Transcript Sets - Wiley

CHAPTER 20
Advanced Data Structures
CHAPTER GOALS
• To learn about set and map data types
• To understand the implementation of hash
tables
• To be able to program hash functions
• To learn about binary trees
• To be able to use tree sets and tree maps
Sets
• A set is an unordered collection of
distinct elements
• Sets don't have duplicates
• Trying to add a duplicate has no effect
Fundamental Operations on a Set
• Adding an element
• Removing an element
• Containment testing (Does the set contain a
given object?)
• Listing all elements ( in arbitrary order)
Sets
• Classes that realize the set interface
o
o
HashSet
TreeSet
Iterator
• Use an iterator to visit all elements in a set
• A set iterator does not visit the elements in
the order in which they were inserted
• An element can not be added to a set at an
iterator position
• A set element can be removed at an iterator
position
Set Classes and Interface in the
Standard Library
Code for Creating and Using a
HashSet
• //Creating a hash set
Set names = new HashSet();
• //Adding an element
names.add("Romeo");
• //Removing an element
names.remove("Juliet");
• //Is element in set if
(names.contains("Juliet") {. . .}
Listing All Elements with an Iterator
Iterator iter = names.iterator();
while (iter.hasNext())
{
String name = (String)
iter.next();
//do something here
}
File SetTest.java
01: import java.util.Iterator;
02: import java.util.Set;
03:
04: /**
05: This program tests the hash set class.
06: */
07: public class SetTest
08: {
09: public static void main(String[] args)
10: {
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Set names = new HashSet(101); // 101 is a prime
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names.add("Sue");
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names.add("Harry");
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names.add("Nina");
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names.add("Susannah");
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names.add("Larry");
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names.add("Eve");
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names.add("Sarah");
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names.add("Adam");
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names.add("Tony");
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names.add("Katherine");
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names.add("Juliet");
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names.add("Romeo");
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names.remove("Romeo");
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names.remove("George");
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Iterator iter = names.iterator();
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while (iter.hasNext())
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System.out.println(iter.next());
31: }
32: }
Maps
• A map keeps associations between key and
value objects
• Every key in a map has a unique value.
• A value may be associated with several keys
Maps
• Classes that realize the Map interface
o HashMap
o TreeMap
An Example of a Map
Code for Creating and Using a
HashMap
• //Creating a HashMap
Map favoriteColors = new HashMap();
• //Adding an association
favoriteColors.put("Juliet", Color.pink);
• //Changing an existing association
favoriteColor.put("Juliet",Color.red);
Code for Creating and Using a
HashMap
• //Getting the value associated with a key
Color julietsFavoriteColor =
favoriteColors.get("Juliet");
• //Removing a key and its associated value
favoriteColors.remove("Juliet");
Printing key/value Pairs
Set keySet = favoriteColors.keySet();
Iterator iter = keySet.iterator();
while (iter.hasNext())
{
Object key = iter.next();
Object value = favoriteColors.getkey();
System.out.println(key + "->" + value);
}
Map Classes and Interface in the
Standard Library
File MapTest.java
01: import java.awt.Color;
02: import java.util.HashMap;
03: import java.util.Iterator;
04: import java.util.Map;
05: import java.util.Set;
06:
07: /**
08: This program tests a map that maps names to colors.
