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Solution for Programming Exercise 9.2

This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.

Exercise 9.2:

Exercise 7.6 asked you to read a file, make an alphabetical list of all the words that occur in the file, and write the list to another file. In that exercise, you were asked to use an ArrayList<String> to store the words. Write a new version of the same program that stores the words in a binary sort tree instead of in an arraylist. You can use the binary sort tree routines from SortTreeDemo.java, which was discussed in Subsection 9.4.2.


In my solution to Exercise 7.6, words are added to an arraylist in the order in which they are encountered. After the file has been completely read, the arraylist is sorted into alphabetical order before the list of words is printed. Since a binary sort tree is designed to store words in alphabetical order at all times, there is no need for sorting. At the end of the program, an inorder traversal of the tree can be used to output the words to the file. Using an inorder traversal guarantees that the words will be output in increasing order.

For my solution to this exercise, I copied the routines treeInsert, treeContains, and countNodes from SortTreeDemo.java. I also copied the declaration of root as a static member variable, since that's the variable that represents the tree itself. (It's unfortunate that root has to be a global variable rather than a local variable in main(), but it's used as a global variable in the treeInsert routine. A better solution to the exercise would define a BinarySortTree class to encapsulate the data and routines needed to represent the tree and to use a variable of type BinarySortTree in the program.)

Only a few changes are needed in the main() routine of the original program. They are shown in red in the solution given below. All-in-all, the substitution of the binary tree for the arraylist is very straightforward.

The Solution

import textio.TextIO;

 * Makes an alphabetical list of all the words in a file selected
 * by the user.  The list can be written to a file.  
 * The words are stored in a binary sort tree.
public class ListAllWordsFromFileWithTree {

   private static TreeNode root;  // Pointer to the root node in a binary tree.
                                  // This tree is used in this program as a 
                                  // binary sort tree.  When the tree is empty, 
                                  // root is null (as it is initially).

   public static void main(String[] args) {
                 This program will ask you to select an input file
              It will read that file and make an alphabetical
              list of all the words in the file.  After reading
              the file, the program asks you to select an output
              file.  If you select a file, the list of words will
              be written to that file; if you cancel, the list
              be written to standard output.  All words are converted
              to lower case, and duplicates are eliminated from the list.
              Press return to begin.

      TextIO.getln();  // Wait for user to press return.
      try {
         if (TextIO.readUserSelectedFile() == false) {
            System.out.println("No input file selected.  Exiting.");
         // ArrayList<String> wordList = new ArrayList<String>(); DELETED LINE
         String word = readNextWord();
         while (word != null) {
            word = word.toLowerCase();  // convert word to lower case
            if ( treeContains(root,word) == false ) {
                  // This is a new word, so add it to the tree
            word = readNextWord();
         int wordsInTree = countNodes(root);
         System.out.println("Number of different words found in file:  " 
               + wordsInTree);
         if (wordsInTree == 0) {
            System.out.println("No words found in file.");
            System.out.println("Exiting without saving data.");
         // selectionSort(wordList);  DELETED LINE
         TextIO.writeUserSelectedFile(); // If user cancels, output automatically
                                         // goes to standard output.
         System.out.println(wordsInTree + " words found in file:\n");
      catch (Exception e) {
         System.out.println("Sorry, an error has occurred.");
         System.out.println("Error Message:  " + e.getMessage());
      System.exit(0);  // Might be necessary, because of use of file dialogs.

