CSC 212 Sequences & Iterators

Announcements Midterm in one week  Will cover through chapter 5 of book  Midterm will be open book, open note (but, closed computer) Will not be at office hours tomorrow  Or at next 3 lectures (lucky you!)  Will hold all-day office hours Tuesday

Sequences Sequences combine List and Vector ADTs Also enables conversion between the two  atRank – returns the position object for a given rank  rankOf – given a position, returns its rank public interface Sequence extends List, Vector { public Position atRank(int r) throws public int rankOf(Position p) throws }

Implementing Sequences Like all ADTs, Sequences do not require any specific implementation  We discuss 2 implementations today: Array-based Sequences Doubly linked list-based Sequences

Linked-list base Sequence Node class implements Position interface  Makes position based calls simple  Using/finding rank requires searching through list nodes/positions elements

Array-based Sequence Ranks are easy to use  But, must create class implementing Position Use array in circular manner  Just like with queues 0123 positions elements S lf

Group Work public class Node implements Position { protected Object elem; protected Node prev; protected Node next;... } public class LLSequence implements Sequence { protected Node head; protected int size;... } public class ArrPos implements Position { protected Object elem; protected int rank;... } public class ArrSequence implements Sequence { protected ArrPos arr[]; protected int first; protected int last;... } Write atRank(r) & rankOf(p) for each class:

Iterator ADT Often process data in a collection in order  Position defines current element  But need collection to get next item  Cannot easily determine when end is reached Cannot iterate through data without accessing the list/sequence  Limits modularity & reusability

Iterator Interface public interface Iterator { boolean hasNext(); Object next(); } Must call next() to get initial item  Future calls to next() step through items Should either of these throw an exception?

Getting Iterators Extend the List (and therefore Sequence) ADT with two methods: /** Iterate through the * positions in the list */ public Iterator positions(); /** Iterate through elements in * the list */ public Iterator elements();

Using Iterators Loops can now rely on using Iterators  Improves modularity As long as new collection provides an iterator, can change ADT used for collection at will  Improves reuse Code relies only on iterator existence and not on any specific implementation details

Review Material for the Midterm Topics we have discussed:  Programming in Java  Class hierarchy and inheritance Field hiding, method overriding, method chaining What assignments are legal and when is typecasting needed  Big-Oh notation Constant, Linear, and Logarithmic time  Recursion

Review Material for the Midterm More topics:  Writing/Using Linked Lists  Stacks & Queues Differences between LIFO and FIFO  Vectors, Lists, & Sequences How rank and position works Iterators

Ideas for Reviewing (Re-)Read book sections you do not understand Look over daily quizzes and the solutions  Make sure you understand why the answers are what they are

Ideas for Reviewing Will post practice midterm on course webpage  Will update with more questions on Saturday  Practice contains questions that could be on an actual midterm  Also posting (separate) answer key

Daily Quiz Due Friday at 4PM me a question you think would be good for the midterm and its solution  I usually include questions from students  Provides a good focus for your studying