Problem of the Day  Simplify this equation: (x - a) * (x - b) * (x - c) * … * (x - z)

Problem of the Day  Simplify this equation: (x - a) * (x - b) * (x - c) * … * (x - z) (x - a) * (x - b) * (x - c) * …*(x – x)*…*(x - z)

Problem of the Day  Simplify this equation: (x - a) * (x - b) * (x - c) * … * (x - z) (x - a) * (x - b) * (x - c) * …*(x – x)*…*(x - z) (x - a) * (x - b) * (x - c) * …* 0 *…*(x - z) = 0

CSC 212 – Data Structures

List ADT  Collection which we can access all elements  Add element after an existing one  Collection can remove any element it contains  Loop over all elements without removing them  List ADTs differ in how they provide access  ArrayList’s indices give quick access to specific position  Good at working at relative location with LinkedList

List ADT

Iterators  Provides data access without knowing more  As an ADT, methods independent of data storage  Access elements 1-by-1 by calling next() method  Like Comparator ; does not store data it uses  Iterator class specific to instance holding data  But means that code using Iterator independent  Why is this important?

Iterators  Provides data access without knowing more  As an ADT, methods independent of data storage  Access elements 1-by-1 by calling next() method  Like Comparator ; does not store data it uses  Iterator class specific to instance holding data  But means that code using Iterator independent  Why is this important?  Breaks up code into separate class for each function  Each part replaceable when better approach found

Perfect Example of Perfect Design

Iterator Interface package java.util; public interface Iterator { boolean hasNext(); E next() throws NoSuchElementException; void remove() throws UnsupportedOperationException; }

Iterator Needs a Cursor

Limit of Iterator  Interface provides remove(), but…  …implementing it is a royal pain in the  Support not required by the interface  Instead throw UnsupportedOperationException  Relying on remove() risky, since it is optional  When in doubt, skip it

Limits to Iterator  I TERATOR ’s cursor differs from cursor in Word, PPT  Cannot add or modify data; (mostly) used to read data  Instance goes through data once, cursor only advances  Moves forward item-by-item only, cannot jump around

Limits to Iterator  I TERATOR ’s cursor differs from cursor in Word, PPT  Cannot add or modify data; (mostly) used to read data  Instance goes through data once, cursor only advances  Moves forward item-by-item only, cannot jump around

Limits to Iterator  Iterator ’s methods are very, very limited  Iterator instance has no data; limits possible uses  While iterating, can be tempting to modify data  Requires actual instance, I TERATOR does not store data  If underlying instance changes, results undefined  May work, may crash, or may do both; caveat emptor

Why Should You Care?

Iterable Interface  So simple makes Iterator look complex  Java’s prettiest feature relies on this interface

Iterable Interface  So simple makes Iterator look complex  Java’s prettiest feature relies on this interface

Iterable Interface  So simple makes Iterator look complex  Java’s prettiest feature relies on this interface package java.lang; public interface Iterable { public Iterator iterator(); }

Iterable Usage  Occasionally use Iterator ; Iterable preferable  Many ADT definitions include Iterable in definition  Makes it possible to interchange ADT when processing  Iterable support built-in to Java language  Like Comparable, interface in java.lang package  For-each loops through Iterable data directly  Translates code to use Iterator (without extra code)

For-Each for the Win  Slightly different loop than normal for loop for (type variableName : IterableVar) List idx; OrderedList profFirst; ArrayList whyNot; // Lots of code is here including assignments to our lists for (Integer i : idx ) { … } for (Costume cost : profFirst) { … } for (Scanner stupid : whyNot) { … }

For-Each Rocks The Hizzy Integer findSum(Iterable able) { Integer retVal = 0; for (Integer datum : able) { retVal += datum; } return retVal; }  able could be (almost) ANY C OLLECTION class  A RRAY L IST & L INKED L IST are examples; many will follow

What Type Should Cursor Be?

Implementing Iterator  Very implementation specific issues for cursor  To iterate over an A RRAY L IST, cursor is index  Cursor is node for L INKED L IST ’s I TERATOR  Iterator’s methods always use similar algorithm  General outline identical, since interface defines task  But implementation changes since cursor use differs

What Type Should Cursor Be?

Same Algorithm, But... Algorithm next() if hasNext() then E retVal = value at cursor’s location Advance cursor to refer to next location return retVal else throw new NoSuchElementException endif end

Same Algorithm, But... Algorithm next() if hasNext() then E retVal = value at cursor’s location Advance cursor to refer to next location return retVal else throw new NoSuchElementException endif end

Implementing Iterator (Start) public class ALIterator { private ArrayList theList; private int cursor; public ALIterator(ArrayList list) { theList = list; cursor = 0; } } public class LLIterator { private LinkedList theList; private DNode cursor; public LLIterator(LinkedList list) { theList = list; cursor = } }

Implementing Iterator (Start) public class ALIterator { private ArrayList theList; private int cursor; public ALIterator(ArrayList list) { theList = list; cursor = 0; } } public class LLIterator { private LinkedList theList; private DNode cursor; public LLIterator(LinkedList list) { theList = list; cursor = } }

Implementing the Iterator  Iterator class needs access to List’s internal fields  Implementationless ADT concept must be violated  Code specific to implementation and not reusable  All CSC212 design concepts get thrown out window

Implementing the Iterator  Iterator class needs access to List’s internal fields  Implementationless ADT concept must be violated  Code specific to implementation and not reusable  All CSC212 design concepts get thrown out window

Implementing for ArrayList L IST

Implementing for ArrayList L IST ALIterator cursor  0 theList

Implementing for ArrayList L IST ALIterator cursor  1 theList

Implementing for ArrayList L IST ALIterator cursor  2 theList

L IST Implementing for ArrayList ALIterator cursor  3 theList

L IST Implementing for ArrayList ALIterator

L IST Implementing for ArrayList ALIterator cursor  4 theList

Implementing for LinkedList  cursor is next node in LinkedList  Need to know class specifics to use node types  No methods in LinkedList provide required info head tail LLIterator cursor theList

Implementing for LinkedList  cursor is next node in LinkedList  Need to know class specifics to use node types  No methods in LinkedList provide required info head tail LLIterator cursor theList

Implementing for LinkedList  cursor is next node in LinkedList  Need to know class specifics to use node types  No methods in LinkedList provide required info head tail LLIterator cursor theList

Implementing for LinkedList  cursor is next node in LinkedList  Need to know class specifics to use node types  No methods in LinkedList provide required info head tail LLIterator cursor theList

Implementing for LinkedList  cursor is next node in LinkedList  Need to know class specifics to use node types  No methods in LinkedList provide required info head tail LLIterator cursor theList

Comparing Implementations ArrayList Iterator LinkedList Iterator  Check if cursor valid: cursor != theList.size ()  Get element at cursor : theList.get ( cursor ) or array[cursor]  Advancing cursor : cursor += 1  Check if cursor valid: cursor != null  Get element at cursor : cursor.getElement ()  Advancing cursor : cursor = cursor. getNext ()

Your Turn  Get into your groups and complete activity

For Next Lecture  Read 8.1 – 8.4 before Friday’s lecture  What is recursion & how does it work?  What happens to locals if method called again?  Why did we spend so much time on tracing methods?  Week #11 assignment available on Angel  No class on Monday  No class on Monday; I will be out of town  As a result, I am delaying quiz until Wednesday