09: */
10: public class MapTest
11: {
12: public static void main(String[] args)
13: {
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Map favoriteColors = new HashMap();
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favoriteColors.put("Juliet", Color.pink);
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favoriteColors.put("Romeo", Color.green);
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favoriteColors.put("Adam", Color.blue);
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favoriteColors.put("Eve", Color.pink);
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print(favoriteColors);
21: }
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23: /**
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Prints the contents of a map
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@param m a map
26: */
27: private static void print(Map m)
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Set keySet = m.keySet();
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Iterator iter = keySet.iterator();
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while (iter.hasNext())
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{
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Object key = iter.next();
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Object value = m.get(key);
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System.out.println(key + "->" + value);
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}
37: }
38: }
Hash Tables
• Hashing can be used to find elements in a data
structure quickly without making a linear
search
• A hash function computes an integer value
(called the hash code) from an object
• A good hash function minimizes collisions,
identical hash codes for different objects
• To compute the hash code of object x: int h =
x.hashcode()
Sample Strings and Their Hash
Codes
Simplistic Implementation of a
Hash Table
• To implement
o Generate hash codes for objects
o Make an array
o Insert each object at the location of its hash
code
• To test if an object is contained in the set
o Compute its hash code
o Check if the array position with that hash
code is already occupied
Simplistic Implementation of a
Hash Table
Problems with Simplistic
Implementation
• It is not possible to allocate an array
that is large enough to hold all
possible integer index positions
• It is possible for two different objects
to have the same hash code
Solutions
• Pick a reasonable array size and reduce the
hash codes to fall inside the array
int h = x.hashCode();
if (h < 0) h = -h;
h = h % size;
• When elements have the same hash code:
o use a link sequence to store multiple
objects in the same array position
o These link sequences are called buckets
Hash Table with Linked Lists to Store
Elements with Same Hash Code
Algorithm for Finding an Object x in
a Hash Table
• Get the index h into the hash table
o Compute the hash code.
o Reduce it modulo the table size
• Iterate through the elements of the bucket at
position h
• For each element of the bucket, check whether it is
equal to x
• If a match is found among the elements of that
bucket, then x is in the set.
• Otherwise, x is not in the set
Hash Tables
• A hash table can be implemented as an array of
buckets
• Buckets are sequences of links that hold elements
with the same hash code
• The table size should be a prime number larger
than the expected number of elements
• If there are few collisions, then adding, locating,
and removing hash table elements takes constant
time
• Big-Oh notation: O(1)
File HashSet.java
001: import java.util.Iterator;
002: import java.util.AbstractSet;
003:
004: /**
005: A hash set stores an unordered collection of objects, using
006: a hash table.
007: */
008: public class HashSet extends AbstractSet
009: {
010: /**
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Constructs a hash table.
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@param bucketsLength the length of the buckets array
013: */
014: public HashSet(int bucketsLength)
015: {
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buckets = new Link[bucketsLength];
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size = 0;
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}
/**
Tests for set membership.
@param x an object
@return true if x is an element of this set
*/
public boolean contains(Object x)
{
int h = x.hashCode();
if (h < 0) h = -h;
h = h % buckets.length;
Link current = buckets[h];
while (current != null)
{
if (current.data.equals(x)) return true;
current = current.next;
}
return false;
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}
/**
Adds an element to this set.
@param x an object
@return true if x is a new object, false if x was
already in the set
*/
public boolean add(Object x)
{
int h = x.hashCode();
if (h < 0) h = -h;
h = h % buckets.length;
Link current = buckets[h];
while (current != null)
{
if (current.data.equals(x))
return false; // already in the set
current = current.next;
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}
Link newLink = new Link();
newLink.data = x;
newLink.next = buckets[h];
buckets[h] = newLink;
size++;
return true;
}
/**
Removes an object from this set.
@param x an object
@return true if x was removed from this set, false
if x was not an element of this set
*/
public boolean remove(Object x)
{
int h = x.hashCode();
if (h < 0) h = -h;
h = h % buckets.length;
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Link current = buckets[h];
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Link previous = null;
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while (current != null)
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{
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if (current.data.equals(x))
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{
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if (previous == null) buckets[h] = current.next;
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else previous.next = current.next;
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size--;
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return true;
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}
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previous = current;
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current = current.next;
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}
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return false;
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Returns an iterator that traverses the elements of this set.
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@param a hash set iterator
*/
public Iterator iterator()
{
return new HashSetIterator();
}
/**
Gets the number of elements in this set.
@return the number of elements
*/
public int size()
{
return size;
}
private Link[] buckets;
private int size;
private class Link
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{
public Object data;
public Link next;
}
private class HashSetIterator implements Iterator
{
/**
Constructs a hash set iterator that points to the
first element of the hash set.
