Class 48: Implementing Lists with Arrays

Held: Wednesday, 23 November 2005

Summary: Today we begin our investigation of the List ADT (actually, a set of List ADTs). Lists provide the iterate feature.

Related Pages:

• EBoard.
• Sample Code.

Assignments

• Exam 3 distributed.

Overview:

• List Basics.
• List ADTs.
• Implementing Lists with Vectors.

Types of Lists

• We now consider our final set of ADTs - those relating to lists.
  • Our List ADT is likely to differ from that of Scheme.
  • In particular, Scheme lists are more data structures than ADTs, since the focus is on the implementation (through pairs).
• What can we say abstractly about lists? That is, what is the philosophy of lists?
• One solution: A list is, in essence, a collection of elements that you can iterate.
  • By iterate, I mean that you can visit the elements of the list one by one.
• We tend to think of three related kinds of non-Scheme lists:
  • Simple Lists are the simplest (as their name suggests). They are collections. They are iterable. That's about it. (In particular, the client does not get to control the order in which the elements are iterated.)
  • Sequenced Lists give the client control over the order in which the elements are iterated. You can talk about adding something before or after an element of an ordered lists.
  • Sorted Lists are iterated in sorted order (typically from smallest to largest).
• It helps to associate Cursors or Positions with lists.
  • Both provide ways of referring to places in the list.
  • Cursors do move. You can advance them to the next element or retreat them to the previous element.
  • Positions don't move: You ask for their successors or predecessors.

List-Related ADTs

A Cursor ADT

Left for class discussion.

A Position ADT

Left for class discussion.

A SimpleList ADT

Left for class discussion.

A SequencedList ADT

Left for class discussion.

A SortedList ADT

Left for class discussion.

Implementing Lists with Arrays or Vectors

• The easiest way to implement simple lists (and sequenced lists) (and sorted lists) is with an array or Vector.
• The cursors or positions simply give numeric indices into the Vector.
• We may look at an example.