CSC153, Class 50: Dictionaries

Overview:
* Notes on Exam 3
* Questions from Friday's class.
* The Dictionary ADT: Basic Design Issues
* Applying Dictionaries.
* The Dictionary ADT: Central Methods
* Dictionaries vs. Databases

Notes:
* Today is "Feed Arjun" day.  Four types of Salsa!
* Exam 3 returned.
* Exam 4 questions?
* Questions on heaps?

----------------------------------------

Notes on Exam 3

Problem 1: Defining Polymorphism
* Citations
* What is a definition?
* What about interfaces?

Problem 2: Weird Comparable Thing
* Order an interface
public interface Order
{
  public boolean mayPrecede(Object alpha, Object beta)
    throws Exception;
}
* Student: a Class (data associated with it)
public class Student {
  double gpa;
}
* OrderStudentsByGPA: Has a clear procedure implementation
  Class or Abstract Class
public class OrderStudentsByGPA
  implements Order // -2
{ 
  public boolean mayPrecede(Object alpha, Object beta)
    throws Exception
  {
    if (!(alpha instanceof Student))
      throw new Exception(alpha + " is not a student");
    if (!(beta instanceof Student))
      throw new Exception(beta + " is not a student");
    return ((Student) alpha).gpa > ((Student) beta).gpa;
  } // boolean mayPrecede(Object, Object)

  public boolean mayPrecede(Object alpha, Object beta)
    throws Exception
  {
    return ((Student) alpha).gpa > ((Student) beta).gpa;
  }

} // OrderStudentsByGPA

----------------------------------------

What's wrong with the following code?  We'll draw pictures to help.

public class BriansQueue
  implements Queue
{
  // Fields
  Node first;
  Node last;
 
  public Object get()
    throws EmptyStructureExcepction 
  {
    if (this.first == null)
      throw new EmptyStructureException;
    Object tmp = this.first.value;
    this.first = this.first.next;
    return tmp;
  }
  
  public void add(Object value)
  {
    this.last.next = new Node(value);
    this.last = this.last.next;
  } // add(Object)

} // class BriansQueue

public class BriansQueue
  implements Queue
{
  // Fields
  Node first;
  Node last;
 
  // Side note: If the queue has one element, do
  // first and last point to the same node?  YES

  // Problem: If we delete the only element in a 
  // one-element queue, we need to set both first
  // and last to null.
  public Object get()
    throws EmptyStructureExcepction 
  {
    if (this.first == null)
      throw new EmptyStructureException;
    Object tmp = this.first.value;
    this.first = this.first.next;
    if (this.first == null) 
      this.last = null;
    return tmp;
  }
 
  // Question:
  //   What does it mean that this.first and this.last are
  //   the same?
  //   this.first == this.last
  //      Both point to the same object
  //   this.first.equals(this.last) 
  //      It depends on how Node.equals is implemented 
  // If the queue is empty, this.last is null.
  // this.last.next is some nasty exception. 
  public void add(Object value)
  {
    if (this.last == null) {
      this.first = this.last = new Node(value);
    }
    else {
      this.last.next = new Node(value);
      this.last = this.last.next;
    }
  } // add(Object)

} // class BriansQueue

----------------------------------------

What is a vector or an array?
A collection that you can index by number
Main operations:
* get ith element
* set ith element
* get length
Expectations that those are O(1) operations

Are there collections you'd like to index, 
but by things other than numbers?
* A traditional dictionary is a collection of definitions
  indexed by words.
* We might index file folders by keyword
* We might index Brian's software by color
  iSoftwareOrganizer
  iSockDrawer

The computer science community has no standard term for
collections indexed by values other than numbers.  I call
them Dictionaries.  Some people call them "Keyed Tables".
Java used to call them Dictionaries.  Java now calls them
Maps.