COMP206-08S General Programming 2 Geoff Holmes and Bernhard

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Transcript COMP206-08S General Programming 2 Geoff Holmes and Bernhard

COMP206-08S General Programming 2

Geoff Holmes and Bernhard Pfahringer

Lectures

Today Collections Continued Sets

Trees Hashing

Department of Computer Science 2

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 Department of Computer Science 3

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) Department of Computer Science 4

Example

import

java.util.HashSet;

import

java.util.Iterator;

import java.util.Set; public class

SetTest { {

public static void

main(String[] args) Set names =

new

HashSet (); names.add(

“Harry”

); // add lots more Department of Computer Science 5

Continued

} names.add("Jane"); names.add("Tom"); names.add("Richard"); names.add("Tom"); names.add("Barbara");

print

(names); Department of Computer Science 6

Printing

private static void print(Set s) { System.out.print("{ "); for (String x : s) { System.out.print(x); System.out.print(" "); } } System.out.println("}");

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TreeSet

 Changing only two lines of code lets us have a Tree behind the scenes  Note the order of items!

Department of Computer Science 8

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 Department of Computer Science 9

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) { Department of Computer Science 10

Continued

Map yellowpages =

new

HashMap (); 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 Department of Computer Science 11

}

Printing out maps

Set keySet = yellowpages.keySet(); {

for

(String key : keySet) } String value = yellowpages.get(key); // value assoc with key } System.

out

.println(key + "->" + value); Department of Computer Science 12

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(); Department of Computer Science 13

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.

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Example

 Consider Set circles = new HashSet(); circles.add(new Circle(5)); if (circles.contains(new Circle(5)) System.out.println(“Circle of radius 5 exists”); Department of Computer Science 15

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) Department of Computer Science 16

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) Department of Computer Science 17

Implementation overview

} } public class BinarySearchTree { // need reference to root (like first in LL) private node root; // inner class for node private class Node { public void addNode(Node newNode) { } public Comparable data; public Node left; public Node right; Department of Computer Science 18

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) Department of Computer Science 19

Trees and Recursion

} { // Note this is a method of the Node inner class public void printNodes( ) if (left != null) left.printNodes( ); System.out.println(data); if (right != null) right.printNodes( ); } { // Method of BinarySearchTree public void print( ) if (root != null) root.printNodes( ); Department of Computer Science 20

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) Department of Computer Science 21

How to choose – ask questions

 How are elements accessed?

 Doesn’t matter  By key (eg a/c #)  Integer index  Does data order matter?

 Doesn’t matter  Must be sorted  Store in insert order (MAP) (ARRAYLIST) (TREESET) (LL, ARRAYLIST) Department of Computer Science 22

COMP206-08S General Programming 2 Geoff Holmes and Bernhard

Transcript COMP206-08S General Programming 2 Geoff Holmes and Bernhard