Chapter: Traversals

If you enter the text of Hamlet into the program above, you will find that you can verify the following data:

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. 1

How do you think I produced the above lines?

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.

Save the countTreeBetter project under a new name "concordance". I want you to add a new button so the screen will look like:

It's not hard to do. The code you need is here (but you'll need to put it in the right place!)

    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 ACTree interface. That's easy: Add this line to the file.

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

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

    /** 
     * traverse -- inorder traversal, printing my data
     */
    public void traverse(java.io.PrintStream ps) {
       ps.println(myData.toString() + " occurs "+count+" times");
       cleft.traverse(ps);
       cright.traverse(ps);
    }

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

Exercise 2