*/
public HashSetIterator()
{
bucket = 0;
previous = null;
previousBucket = buckets.length;
// set bucket to the index of the first nonempty bucket
while (bucket < buckets.length && buckets[bucket] == null)
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bucket++;
if (bucket < buckets.length) current = buckets[bucket];
else current = null;
}
public boolean hasNext()
{
return current != null;
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public Object next()
{
Object r = current.data;
if (current.next == null) // move to next bucket
{
previousBucket = bucket;
bucket++;
while (bucket < buckets.length && buckets[bucket] == null)
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bucket++;
if (bucket < buckets.length)
current = buckets[bucket];
else
current = null;
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else // move to next element in bucket
{
previous = current;
current = current.next;
}
return r;
}
public void remove()
{
if (previous != null)
previous.next = previous.next.next;
else if (previousBucket < buckets.length)
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buckets[previousBucket] = null;
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else
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throw new IllegalStateException();
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previous = null;
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previousBucket = buckets.length;
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}
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private int bucket;
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private Link current;
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private int previousBucket;
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private Link previous;
189: }
190: }
File SetTest.java
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01: import java.util.HashSet;
02: import java.util.Iterator;
03: import java.util.Set;
04: import javax.swing.JOptionPane;
05:
06: /**
07: This program demonstrates a set of strings. The user
08: can add and remove strings.
09: */
10: public class SetTest
11: {
12: public static void main(String[] args)
13: {
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Set names = new HashSet();
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boolean done = false;
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while (!done)
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{
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String input = JOptionPane.showInputDialog("Add name, Cancel when done");
if (input == null)
done = true;
else
{
names.add(input);
print(names);
}
}
done = false;
while (!done)
{
String input = JOptionPane.showInputDialog("Remove name, Cancel when done");
if (input == null)
done = true;
else
{
names.remove(input);
print(names);
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}
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}
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System.exit(0);
42: }
43:
44: /**
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Prints the contents of a set
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@param s a set
47: */
48: private static void print(Set s)
49: {
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Iterator iter = s.iterator();
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System.out.print("{ ");
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while (iter.hasNext())
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{
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System.out.print(iter.next());
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System.out.print(" ");
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}
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System.out.println("}");
58: }
59: }
Hash Function
• A hash function computes an integer hash code
from an object
• Choose a hash function so that different
objects are likely to have different hash codes.
• Bad choice for hash function for a string
o Adding the unicode values of the
characters in the string
o Because permutations ("eat" and "tea")
would have the same hash code
Computing Hash Codes
• Hash function for a string s from
standard library
final int HASH_MULTIPLIER = 31;
int h = 0;
for (int i = 0; i < s.length(); i++)
h = HASH_MULTIPLIER * h + s.charAt(i)
A hashCode method for the Coin
class
• There are two instance fields: String coin name and
double coin value
• Use String's hashCode method to get a hash code
for the name
• To compute a hash code for a floating-point
number:
o Wrap the number into a Double object
o Then use Double's hashCode method
• Combine the two hash codes using a prime number
as the HASH_MULTIPLIER
A hashCode method for the Coin
class
class Coin
{
public int hashCode()
{
int h1 = name.hashCode();
int h2 = new Double(value).hashCode();
final int HASH_MULTIPLIER = 29;
int h = HASH_MULTIPLIER * h1 + h2;
return h;
}
. . .
}
Creating Hash Codes for your
classes
• Use a prime number as the HASH_MULTIPLIER
• Compute the hash codes of each instance field
• For an integer instance field just use the field
value
• Combine the hash codes
int h = HASH_MULTIPLIER * h1 +h2;
h = HASH_MULTIPLIER * h + h3;
h = HASH_MULTIPLIER *h + h4;
. . .
return h;
Creating Hash Codes for your
classes
• Your hashCode method must be compatible
with the equals method
if x.equals(y) then x.hashCode() == y.hashCode()
• In general, define either both hashCode and
equals methods or neither
HashMap
• In a hash map, only the keys are hashed
• The keys need compatible hashCode and
equals method
File Coin.java
01: /**
02: A coin with a monetary value.
03: */
04: public class Coin
05: {
06: /**
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Constructs a coin.
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@param aValue the monetary value of the coin.
