Traversals

Chapter: Traversals

If you enter the text of Hamlet into a Map structure, you might like to use that structure to produce a Concordance, that, traditionally, would begin something like this: 

a occurs 571 times
abate occurs 1 times
abatements occurs 1 times
abhorred occurs 1 times
ability occurs 1 times
able occurs 1 times
aboard occurs 3 times
abominably occurs 1 times
about occurs 18 times
above occurs 4 times
abridgment occurs 1 times
abroad occurs 1 times
absent occurs 1 times
absolute occurs 2 times
abstinence occurs 1 times
abstract occurs 1 times
absurd occurs 2 times
abus occurs 1 times
abuse occurs 1 times
abuses occurs 1 times
accent occurs 2 times
accepts occurs 1 times
access occurs 1 times

Q. 4
How do you think I produced the above lines?

Exercise 7

Copy your entire package countTreeMap into a new project called countTreeMap2. Add an instance variable protected int occurrences to Tree, and make sure it reflects the number of times the item has been inserted. Add a method public abstract void inOrder() to ATree. It will basically perform an inorder traversal of a binary search tree. In ETree there will be nothing to do in inOrder(), but in Tree you will need to:
  1. perform inOrder() on the left tree,
  2. print myData and the number of its occurrences,
  3. perform inOrder() on the right tree,

I will use the word "concordance" to describe a file that lists all the words in alphabetic order together with a count of how often they occurred. Above you see an early part of the concordance I want you to produce for Hamlet.

I want you to add a new button so you can produce a concordance in the Console.

Then let's modify it so you print the concordance to a file. It's not hard to do. The code you need is here (but you'll need to put it in the right place!)

traverse will be just like inOrder but you'll add a parameter for a PrintStream.

    private concordance;
        concordance = new JButton("Make a concordance");
        buttonPanel.add(concordance);

And, of course, you'll need a listener so the button works. Here's that code:

         concordance.addActionListener(new ActionListener() {
            public void actionPerformed (ActionEvent e) {
                PrintStream ps = null;
                try {
                    ps = new PrintStream(new File(data.getText()));
                }
                catch (Exception e2) {}
                myTree.traverse(ps);
                ps.close();
            }
         });

You may want to be a little more polite than I am about catching the exception. Notice what this action is: When the button is clicked:

  1. A file is created under the name found in the data TextField,
  2. A PrintStream that can be used for output to that file is created,
  3. that PrintStream is passed to the traverse method of the tree
  4. the PrintStream is closed

Observe that in order for this class to compile now you will need to add the traverse method to the ATree interface. That's easy: Add this line to the file. 

    public void traverse(java.io.PrintStream ps);

Now, of course, ETree and Tree must implement the traverse method. Here's my code for Tree: 

    /** 
     * traverse -- inorder traversal, printing my data
     */
    public void traverse(java.io.PrintStream ps) {
       ps.println(myData.toString() + " occurs "+count+" times");
       left.traverse(ps);
       right.traverse(ps);
    }
The only problem is that I've given you code for a preorder traversal. That's not what you need if you want your concordance to be printed in alphabetic order. (Hint: you need to reorder the lines of code to get the traversal you want.)

I'm going to let you write your own code for the traverse method in the ETree class. (Hint: That's not asking very much at all, is it?)

Exercise 8

Make the concordance work correctly. Almost all the code you need is given above. The major change is to get away from the preorder traversal to the kind of traversal you really need. Check that your output contains approximately the segment I gave when you build a tree from the text of Hamlet.

rhyspj@gwu.edu