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)

Problem of the Day


(x - a) * (x - b) * (x - c) * … * (x - z)

(x - a) * (x - b) * (x - c) * …*(x – x)*…*(x - z)

Problem of the Day


(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

LECTURE 30:ITERATOR AND ITERABLE

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

Collection which we can access all elements Add element before an existing one Collection can remove any element it





LinkedList

List ADT






LinkedList

List ADT






LinkedList

Must violate ADT to access List’s elements

List ADT





specific Good at working at relative location with

LinkedList

Must violate ADT to access List’s elements

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

Example of Perfect Design

Iterator Interface

package java.util;public interface Iterator<E> {

boolean hasNext();

E next() throws NoSuchElementException;

void remove() throws UnsupportedOperationException;

}

Iterator Needs a Cursor

Uses cursor like the one in Eclipse, Word, PPT, … Like in Eclipse: cursor marks location to be

used Data EXTERNAL to ITERATOR, like

normal cursor When run, application uses and moves

cursor around Cursor not part of document and not found

in file Only matters when editing: cursor useful

only with data

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

ITERATOR’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 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, ITERATOR 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

Iterable Interface

So simple makes Iterator look complex

Java’s prettiest feature relies on this interface

Iterable Interface



Iterable Interface



package java.lang;

public interface Iterable<E> { public Iterator<E> 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 loopfor (type variableName : IterableVar)

List<Integer> idx;OrderedList<Costume> profFirst;ArrayList<Scanner> 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<Integer> able) {Integer retVal = 0; for (Integer datum : able) { retVal += datum; }return retVal;

}

able could be (almost) ANY COLLECTION class ARRAYLIST & LINKEDLIST are examples;

many will follow

What Type Should Cursor Be?


Implementing Iterator

Very implementation specific issues for cursor To iterate over an ARRAYLIST, cursor is index Cursor is node for LINKEDLIST’s ITERATOR

Iterator’s methods always use similar algorithm General outline identical, since interface

defines task But implementation changes since cursor use

differs


Same Algorithm, But...

Algorithm next()if hasNext() then E retVal = value at cursor’s location Advance cursor to refer to next location return retValelse throw new NoSuchElementExceptionendif

end

Same Algorithm, But...

Algorithm next()if hasNext() then E retVal = value at cursor’s location Advance cursor to refer to next location return retValelse throw new NoSuchElementExceptionendif

end

Implementing Iterator (Start)public class ALIterator<E> { private ArrayList<E> theList; private int cursor; public ALIterator(ArrayList<E> list) { theList = list; cursor = 0; }}

public class LLIterator<E> { private LinkedList<E> theList; private DNode<E> cursor; public LLIterator(LinkedList<E> list) { theList = list; cursor = }}

Implementing Iterator (Start)public class ALIterator<E> { private ArrayList<E> theList; private int cursor; public ALIterator(ArrayList<E> list) { theList = list; cursor = 0; }}

public class LLIterator<E> { private LinkedList<E> theList; private DNode<E> cursor; public LLIterator(LinkedList<E> 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

Cursor after iterated data to make tasks simpler When next() called, value at index of cursor

returned Very implementation specific issues for

cursor To iterate over an ARRAYLIST, cursor is

index (int) Cursor is NODE for LINKEDLIST’s ITERATOR

LIST






LIST

ALIteratorcursor 0theList






LIST







LIST


LIST







LIST






ALIteratorcursor ?theList

LIST







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




required info

head tail





required info

head tail





required info

head tail





required info

head tail


Comparing Implementations

ArrayList Iterator LinkedList Iterator

Check if cursor valid:

cursor!=theList.size()

Get element at cursor:

theList.get(cursor) orarray[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; I will be out of town As a result, I am delaying quiz until

Wednesday

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

Documents