public StkUsingArray() { data = new OurThing[10]; size = 0; capacity = 10; public OurThing () { size--; return data[size];

2 Stack Implemented (Array) cont. public OurThing peek() { return data[size-1]; public boolean isempty() { if (size == 0) else return false; public boolean push(ourthing ot) { if (size == capacity) // make temp, copy over, // data = tmp, update capacity data[size] = ot; size++; Stack Implemented (LL) cont. public OurThing () { OurThing orthg = head.getourthing(); head = head.getnext(); return orthg; public boolean isempty() { if (head == null) else return false; public OurThing peek() { OurThing orthg = head.getourthing(); return orthg; Stack implemented with a Linked List Idea: top of stack is head, bottom tail public class StkUsingLL implements Stack { Node head; public StkUsingLL() { head = null; public boolean push(ourthing ot) { Node nd = new Node(ot); nd.setnext(head); head = nd; Stack Application 1 - Valid Bracketing Input: Java Program Output: Is the bracketing correct? Question with blank: Is the bracketing in.java correct? Have a program and you want to check if the symbols [, {, (, ),, ] match up properly. [ ( ) ( ) ] { ( ) [ ( ) ] is improper no match for { [ { ] is improper (ranges intersect improperly) [ { ] is improper (close before open)

3 Idea of Solution Pseudo-Code Solution ([]{[] push ( push [ push { push [ [ ( ( No match for ( { ( ( [ { ( { ( ( public boolean propbrackets(string st) { while ( st has more characters ) { get next character; if ( character is "[", "(", or "{" ) thestk.push(character); else if ( character is "]", ")", or "" ) { if (thestk.isempty()) return false; x = thestk.(character); if (x is not matching opener) return false; if (thestk.isempty()) else return false; Stack Application 2 - Input with backspacing Imagine an editor which allows typing, backspacing, but nothing else. How can we store the input? Check that we don t delete too often? public boolean handleinput() { while( more typing ) { get a char; if (char == backspace) { if (thestk.isempty()) return false; thestk.(); else thestk.push(char); Stack Application 3 - Method Calls How can a program have methods that call (other) methods? How does it know where to return to? How does it remember what values the variable had? How does it handle local variables? Every time method called frame created. Containing: which method which step in method all local variables (with values) Placed on stack. When finished removed from stack.

4 Stacks Plus Recursion What does unknown accomplish? Trace 1 public boolean unknown(int i) { if (thestk.isempty()) return false; if (i==0) { thestk.(); i= i=3 i=2 i=1 ot= ot= ot= ot= value= value= value= value= True True True True True i=0 OurThing ot = thestk.(); boolean value = unknown(i-1); thestack.push(ot); return value; What does unknown accomplish? Trace 2 Queues i=3 ot= i=2 ot= i=1 ot= value= True True value= value= True True i=0 Similar to stack, but IO instead of LIO Queue = line (join at back, leave from front) Operations: enqueue (join the queue) dequeue (remove item from queue) isempty (check empty) first? (not quite standard)

5 Queue as an ADT Queue Example public interface Queue { public boolean enqueue(ourthing ot); public OurThing dequeue(); public boolean isempty(); public OurThing first(); enqueue enqueue enqueue enqueue 5 5 Useful for simulations/process scheduling. enqueue dequeue 5 1 enqueue How to implement? Priority Queue Linked List w/ head & tail pointer most natural Circular Linked List w/ tail pointer little complicated Vector (array) with shifting inefficient Vector (array) without shifting? (?sneaky?) Queue except enqueue based on priority (higher priority more important?) (lower more important? first priority, second priority...) public class MyPQ extends MyQ { // overrides enqueue method (explain) public boolean enqueue(ourthing ot) { // insert based upon priority // blah, blah, blah //everything else inherited (explain) Mention private/protected.

6 Priority Queue Example enqueue enqueue enqueue enqueue 5 enqueue 1 dequeue enqueue Applications Enqueue for MyPQ Sets (in a linked list implementation) public boolean enqueue(ourthing ot) { if ( head == null head.getot().getprty() < ot.getprty() ) { head = new Node(ot, head); Node nd = head; while(nd.getnext()!=null && nd.getnext().getprty() > ot.getprty() ) nd = nd.getnext(); nd.setnext(new Node(ot,nd.getNext()); Bookstore here (priority = job) Your homework (priority = due date) Process scheduling (priority = nice value) Normal queue (priority = arrival time) most generic of all the data structures a set is just a collection of items no organization except no duplicates (if you allow duplicates called multi-set/bag) no (natural) notion of ith item more advanced ops

7 Set as an Idea Operations b e d a c S = {a,b,c,d,e a, b, c, d, e are instances of OurThing A is a set (containing 5 elements - instances of OurThing) addelement = put new item into set deleteelement = remove item from set iselement = check if element is in the set union = join two sets together intersection = limit to elements in both setdifference = items in one, but not other size = number of items in the set Operations By Picture Set as an ADT S e a b c d S T = {a,b,c,d,e,f,g S T = {a,c S T = {b,d,e T S = {f,g S = 5 f g T S = {a,b,c,d,e T={a,c,f,g public interface Set { public boolean addelement(ourthing ot); public boolean deleteelement(ourthing ot); public boolean iselement(ourthing ot); public Set union(set set); public Set intersection(set set); public Set setdifference(set set); public int size();

8 Implementations? Vector Ordered Vector Linked List Other? Time complexity of operations? Proving Set Equivalence Claim: A B = A (A B). Proof: x A B x A & x B x A & x / A B x A (A B). x A (A B) x A & x / A B x A & (x / A or x B) x A & x B x A B.

Insertions and removals follow the Fist-In First-Out rule: Insertions: at the rear of the queue Removals: at the front of the queue

Queues CSE 2011 Fall 2009 9/28/2009 7:56 AM 1 Queues: FIFO Insertions and removals follow the Fist-In First-Out rule: Insertions: at the rear of the queue Removals: at the front of the queue Applications,