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@param aName the name of the coin
10: */
11: public Coin(double aValue, String aName)
12: {
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value = aValue;
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name = aName;
15: }
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Gets the coin value.
@return the value
*/
public double getValue()
{
return value;
}
/**
Gets the coin name.
@return the name
*/
public String getName()
{
return name;
}
public boolean equals(Object otherObject)
{
if (otherObject == null) return false;
if (getClass() != otherObject.getClass()) return false;
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Coin other = (Coin)otherObject;
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return value == other.value && name.equals(other.name);
41: }
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43: public int hashCode()
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int h1 = name.hashCode();
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int h2 = new Double(value).hashCode();
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final int HASH_MULTIPLIER = 29;
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int h = HASH_MULTIPLIER * h1 + h2;
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return h;
50: }
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52: public String toString()
53: {
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return "Coin[value=" + value + ",name=" + name + "]";
55: }
56:
57: private double value;
58: private String name;
59: }
File HashCodeTest.java
01: import java.util.HashSet;
02: import java.util.Iterator;
03: import java.util.Set;
04:
05: /**
06: A program to test hash codes of coins
07: */
08: public class HashCodeTest
09: {
10: public static void main(String[] args)
11: {
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Coin coin1 = new Coin(0.25, "quarter");
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Coin coin2 = new Coin(0.25, "quarter");
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Coin coin3 = new Coin(0.05, "nickel");
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System.out.println("hash code of coin1="
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+ coin1.hashCode());
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System.out.println("hash code of coin2="
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+ coin2.hashCode());
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System.out.println("hash code of coin3="
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+ coin3.hashCode());
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Set coins = new HashSet();
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coins.add(coin1);
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coins.add(coin2);
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coins.add(coin3);
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Iterator iter = coins.iterator();
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while (iter.hasNext())
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System.out.println(iter.next());
31: }
32: }
Binary Search Trees
• Binary search trees allow for fast insertion and
removal of elements
• A binary tree consists of two nodes, each of which
has two child nodes
• All nodes in a binary search tree fulfill the property
that:
o Descendants to the left have smaller data values than the
node data value
o Descendants to the right have larger data values than the
node data value
A Binary Search Tree
A Binary Tree That Is Not a Binary
Search Tree
Implementing Tree Classes
• Implement a class for the tree containing a
reference to the root node
• Implement a class for the nodes
o A node contains two references ( to left and
right child nodes)
o A node contains a data field
o The data field has type Comparable, so that
you can compare the values in order to place
them in the correct position in the binary
search tree
Public interface for Tree and Node
classes
class Tree
{
public Tree() { . . . }
public void insert(Comparable obj) { . . . }
public void print() { . . . }
private Node root;
private class Node
{
public void insertNode(Node newNode) { . . . }
public void printNodes() { . . . }
public Comparable data;
public Node left;
public Node right;
}
}
Insertion Algorithm
• If you encounter a non-null pointer, look at its
data value
o If the data value of that node is larger than
the one you want to insert, continue the
process with the left subtree
o If the existing data is smaller, continue the
process with the right subtree
• If you encounter a null node pointer, replace
it with the new node
Insertion Algorithm - Tree class
class Tree
{ . . .
public void insert(Comparable obj)
{
Node newNode = new Node();
newNode.data = obj;
newNode.left = null;
newNode.right = null;
if (root == null) root = newNode;
else root.insertNode(newNode);
}
. . .
}
Insertion Algorithm - Node class
private class Node
{
public void insertNode(Node newNode)
{
if (newNode.data.compareTo(data) < 0)
{
if (left == null) left = newNode;
else left.insertNode(newNode);
}
else
{
if (right == null) right = newNode;
else right.insertNode(newNode);
}
}
. . .
}
Binary Search Tree After Four Insertions
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The tree before the insertion of node with data "Romeo"
Binary Search Tree After Five Insertions
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The tree after the insertion of node with data "Romeo"
Algorithm for Printing Elements in
Sorted Order
• Print the left subtree
• Print the data
• Print the right subtree
Tree
Class Method to Implement
Printing in Sorted Order
class Tree
{
public void print()
{
if (root != null)
root.printNodes();
}
. . .
