CSECE 268

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Chapter 17 Maps

• CS/ECE 268
• Kristin Stechschulte
• Sophomore Computer Science

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Maps

• What is a Map?
• Comparison to Vectors/arrays.
• Everyday usages of Maps.
• The methods of a Map.
• The adapter Association.
• Running times of Maps with adapters.
• Pros v. Cons of Maps.

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Introduction

• Most of the data structures dealt with in the
  class thus far have been ones that place emphasis
  on the single independent values.
• Bag, Stack, Queue, Deque, Etc.
• For example the Bag it holds a collection of
  values and can tell whether a specific value is
  part of the collection.
• But sometimes is would be convenient to use a key
  or a certain value to find what you are searching
  for within a collection.

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What is a Map?

• A Map is a data abstraction which is specialized
  for the specific purpose of making referencing
  easier.
• It may also be referred to as table, search
  table, dictionary, associative array, or
  associative container.
• Elements of a Map are contained in pairs.
• Key a unique identifier that connects to one
  associated entry (value).
• Value an entry in the Map which is accessed by
  a key.

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Map Information

• Two keys can point to the same information, but

Color A
Blue
Color B
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Map Information Continued

• A single key can not point to two separate values.

Blue
Color A
Purple
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Vector/array Comparison

• The Map, which is a keyed container, can be
  contrasted with an indexed container, like a
  Vector or an array.
• In a Vector or array the indexed values are
  associated to the entries, like a key in a Map.
• But this has a set back, because there is no easy
  way to remember which values are associated with
  which indexes.
• All the indexes in a Vector/array type are
  restricted to the integer type.

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Vector/array Comparison

• Index Values Connected to the Index

0
For God so loved the world that He gave one and
only Son, that whosoever believes in Him shall
not perish but have eternal life.
The thief does not come except to steal, to kill,
and to destroy. I have come that they may have
life, and have it more abundantly.
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For the wages of sin is death, but the gift of
God is eternal life in Christ Jesus our Lord!
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God demonstrates His love towards us, in that
while we were yet sinners, Christ died for us.
3
Jesus said to him, I am the way, and the truth,
and the life no one comes to the Father but
through Me.
4
As many as received Him, to them He gave the
right to become children of God, even to those
who believe in His name.
5
By grace you have been saved through faith and
that not of yourselves, it is the gift of God
not as a result of works that no one should boast.
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Advantages of Maps

• With a key it is easier to remember the
  connections, since it is not just an integer
  value but it has a specific or easily
  identifiable link to the value.
• The Map is optimized for easy retrieval of values
  when the key is input and known to the structure.
• A Map does not limit the the number of elements
  that can be held in the structure.

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Map Examples

• There are many common example for the usages of
  Maps as given over the past few days in class and
  also presented in the book.
• Dictionary key is the word, value is the
  definition.
• Encyclopedia key is the topic, value is the
  information.
• Address Book key is the name, value is the
  address.
• Bank Accounts key is the account number, value
  is amount held in the savings or checking
  account.
• Student Accounts key is the student
  identification number, value is the student name
  and other info.

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More Map Examples

• Another great example is the Bible.
• The key are the Scripture references, and the
  values are the Scripture verses.

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Map Methods

• The Map container implements the Map interface,
  and has the properties associated with the normal
  ADT.
• int size()
• input - nothing
• output returns an integer value
• boolean isEmpty()
• input - nothing
• output returns a boolean value, true or false

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Map Methods continued

• There are also several methods that are unique to
  the Map ADT.
• void set(key, value)
• input a key object and a value object
  associated with the key
• output - nothing
• Object get(key)
• input a key object
• output the value object associated with the
  key, if the key does not exist a
  NoSuchElementException will be thrown
• void removeKey(key)
• input a key object
• output it deletes the key and the value
  associated with it, if the element does not exist
  NoSuchElementException is thrown

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Map Methods continued

• boolean containsKey (key)
• input a key object
• output a boolean value (true or false) is
  returned depending on whether the key object is
  contained in the collection or not
• Enumeration elements()
• input - nothing
• output an enumeration of the keys in the
  collection, but the order is not specified or
  defined

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Map Example...
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Map Implementation

• Because a Map is a ADT you cant use it directly.
• You must implement it through a sort of adapter
  that is built on top of an underlying container.
• The book points out how different effects can be
  produced by changing the underlying data
  structure.
• By using a LinkedList, SortedVector, SortedList,
  SkipList, AVLTree, OpenHashTable, HashTable, Etc.
• The author uses a MapAdapter that manages
  key/value pairs through the inner class named
  Association.

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Association

• An Association holds a key and a value in data
  fields that are like the class Object.
• Comparisons between two associations are based
  strictly on the keys and have nothing to do with
  the values associated to the keys.
• This is done by a Comparator object.
• Implementing MapAdapter from this class is
  relatively simple.
• The MapAdapter can also be built on top of any
  other data structure that also supports the Bag
  interface.

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Running Times

• The running time for the various operations of
  the Map interface is dependent on the underlying
  container.

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Running Times continued

• The simplest underlying container is the
  LinkedList.
• But the performance of all the functions in this
  are slower, O(n), because it is linear.
• The MapAdapter DOES NOT require ordering, but it
  may help in the performance of the container when
  you are searching for an object.
• The AVLTree and the SkipList are considerably
  faster than LinkedList, SortedList, or the
  SortedVector, but the setback is that the
  elements are required to be ordered if these
  structures are used.

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Running Times continued

• The HashTable containers are overall the fastest,
  and the elements are not required to be ordered.
  But their speed is also dependent on the size of
  the hash table and the load factor of the hash
  table.

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Pros and Cons

• A Map is useful, because it reduces the work
  needed to be done in performing a search over a
  collection of keys and values.
• It eliminates the need to iterate through the
  collection while performing a search.
• Able to find a Scripture by looking at the
  reference, faster.
• But there is a problem with it when you want to
  go from the other direction and use the value to
  find the key.
• Cant type in the verse to find the reference
  without going through each record step-by-step,
  this takes a lot of time.

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To Wrap it up...

• A Map is an ADT that uses a pair of objects,
  key/value, to store information.
• It is similar to arrays and vectors, except that
  it is easier to identify with the value by using
  the key instead of being forced to use an index
  number.
• There are many everyday examples that use the map
  structure
• Dictionary, Student Account, and Bible, etc.
• To implement the Map structure an adaptor must be
  used and it must be built on top of a data
  structure
• LinkedList, SortedVector, HashTable, etc.

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To Wrap it up...

• Performing different functions in the Map are
  dependent on the running times of the underlying
  containers.
• The Map is a useful data structure when you want
  and easy way to remember and search the values
  that are stored.