Hash Tables

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Hash Tables

• Computer Science and Engineering
• Chapter 8 of Goodrich and Tomassias Text

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Topics

• Hashing
• Hash functions
• Hash Tables
• Collision
• Collision resolution
• Chaining, linear probing, quadratic probing,
  double hashing
• Java Hash table
• Example

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Hashing Concept

• Another approach for storing and searching
  elements.
• Used in applications where add and delete,
  besides search, are used.
• Worst case linear but can get O(1) best case.

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Hash Function

• A hash function h maps keys of a given type into
  an integer interval 0, N-1
• A simple function
• h(x) x mod N
• A good hash function will uniformly disperse the
  keys in the range 0, N-1

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Hash Table

• Hash table ADT
• Has a hash function h
• Array of size N
• Collision occurs when two keys map to the same
  array index.
• Two major collision resolution schemes are
  chaining and open addressing

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Example

• Item(name, ssn) where ssn is 9 digit positive
  integer.
• Hash table N 10000.
• Hash function last four digits of of x
• Use chaining to handle collision.

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Hash Table Example
0 1 2 3
045-34-0002
. . . .
9996 9997 9998 9999
567-34-9996
045-35-9996
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Hash Functions

• Usually specified as two components
• Component1 called the hash code map, collects the
  parts of the data and maps them to integers
  (numeric data)
• H1 keys ? integers
• Component2 called the compression map, takes the
  integers and maps them to 0 to N-1.
• H2 integers? 0, N-1

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Hash Code Maps

• Memory address of data
• Integer cast of non-numeric data bit/byte
  pattern of data
• Sum of all components add the ascii values of
  your last name
• Polynomials of various parts of data.

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Compression maps

• Reminder of division (mod)
• h(y) y mod N
• Multiple, Add and Divide (MAD)
• h(y) (ay b) mod N where
• a mod N ltgt 0 (otherwise everything will map to
  b!)

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Linear Probing

• Linear probing resolves collision by placing the
  colliding item in the next available empty cell.
• Each entry inspected is referred to as the
  probe
• Example h(x) x mod 13
• Insert keys 18, 41, 22, 44, 59, 32, 31, 73, in
  that order

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Search with linear probing find(k)

• j h(k) probe 0
• while (p lt N)
• 2.1 c Aj
• 2.2 if (c null) return NOT_FOUND
• else if (c.key() k)
• return c.element()
• else j (j1) mod N
• p p 1
• 3. return NOT_FOUND

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Double Hashing

• h1 primary hash function
• If it results in collision, resolve by applying
  another hash function, secondary hash function
  here is an example of such a function.
• d(k) q k mod q
• Where q lt N,
• Possible values of d(k) are 1,2,3,..q
• It cannot be 0
• When collision occurs (h(k)jd(k)) mod N
• j 0, 1, ..N-1

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Example

• h(k) k mod 13
• d(k) 7 k mod 7
• Insert 18, 41, 22, 44, 59, 32, 31, 73.

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Java support for Hashing

• Java1.4 API
• Hashtable since jdk1.0
• HashMap since jdk1.2

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Hashtable/HashMap

• Array of Entry
• Chaining is the collision resolution policy
• Entry is an inner class Entry is a list
• Entry holds a key, value and a link to next Entry
• Key Object should implement hashcode() and
  equals() methods
• hashcode() of the Key class is the hash code map
  that converts key attributes to an integer
• Compression map/function is hashcode mod
  array.length
• Loadfactor (how full it can get before
  expanding), initial capacity can be specified

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Hashtable vs HashMap
AbstractMap
extends
Hashtable Map, Cloneable,
Serailizable
implements
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Hashtable vs HashMap

• Hashtable implements Dictionary which is
  deprecated and has been superceded by AbstractMap
• Hashtable implementation is a synchronized for
  concurrent access. HashMap leaves synchronization
  to the application.
• Hashtable is slower (due to synchronized
  implementation)

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HashMap
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Using HashMap

• Problem Consider employee data that has last
  name, first name, employee number (5 digit
  integer), and salary. We want to maintain a data
  structure of 100 temporary employees for easy
  add, remove and search.
• Solution HashMap

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Using HashMap

• Problem An employee data is made up of Name
  (first, last), employee number (Integer), and
  salary. Maintain a set of 50 employee data for a
  temp agency so that employees can be added,
  removed and searched easily.
• Solution Choice of data structure HashMap
• Key? Lets consider employee number as key in the
  first version
• In the second version we will consider name as
  the key
• Name is a class of two attributes first, last

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TempAgency Design
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More hashCode() method

• hashCode() is predefined in Object class for all
  classes.
• The default value that hashCode() returns is the
  memory address of the object.
• Where is hashCode() called in the HashMap?
• In the put(), get(), remove() and others that
  compute the hash index in the two phase hashing
  (hash code map and compression map)

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Summary

• Hashing offers a convenient way to store and
  search for data.
• The data considered in the form of key, value
  pair.
• The location of a data depends only on its key,
  not on its successor and/or predecessor key.