}
Node
Class Method to Implement
Printing in Sorted Order
private class Node
{
public void printNodes()
{
if (left != null)
left.printNodes();
System.out.println(data);
if (right != null)
right.printNodes();
}
. . .
}
Binary Search Tree
• There is no insertion position
o You can not select the position at which to
insert an element
• The structure is self-organizing
o Each element finds its own place
• If it is approximately balanced, then adding an
element takes O(log(n)) time
File TreeTest.java
01: /**
02: This program tests the binary search tree class.
03: */
04: public class TreeTest
05: {
06: public static void main(String[] args)
07: {
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Tree names = new Tree();
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names.insert("Romeo");
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names.insert("Juliet");
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names.insert("Tom");
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names.insert("Dick");
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names.insert("Harry");
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names.print();
16: }
17: }
File Tree.java
01: /**
02: This class implements a binary search tree whose
03: nodes hold objects that implement the Comparable
04: interface.
05: */
06: public class Tree
07: {
08: /**
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Constructs an empty tree.
10: */
11: public Tree()
12: {
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root = null;
14: }
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Inserts a new node into the tree.
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@param obj the object to insert
*/
public void insert(Comparable obj)
{
Node newNode = new Node();
newNode.data = obj;
newNode.left = null;
newNode.right = null;
if (root == null) root = newNode;
else root.insertNode(newNode);
}
/**
Prints the contents of the tree in sorted order.
*/
public void print()
{
if (root != null)
root.printNodes();
}
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private Node root;
/**
A node of a tree stores a data item and references
of the child nodes to the left and to the right.
*/
private class Node
{
/**
Inserts a new node as a descendant of this node.
@param newNode the node to insert
*/
public void insertNode(Node newNode)
{
if (newNode.data.compareTo(data) < 0)
{
if (left == null) left = newNode;
else left.insertNode(newNode);
}
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else
{
if (right == null) right = newNode;
else right.insertNode(newNode);
}
}
/**
Prints this node and all of its descendants
in sorted order.
*/
public void printNodes()
{
if (left != null)
left.printNodes();
System.out.println(data);
if (right != null)
right.printNodes();
}
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public Comparable data;
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public Node left;
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public Node right;
81: }
82: }
TreeSet
and HashSet
• Both implement the Set interface
• With a good hash function, hashing is
generally faster than tree-based algorithms
• TreeSet's balanced tree guarantees
reasonable performance
• TreeSet's iterator visits the elements in
sorted order rather than the HashSet's
random order
To use a TreeSet
• Either your objects must implement
Comparable interface
• Or you must provide a Comparator object
To use a TreeMap
• Either the keys must implement the
Comparable interface
• Or you must provide a Comparator object
for the keys
• There is no requirement for the values
File TreeSetTest.java
This program constructs a TreeSet of Coins using the
CoinComparator object
01: import java.util.Comparator;
02: import java.util.Iterator;
03: import java.util.Set;
04: import java.util.TreeSet;
05:
06: /**
07: A program to test hash codes of coins
08: */
09: public class TreeSetTest
10: {
11: public static void main(String[] args)
12: {
13:
Coin coin1 = new Coin(0.25, "quarter");
14:
Coin coin2 = new Coin(0.25, "quarter");
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Coin coin3 = new Coin(0.01, "penny");
Coin coin4 = new Coin(0.05, "nickel");
class CoinComparator implements Comparator
{
public int compare(Object firstObject, Object secondObject)
{
Coin first = (Coin)firstObject;
Coin second = (Coin)secondObject;
if (first.getValue() < second.getValue()) return -1;
if (first.getValue() == second.getValue()) return 0;
return 1;
}
}
Comparator comp = new CoinComparator();
Set coins = new TreeSet(comp);
coins.add(coin1);
coins.add(coin2);
coins.add(coin3);
35:
coins.add(coin4);
36:
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Iterator iter = coins.iterator();
38:
while (iter.hasNext())
39:
System.out.println(iter.next());
40: }
41: }