unit 11 1 Unit 11: Data Structures H We explore some simple techniques for organizing and managing information H This unit focuses on: Abstract Data Types (ADTs) queues stacks dynamic structures linked lists basic programming concepts object oriented programming topics in computer science syllabus

unit 11 2 Abstract Data Types H An abstract data type (ADT) is an organized collection of information and a set of operations used to manage that information H The set of operations define the interface to the ADT H As long as the ADT accurately fulfills the promises of the interface, it doesn't really matter how the ADT is implemented

unit 11 3 Queues H A queue is similar to a list but adds items only to the end of the list and removes them from the front H It is called a FIFO data structure: First-In, First-Out H Analogy: a line of people at a bank teller’s window enqueue dequeue

unit 11 4 Interface for ADT Queue Queue public interface Queue { public int size(); public boolean isEmpty(); public Object front() throws QueueEmptyException; public void enqueue(Object o); public Object dequeue() throws QueueEmptyException; }

unit 11 5 Stacks H A stack ADT is also linear, like a list or queue H Items are added and removed from only one end of a stack H It is therefore LIFO: Last-In, First-Out H Analogy: a stack of plates

unit 11 6 Stacks H Stacks are often drawn vertically: poppush

unit 11 7 Interface for ADT Stack Stack public interface Stack { public int size(); public boolean isEmpty(); public Object peek() throws StackEmptyException; public void push(Object o); public Object pop() throws StackEmptyException; }

unit 11 8 Push Implementation top: 5 elements

unit 11 9 Push Implementation top: 5 elements obj if (isFull()) throw Exception;

unit Push Implementation top: 5 elements obj if (isFull()) throw Exception; elements[top] = obj;

unit Push Implementation top: 6 elements if (isFull()) throw Exception; elements[top] = obj; top++;

unit Queue Implementation - Musical Chairs H Use an array of size n, and a rear index H Elements are always inserted to rear H Elements are removed from front, all other elements move as in musical chairs

unit Circular Queue - Insert frontrear

unit Circular Queue - Insert frontrear

unit Circular Queue - Insert frontrear

unit Circular Queue - Insert frontrear

unit Circular Queue - Remove frontrear

unit Circular Queue - Remove frontrear

unit Circular Queue - Insert frontrear

unit Circular Queue - Insert frontrear

unit Circular Queue - Insert frontrear

unit Circular Queue - Insert frontrear

unit Circular Queue - Insert frontrear

unit Circular Queue - Remove frontrear

unit Circular Queue - Insert frontrear

unit Circular Queue - Insert frontrear what will happen if we add one more element?

unit Circular Queue - Operations H front and rear are always incremented modulu n H queue is full if (rear + 1) % n = front H queue is empty if rear = front H in an array of size n, only n-1 cells can be utilized

unit Static vs. Dynamic Structures  A static data structure has a fixed size (this meaning is different than those associated with the static modifier) H Arrays are static; once you define the number of elements it can hold, it doesn’t change H A dynamic data structure grows and shrinks as required by the information it contains H Example of dynamic data structures: linked lists

unit Object References H Object reference is a variable that stores the address of an object H A reference can also be called a pointer H They are often depicted graphically: student John Smith

unit References as Links H Object references can be used to create links between objects  Suppose a Student class contained a reference to another Student object John Smith Jane Jones

unit References as Links H References can be used to create a variety of linked structures, such as a linked list: studentList element next } node

unit “Generic” Node Class Node class Node { private Object element; private Node next; // two overloaded constructors public Node() { this(null,null); } public Node(Object element, Node next) { this.element = element; this.next = next; } // functionality methods void setElement(Object element) { //...} void setNext(Node next) { //...} Object getElement() { //...} Node getNext() { //...} }

unit A Linked Node Chain head next element null SingleLinkedList

Stack Implementation: linked list size: 0 top: null

size: 0 top: push(Object obj) obj

size: 0 top: n push(Object obj) { Node n = new Node(); obj push(Object obj)

size: 0 top: n push(Object obj) { Node n = new Node(); n.setElement(obj); obj

size: 0 top: n push(Object obj) { Node n = new Node(); n.setElement(obj); n.setNext(top); obj

size: 0 top: n push(Object obj) { Node n = new Node(); n.setElement(obj); n.setNext(top); top = n; obj

size: 1 top: n push(Object obj) { Node n = new Node(); n.setElement(obj); n.setNext(top); top = n; size++; } obj

size: 1 top:

size: 1 top: obj push(Object obj)

size: 1 top: n obj push(Object obj) { Node n = new Node();

size: 1 top: n obj push(Object obj) { Node n = new Node(); n.setElement(obj);

size: 1 top: n obj push(Object obj) { Node n = new Node(); n.setElement(obj); n.setNext(top);

size: 1 top: n obj push(Object obj) { Node n = new Node(); n.setElement(obj); n.setNext(top); top = n;

push(Object obj) { Node n = new Node(); n.setElement(obj); n.setNext(top); top = n; size++; } size: 2 top: n obj

size: 2 top:

size: 2 top: Object pop() { if (isEmpty()) throw Exception;

Object pop() { if (isEmpty()) throw Exception; Object temp; size: 2 top: temp

Object pop() { if (isEmpty()) throw Exception; Object temp; temp = top.getElement(); size: 2 top: temp

Object pop() { if (isEmpty()) throw Exception; Object temp; temp = top.getElement(); top = top.getNext(); size: 2 top: temp

Object pop() { if (isEmpty()) throw Exception; Object temp; temp = top.getElement(); top = top.getNext(); size--; size: 1 top: temp

Object pop() { if (isEmpty()) throw Exception; Object temp; temp = top.getElement(); top = top.getNext(); size--; return temp; } size: 1 top: temp

size: 1 top: temp

size: 1 top: temp

size: 1 top: temp

interface Stack ArrayStack implements Stack ArrayStack(int size) push() throws StackFullException pop() isFull() size()... SinglyLinkedStack implements Stack push() pop() size()... push() pop() size()...

unit Using a Stack: Balanced Strings H A single character string which is not an opening or a closing character is a balanced string H If the strings s 1 and s 2 are balanced, then s 1 + s 2 (i.e., the concatenation of s 1 and s 2 ) is a balanced string H If a string s is balanced, and c op,c cl is a matching pair of opening and closing characters, then c op + s + c cl is a balanced string

unit Balanced String Verification Algorithm boolean isBalanced(String str) for each character c in str[0]... str[length-1]: if c is an opening character then push it onto the stack; if c is a closing character then if the stack is empty, return false; else // stack not empty pop the topmost character t from the stack; if t and c do not match return false otherwise // c is not a closing character do nothing (skip the character). if the stack is empty, return true, else return false.

unit Using a Queue: ComputationSimulation  public abstract void computeOneStep() - Performs a single computation step  public abstract boolean isCompleted() - Returns true if the computation is completed, false otherwise  public Object getResult() - Returns the result of this computation, null if computation is not completed H …

unit An Iteration of a Time Sharing Machine computation queue

unit computation queue

unit computation queue computeOneStep() K times

unit computation queue isCompleted() ?

unit computation queue No ?

unit computation queue Yes ? getResult()

unit Collection Classes H The Java 2 platform contains a Collections API H This group of classes represent various data structures used to store and manage objects  Their underlying implementation is implied in the class names, such as ArrayList and LinkedList  Several interfaces are used to define operations on the collections, such as List, Set, SortedSet, Map, and SortedMap

unit When do we use dynamic structures? static H insert and remove items from unsorted lists H examine elements of lists dynamic H insert and remove items from sorted lists H length of list can change in orders of magnitude