Lab 10: Hash Tables
CS 241: Data Structures, Fall 2007

Goals for this assignment are to:

• Understand Hash Table Collision Detection Algorithms
• Demonstrate how Hash Tables are used.

Reading

Review Chapter 11.4

Part 1: A Simple Hash Table with Collision Detection

• In this part of the lab, you are to write, from scratch, a very simple hash table program. Your program should be much simpler than the one given in the text.

• Your Hash Table class will store only non-negative integers in a simple integer array (the user should be able to set the size). Initialize the elements of the hash table array to -1 to indicate an "empty" slot.

  If you want, you may start with the skeleton program. In the HashTable class you need to implement the methods:

  1. public int insert(int n) - inserts an item using linear probing. Returns the number of collisions encountered while trying to insert the item. Returns -1 if table is full. You will need to think about how to count collisions.
  2. public int contains(int n) - checks to see if an item is in the table, assuming that the item was entered using linear probing. Returns -1 if item is not in the table otherwise return the number of collisions.
  3. public int remove(int n) - checks to see if an item is in the table, assuming that the item was entered using linear probing. If yes, remove the item. Returns -1 if item is not in the table otherwise return the number of collisions.
  4. public String toString() - return a string representation of the hash table indicating what items are in which slots and which slots are empty. Differentiate between slots that are empty because they never contained an item and those that are empty because an item was removed.

• Think about how the above methods would change if you instead used quadratic probing.

Part 2: A SpellChecker Program

• In this part of the lab, you will use the java.util.Hashtable class available in the Java libraries to write a spell checker application.
• Your program should allow the user to enter text in a TextArea. When a button is clicked, the program checks to see if there are any misspelled words. The misspelled words should be printed out in a second TextArea.
• When the SpellCheck object is initially created (i.e. in the constructor), it should load the "dictionary" into a HashTable from the textfile words.txt which can be found here. Note that this list of words has roughly 25K words but is hardly complete.The code for reading the textfile is similar to the Anagrams constructor given in your text on p. 289. Feel free to add words if you think important ones are missing.
• To check the spelling, parse the words that the user has entered (look at how the StreamTokenizer was used in the your calculator program). For each word parsed, check to see if it is in the dictionary (i.e. hash table). If it is, then it must be spelled correctly. If it isn't, then assume it is misspelled. Note that punctuation can mess things up. For example, to remove a "." just set it as an ordinary character as follows:

  StreamTokenizer st = new StreamTokenizer(new StringReader(s));
  st.ordinaryChar('.');
  

Contractions won't be parsed correctly (e.g. didn't, can't, etc). This could be fixed by using regular expressions, however, this is a bit beyond the scope of this lab.

Evaluation

By no later than class on Wednesday November 28, you should zip together both projects, including a jar files, and email the code as an attachment to gorr@willamette.edu. Place CS241 Lab 10 in the subject line.

Demonstrate your the hash table program (part 1) and and the spell check program (part 2).