Outline of Class 16: Experimenting with Vectors and Dictionaries

Held: Friday, February 13, 1998

Administrivia

• Assignment three is now ready and is due next Friday.
• Assignment four is also ready and is due the following Tuesday.
• We'll spend most of today's class on lab 7.
• For Monday, read section 9.1 of Bailey.

Dictionaries

• In our investigation of vectors, we've seen a structure that collects objects in which the objects can be indexed by numbers.
• Are there other ways to index collections? Yes, we could index collections by "any object".
• Collections of objects indexed by objects (e.g., strings) are called dictionaries.
• Why do we do we want dictionaries? They make many tasks more convenient.
  • Keeping track of objects we've seen (if an object is an index in the dictionary, we've seen it; if it's not an index in the dictionary, we haven't seen it).
  • Counting things (associate a counter with each object; when you see another copy, increment the counter).
• How do we do build dictionaries? Right now, we'll just use dictionaries and won't worry about how they work. Later, we'll look at how to build them.
• Some terminology:
  • The objects we're using as indices are called keys.
  • The objects we're looking up are called values.
• What features do we need in a dictionary? Some are similar to those we need for Vectors.
  • Add something to the dictionary (requires a key and value).
  • Look up something in the dictionary (requires a key, returns a value).
  • Remove something from the dictionary.
• Others correspond more to the different structure of dictionaries.
  • List all the keys in the dictionary.
  • List all the values in the dictionary.

Hash Tables

• Hash tables are one mechanism for implementing dictionaries. They provide an implementation in which it is likely, but not guaranteed, that access to particular elements will be quick.
• In Java, java.util.Hashtable implements the interface java.util.Dictionary.
  • Well ... Dictionary isn't really an interface; it's an abstract class. Nonetheless, it's similar enough for our purposes.
• For now, you can think of hash tables as equivalent to dictionaries. When you need a dictionary, create a hash table.