Problem Of The Day  Two missiles speed directly toward each other  One goes 9,000 miles per hour  Other goes 21,000 miles per hour.  If they start 1,317 miles apart, how far apart are they 1 minute before colliding?

Problem Of The Day  Two missiles speed directly toward each other  One goes 9,000 miles per hour  Other goes 21,000 miles per hour.  If they start 1,317 miles apart, how far apart are they 1 minute before colliding? They are closing at 30,000MPH ; 1 minute of this is: 30,000/60 = 3000/6 = 500 miles!

CSC 212 – Data Structures

I NDEX L IST ≠ array  Extends array concepts, like using indices, but…  I NDEX L IST s do not have constant size  Element’s index changes as data added & removed  ADTs remain completely implementation-free  Using non-tenured faculty still perfectly acceptable

I NDEX L IST.add()  add( i, e ) “shifts” elements to make space  Traverse linked list or move array elements  Can take O(n) time for this shifting  Worst case occurs in both array- & linked list-based  May only take O(1) to add to end of list 012 e n-1 i

I NDEX L IST.remove()  remove( i ) “shifts” elements down to fill hole  Not implementation-specific – only indices are shifted  O(n) time required in the general case  But for specific situations could take only O(1) time  But must consider worst case when computing big-Oh 012 e n-1 i

I NDEX L IST D EQUE Q UEUE S TACK add() addFront() addLast() enqueue()push() get() getFront() getLast() front()top() remove () removeFront() removeLast() dequeue()pop() set() None! Operations To Date

 Collection which we can access all elements  Add element before an existing one  Return the Collection’s 3 rd element  Loop over all elements without removing them  L IST ADTs differ in how they provide access  I NDEX L IST uses indices for absolution positioning  Can only use relative positions in N ODE L IST List ADT

Does Absolute Position Matter?

Relativity Can Be Important

 Based upon a conceptual linked list  No implementation specified  No implementation specified: still an ADT  Relies upon Node s implementing P OSITION ADT  P OSITION s at relative locations in the L IST  first() & last() P OSITION s only absolutes  Traverse using next(p) & prev(p) methods P OSITION L IST with 3 elements N ODE L IST ADT first last

 Abstracts Node idea for use anywhere  Get the P OSITION 's value using element()  Cannot change element using interface's methods  Position is ADT & works with array elements class ArrayPos implements Position { private E[] theArray; private int index; public ArrayPos(E[] arr, int idx) { … } public E element(){ return theArray[index]; } } P OSITION Interface

public interface PositionList extends Collection { Position first(); Position last(); Position next(Position p) throws /* … */ ; Position prev(Position p) throws /* … */ ; E set(Position p, E elem) throws /* … */ ; E remove(Position p) throws /* … */ ; void addFirst(E elem); void addLast(E elem); void addBefore(Position p, E e) throws /*…*/ ; void addAfter(Position p, E e) throws /* …*/ ; } Memorization Limits

 Good news: remove & set the same  Now take P OSITION as parameter and not an index  Whatever it had as its value before call returned  Relative location used to add elements to L IST  addFirst & addLast are relative to all others  e in new P OSITION before p via addBefore  addNext creates new P OSITION after p for e N ODE L IST Methods

 Linked list implements N ODE L IST in most cases  This is not required, but is most reasonable  Few methods free with doubly-linked lists  Use next & prev fields to move between P OSITION s  Call Node. setElement(e) to implement set  Most other methods we've already implemented  Just adding & removing Node s from the linked list Implementing N ODE L IST

 Linked list implements N ODE L IST in most cases  This is not required, but is most reasonable  Few methods free with doubly-linked lists  Use next & prev fields to move between P OSITION s  Call Node. setElement(e) to implement set  Most other methods we've already implemented  Just adding & removing Node s from the linked list But how to do this with P OSITION s? Implementing N ODE L IST

Typecasting Explained

 Used when we implement PositionList  Class allocates instance, so typecasting is safe private Node checkPosition(Position p) throws InvalidPositionException { if ((p == null) || !(p instanceof Node)) { throw new InvalidPositionException(); } return (Node )p; } From Position to Node

Your Turn  Get into your groups and complete activity

For Next Lecture  Before Wednesday’s lecture read GT 6.3  What is an Iterator ? How is it used?  How are Iterable classes related to List s?  Are Iterable & Iterator s related? If so, how?  Programming Assignment #2  Programming Assignment #2 available now good  javadoc is your friend; please write good comments