Outline of Class 40: Implementing Trees with Nodes

Held: Tuesday, April 14, 1998

Administrivia

• Any questions on assignment six? Don't forget that it's due this Friday.
• Today and tomorrow, we'll be spending most of the class on lab 14. You should make sure to read over (and think about) the last part of the lab before tomorrow's class.
• A reminder: on Friday, we'll be starting to discuss the history of computing in conjunction with the Evolution of Technology class in Anthropology (and Technology Studies).
• In preparation for the discussion of the history of computing, you should take a survey on the history of computing sometime after class and before midnight on Wednesday.

Tree Nodes, Revisited

• As with lists, it is possible to implement trees directly with a TreeNode class and without a wrapper class.
• While TreeNodes will have many similarities to ListNodes, there will be some differences:
  • We need to support more than one next element (child). We could do that by creating a separate variable for each child or by building an array or vector of children.
  • We need to keep track of how many children we have. We can use the size of the array or vector, but it may be useful to have separate access to that information.
  • We may need to consider the arity of our objects in the constructor.
• We also have to make some of the same decisions we made for list nodes.
  • Do we want unidirectional or bidirectional links? Bidirectional links support a parent() method, but unidirectional links (parent to child) permit reuse of the same subtree in multiple trees and may be easier to support.
  • Should each node keep track of its depth? This makes computation easier, but modification harder.
• We also have a number of questions to ask ourselves in the design, such as whether the arity of a node can change. If so, how do you determine the current arity of a node?
• If we allow variable-arity nodes, it becomes important to consider the appropriate preconditions for setChild(). For example, can you set the fifth child of a node with no children?
• If we want to implement binary trees and don't want to support a parent link, we can simply use the BinaryNode class we've already developed.