Geoff Holmes and Bernhard Pfahringer COMP206-08S General Programming 2.

Geoff Holmes and Bernhard Pfahringer

COMP206-08SGeneral

Programming 2

Department of Computer Science 2

Lectures

- Today - Collections Continued- Sets

- Trees- Hashing


So far Arrays Arraylists LinkedList

Stacks Queues

Items are all placed at a position in the data structure LinkedList has an iterator with an “add” method


Sets A set is a collection of items that does not permit

duplicates Adding an existing item has no effect Removing an item not in the set has no effect

Java has a Set interface with two implementations HashSet (uses hashing to store items) TreeSet (uses a tree to store items – Sorted)


Exampleimport java.util.HashSet;import java.util.Iterator;import java.util.Set;

public class SetTest {public static void main(String[] args){

Set<String> names = new HashSet<String> ();

names.add(“Harry”); // add lots more


Continued

names.add("Jane");

names.add("Tom");

names.add("Richard");

names.add("Tom");

names.add("Barbara");

print(names);

}


Printingprivate static void print(Set<String> s){

System.out.print("{ ");

for (String x : s){

System.out.print(x);System.out.print(" ");

}

System.out.println("}");}


TreeSet

Changing only two lines of code lets us have a Tree behind the scenes

Note the order of items!


Maps Similar idea can be developed to make

associations Two sets are kept in association Set of keys -> Set of Values Two keys may refer to same value Names and phone numbers Java has a Map Interface Two maps are implemented

HashMap TreeMap


Test program for maps

import java.util.HashMap;

import java.util.Iterator;

import java.util.Set;

import java.util.Map;

public class maps {

public static void main(String[] args) {


Continued

Map<String, String> yellowpages = new HashMap<String, String> ();

yellowpages.put("Harry", "123456");

yellowpages.put("Jane", "012345");

yellowpages.put("Tom", "123458");

yellowpages.put("Richard", "123457");

yellowpages.put("Barbara", "123454");

yellowpages.put("Barbara", "987654"); // assignment


Printing out maps

Set<String> keySet = yellowpages.keySet();for (String key : keySet){String value = yellowpages.get(key); // value assoc with keySystem.out.println(key + "->" + value);}}

}


What is hashing? Technique for finding elements without making a

linear search through all elements Uses a hash function is a function that computes

an integer value (called a hash code) from an object (different objects should have different hash codes)

Object class has a hashCode method int h = x.hashCode();


Hash functions Should avoid collisions (two or more different

objects with the same hash code) If you have your own objects you write:

public int hashCode( ) When adding x.equals(y) => x.hashCode() ==

y.hashCode (avoid duplicates) Eg for a circle so that circles of different radii are

stored separately. Forgetting to implement hashCode means the one

assoc with Object is used – not a good idea.


Example Consider

Set<Circle> circles = new HashSet<Circle>();

circles.add(new Circle(5));

if (circles.contains(new Circle(5))

System.out.println(“Circle of radius 5 exists”);


Binary Search Trees Finding things is O(log N) with binary search With arrays insertion and deletion are O(N) Binary search trees are fast at everything Nodes of a binary search tree have two children Data values of all descendants to the left of any

node are less than the data value stored at that node (similarly right and greater)


Tree of names

For the list {Adam, Eve, Harry, Jane, Richard, Tom}

Jane (root) – children Eve and Richard Eve – children Adam and Harry Richard – children Tom and null Adam, Harry and Tom are leaf nodes (both

children null)


Implementation overviewpublic class BinarySearchTree {// need reference to root (like first in LL)private node root;// inner class for nodeprivate class Node { public void addNode(Node newNode) { } public Comparable data; public Node left; public Node right;}}


Tree Operations Tree Traversal (sorted order)

Print left subtree Print data Print right subtree

Called Inorder traversal Preorder traversal (root, left, right) Postorder traversal (left, right, root) (3+4*5)


Trees and Recursion// Note this is a method of the Node inner classpublic void printNodes( ){

if (left != null) left.printNodes( );System.out.println(data);if (right != null) right.printNodes( );

}

// Method of BinarySearchTreepublic void print( ){

if (root != null) root.printNodes( );}


HashSet or TreeSet

If you have a good hash function for your objects then performance will be better than trees

Hash sets are at the mercy of the hash function Iterators visit TreeSet data in sorted order (hash

order is random) TreeSet requires objects to implement

Comparable (or Comparator)


How to choose – ask questions How are elements accessed?

Doesn’t matter By key (eg a/c #) (MAP) Integer index (ARRAYLIST)

Does data order matter? Doesn’t matter Must be sorted (TREESET) Store in insert order (LL,

ARRAYLIST)


More questions What operations need to be fast?

Doesn’t matter (small dataset) Add/remove (LL) Search (SET)

(DM = ARRAYLIST) Hash or Tree?

Strings should be hashed Own class need to check hashCode and equals Trees may need a Comparator

Geoff Holmes and Bernhard Pfahringer COMP206-08S General Programming 2.

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