    * Read the next word from TextIO, if there is one.  First, skip past
    * any non-letters in the input.  If an end-of-file is encountered before 
    * a word is found, return null.  Otherwise, read and return the word.
    * A word is defined as a sequence of letters.  Also, a word can include
    * an apostrophe if the apostrophe is surrounded by letters on each side.
    * @return the next word from TextIO, or null if an end-of-file is encountered
   private static String readNextWord() {
      char ch = TextIO.peek(); // Look at next character in input.
      while (ch != TextIO.EOF && !Character.isLetter(ch)) {
         TextIO.getAnyChar();  // Read the character.
         ch = TextIO.peek();   // Look at the next character.
      if (ch == TextIO.EOF) // Encountered end-of-file
         return null;
      // At this point, we know that the next character, so read a word.
      String word = "";  // This will be the word that is read.
      while (true) {
         word += TextIO.getAnyChar();  // Append the letter onto word.
         ch = TextIO.peek();  // Look at next character.
         if ( ch == '\'' ) {
               // The next character is an apostrophe.  Read it, and
               // if the following character is a letter, add both the
               // apostrophe and the letter onto the word and continue
               // reading the word.  If the character after the apostrophe
               // is not a letter, the word is done, so break out of the loop.
            TextIO.getAnyChar();   // Read the apostrophe.
            ch = TextIO.peek();    // Look at char that follows apostrophe.
            if (Character.isLetter(ch)) {
               word += "\'" + TextIO.getAnyChar();
               ch = TextIO.peek();  // Look at next char.
         if ( ! Character.isLetter(ch) ) {
               // If the next character is not a letter, the word is
               // finished, so bread out of the loop.
         // If we haven't broken out of the loop, next char is a letter.
      return word;  // Return the word that has been read.
   //------------- Binary Sort Tree data structures and methods ------------------
   //------------- (Copied from SortTreeDemo.java) -------------------------------
    * An object of type TreeNode represents one node in a binary tree of strings.
   private static class TreeNode {
      String item;      // The data in this node.
      TreeNode left;    // Pointer to left subtree.
      TreeNode right;   // Pointer to right subtree.
      TreeNode(String str) {
             // Constructor.  Make a node containing the specified string.
             // Note that left and right pointers are initially null.
         item = str;
   }  // end nested class TreeNode

    * Add the item to the binary sort tree to which the global variable 
    * "root" refers.  (Note that root can't be passed as a parameter to 
    * this routine because the value of root might change, and a change 
    * in the value of a formal parameter does not change the actual parameter.)
   private static void treeInsert(String newItem) {
      if ( root == null ) {
             // The tree is empty.  Set root to point to a new node containing
             // the new item.  This becomes the only node in the tree.
         root = new TreeNode( newItem );
      TreeNode runner;  // Runs down the tree to find a place for newItem.
      runner = root;   // Start at the root.
      while (true) {
         if ( newItem.compareTo(runner.item) < 0 ) {
                // Since the new item is less than the item in runner,
                // it belongs in the left subtree of runner.  If there
                // is an open space at runner.left, add a new node there.
                // Otherwise, advance runner down one level to the left.
            if ( runner.left == null ) {
               runner.left = new TreeNode( newItem );
               return;  // New item has been added to the tree.
               runner = runner.left;
         else {
                // Since the new item is greater than or equal to the item in
                // runner it belongs in the right subtree of runner.  If there
                // is an open space at runner.right, add a new node there.
                // Otherwise, advance runner down one level to the right.
            if ( runner.right == null ) {
               runner.right = new TreeNode( newItem );
               return;  // New item has been added to the tree.
               runner = runner.right;
      } // end while
   }  // end treeInsert()

    * Return true if item is one of the items in the binary
    * sort tree to which root points.   Return false if not.
   static boolean treeContains( TreeNode root, String item ) {
      if ( root == null ) {
             // Tree is empty, so it certainly doesn't contain item.
         return false;
      else if ( item.equals(root.item) ) {
             // Yes, the item has been found in the root node.
         return true;
      else if ( item.compareTo(root.item) < 0 ) {
             // If the item occurs, it must be in the left subtree.
         return treeContains( root.left, item );
      else {
             // If the item occurs, it must be in the right subtree.
         return treeContains( root.right, item );
   }  // end treeContains()

    * Print the items in the tree in inorder, one item to a line.  
    * Since the tree is a sort tree, the output  will be in increasing order.
   private static void treeList(TreeNode node) {
      if ( node != null ) {
         treeList(node.left);             // Print items in left subtree.
         System.out.println("  " + node.item);  // Print item in the node.
         treeList(node.right);            // Print items in the right subtree.
   } // end treeList()

    * Count the nodes in the binary tree.
    * @param node A pointer to the root of the tree.  A null value indicates
    * an empty tree
    * @return the number of nodes in the tree to which node points.  For an
    * empty tree, the value is zero.
   private static int countNodes(TreeNode node) {
      if ( node == null ) {
            // Tree is empty, so it contains no nodes.
         return 0;
      else {
            // Add up the root node and the nodes in its two subtrees.
         int leftCount = countNodes( node.left );
         int rightCount = countNodes( node.right );
         return  1 + leftCount + rightCount;  
   } // end countNodes()

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