Class 48: LInear Structures

Held: Friday, 25 April 2003

Summary: Today we consider a variety of linear structures, among the simplest forms of collections.

Related Pages:

• EBoard.
• Linear.java.
• FullStructureException.java.
• EmptyStructureException.java.
• Queue.java.

Assignments:

• Read the chapter on linear structures in Bailey.

Notes:

• CSC151 project presentations Monday at 2:15.

Overview:

• About linear structures.
• Stacks.
• Queues.
• Implementation notes.

About Linear Structures

• Linear Structures are one of the simplest forms of collections.
• Linear structures typically provide two basic operations:
  • put, add an element to the collection
  • get, get an element from the collection, removing it from the collection
• Since get requires a non-empty collection, most collections provide a isEmpty predicate.
• The choice of what get returns is up to the policy of the particular kind of linear structure.
  • Stacks implement a last in, first out (LIFO) policy.
  • Queues implement a first in, first out (FIFO) policy.
  • Priority queues implement a best first policy.
• There are a few related operations that you may find helpful:
  • peek returns the object to be returned by get, but without deleting it.
  • size returns the number of elements in the structure.
• It might also be useful to have an isFull operation.
• The individual operations traditionally have different names in the different types of linear structures.
  • Why? The structures were invented first. Then someone realized they had a commonality. By that point, the names were ingrained.

Stacks

• LIFO policy.
• LIFO mimics many common operations, such as what happens with procedure calls in most programming languages.
• add is typically called push.
• get is typically called pop.
• peek is typically called top.

Queues

• FIFO policy.
• add is typically called enqueue.
• get is typically called dequeue.
• peek is typically called front.

Implementation Notes

• As we noted earlier, you can implement lists with arrays or nodes.
• Similarly, we can implement linear structures with arrays or nodes (or even with lists).
• If we're implementing a queue with nodes, we probably want to keep a pointer to both the front and the back of the queue.
  • We'll probably look at that implementation in more detail.