CSC 212 – Data Structures Lecture 23: Iterators

Question of the Day Thieves guild states it will sell to members: lock picking kits  $0.67 each 40’ rope  $2.12 each Wire cutters  $4.49 each How much would one pay for 4 lock picking kits, 8 ropes, and 5 wire cutters? Nothing, they’d steal it.

List ADT Accesses all elements in a Collection  Can access all elements without removals  set added to accessors and removals Lists differ how they provide access  IndexList access via integer ranks  PositionList access via relative Position What then defines a List?  Lists are Iterable

Iterators Scans collection’s elements  Start processing with first element…  …then process second element…  …then the third element…  …and so on until processed last element (Obviously) Associated with other ADTs  Provides mean of processing elements  Debug structures like Stack, Queue, & Deque

Using Iterators Loops can rely on using Iterators Improves modularity  Use code with any collection providing iterator Improves reuse  Leave ADT implementation issues to Iterator  Write very generic, simple code

Iterator Interface Important interface defined by Java: import java.util.Iterator;... public class It implements Iterator {…} Interface defines 3 methods: E next(); boolean hasNext() throws NoSuchElementException; void remove() throws UnsupportedOperationException; First call to next() returns first element  Later calls advance through structure

Cursor How Iterators Work Iterators keep cursor with current location Cursor is very implementation specific  Array-based structure – cursor could be index  Linked list-based structure – cursor is Node(?) elements Cursor Ø

Iterable Data Structures Data structure for which have an iterator  Interface defined as java.lang.Iterable Used by List ADT import java.util.Iterator; import java.lang.Iterable;... public interface List extends Iterable { public Iterator iterator(); } Marks any instances will have an Iterator

Why Bother? Java provides nice shortcut for Iterables List myList;... for (Integer i : myList) {... } does the same thing as if we wrote: List myList;... for (Iterator j=myList.iterator(); j.hasNext();){ Integer i = j.next();... }

Limit of Iterator May not want to iterate over elements  Cannot change element stored at locations  Interface provides remove() operation, but…  …that is slow, ugly, and painful (at best) Often define Iterator over positions  In turn, returns each position  With the position, can make changes desired Adds method to PositionList interface: public Iterator > positions();

Your Turn Get back into groups and do activity

Before Next Lecture… Keep up with your reading! Start Week #10 Assignment Work on Programming Assignment #2