Class 35: Lists, Concluded

Held Monday, November 1, 1999

Overview

Today, we will conclude our discussion of lists by considering other issues in the design of linked lists.

Notes

• I won't be available for office hours today. Sorry.
• There's an interesting talk tomorrow at 4:30 by Vivek Venugopal and Jared Seaman on their summer research. I encourage you to attend.
• I've had some questions about continuing your studies of CS.
  • You cannot take CSC302 next semester. 302 as a 301 prereq.
  • You shouldn't be able to take CSC364, which has a 211/213 prereq. However, we have decided to waive that prereq so that there is an avialable course.
  • You should take MAT218 next semester (or MAT215, if you haven't done so already).
  • Those of you taking MAT218 next semester might want to consider the 2-credit Games Theory special topics course being offered.

Contents

• Doubly-Linked Lists
• List Cursors
• Doubly-Linked Lists
• Vectors

Handouts

• Lab X2
• Assignment 4

Summary

• Assigned: assignment 4: linked lists (Due November 8)
• Cursored lists
• Doubly-linked lists
• Vectors, revisited

Doubly-Linked Lists

List Cursors

• Rather than providing the three iterator methods (nextElement, hasMoreElements, and reset), some designers of lists provide an explicit ``current element''.
• With an explicit current element, you can
  • advance the current element
  • back-up the current element
  • delete the current element
  • add an element before or after the current element
  • find an element and move the cursor to that element
  • move the cursor to the front or back of the list
  • ...
• What are the implications of including a cursor?
  • The list must be ordered
  • You have to be very careful in documenting your preconditions and postconditions for these and other methods. (We'll look at some.)

Doubly-Linked Lists

• Note that our cons-based implementation of lists (typically called linked lists) have an interesting assymetry:
  • It is O(1) to add to the front of the list
  • It is O(n) to add to the back of the list
• Similarly,
  • It is O(1) to add after the current element
  • It is O(n) to add before the current element
• To eliminate this problem, some implementers put the links in both directions. That is, in addition to having a next link, each cons cell also has a previous link.

Vectors

• Java provides a data type for expandable, array-like (and also list-like) objects that are accessible by element.
• These structures are called Vectors.
• Do Lab X2.2