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Solution for Programming Exercise 10.5

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

Exercise 10.5:

An example in Subsection 10.4.2 concerns the problem of making an index for a book. A related problem is making a concordance for a document. A concordance lists every word that occurs in the document, and for each word it gives the line number of every line in the document where the word occurs. All the subroutines for creating an index that were presented in Subsection 10.4.2 can also be used to create a concordance. The only real difference is that the integers in a concordance are line numbers rather than page numbers.

Write a program that can create a concordance. The document should be read from an input file, and the concordance data should be written to an output file. You can use the indexing subroutines from Subsection 10.4.2, modified to write the data to TextIO instead of to System.out. (You will need to make these subroutines static.) The input and output files should be selected by the user when the program is run. The sample program WordCount.java, from Subsection 10.4.4, can be used as a model of how to use files. That program also has a useful subroutine that reads one word from input.

As you read the file, you want to take each word that you encounter and add it to the concordance along with the current line number. Keeping track of the line numbers is one of the trickiest parts of the problem. In an input file, the end of each line in the file is marked by the newline character, '\n'. Every time you encounter this character, you have to add one to the line number. WordCount.java ignores ends of lines. Because you need to find and count the end-of-line characters, your program cannot process the input file in exactly the same way as does WordCount.java. Also, you will need to detect the end of the file. The function TextIO.peek(), which is used to look ahead at the next character in the input, returns the value TextIO.EOF at end-of-file, after all the characters in the file have been read.

Because it is so common, don't include the word "the" in your concordance. Also, do not include words that have length less than 3.


Solving this exercise, for the most part, means collecting subroutines that were presented in Subsection 10.4.2 into a complete program. I copied the method addReference() directly, just making it static and changing the names of the parameters to names that are more appropriate for a concordance. I copied printIntegers(), changed its name to printConcordance, made it static, and changed "System.out.print" to "TextIO.put" so that it would send its output to the file selected by the user.

In Subsection 10.4.3, I discussed the problem that arises when comparing strings that differ only in their use of upper and lower case. In that subsection, I used a Comparator to solve the problem. In my solution to this exercise, however, I used the simpler solution of converting all words to lower case immediately after reading them from the file. For lower case words, the default ordering is alphabetical order, so I don't need a Comparator.

It remains to write the main() routine, which is similar in outline to the main() routine in WordCount.java. One difference is that we have to keep track of line numbers. We can do this by looking for new-line characters as we skip over the characters between words. While we do this, we have to be on the alert for the special character TextIO.EOF that is returned by TextIO.peek() when the end of the file is reached. A pseudocode algorithm can be given as:

Open the input files.

Let lineNum = 1.

      // Skip over any non-letters in the input, stopping when a
      // letter (marking the beginning of a word) or EOF is found.
      while the next character is not end-of-file or a letter:
         Read the next character.
         If it is a new line character:
            Count the line by adding 1 to lineNum
      // After the while loop, we are looking at either the end of file
      //    or at a letter that is the beginning of the next word.
      If at end-of-file:
         Exit from the loop.
      Get the next word from the input file.
      Convert the word to lower case.
      if the word is not "the" and has length > 2:
         Add the word and lineNum to the concordance.

Open the output file.

Print the concordance.

This can be translated directly (using some code from WordCount.java) to give the main() routine. For reading individual words from input, the method readNextWord() can be copied directly from WordCount.java. (In my program, I used a slightly simpler method named readWord() which uses the fact that when readWord() is called by the main() routine, the next character in input is already known to be a letter.)

The Solution

import java.util.Map;
import java.util.TreeMap;
import java.util.TreeSet;

 * This program makes a concordance for a file.  A concordance
 * lists all the words that occur in the file, along with all
 * the line numbers on which each word occurs.  (Words of
 * length less than 3 are omitted, and "the" is omitted.)  The
 * concordance is written to an output file.  The user selects
 * the input and output files using file dialog boxes.  This
 * program uses the non-standard class, TextIO.
public class Concordance {

 * This TreeMap holds the concordance.  Words from the file
 * are used as keys in the map.  The value associated with
 * each word is a set that contains the line numbers on which
 * the word occurs in the file.  The set contains values
 * belonging to the wrapper class, Integer.
private static TreeMap<String, TreeSet<Integer>>  concordance;

public static void main(String[] args) {

   System.out.println("\n\n   This program will ask you to select an input file.");
   System.out.println("It makes a list of all the words that occur in the file");
   System.out.println("along with the line number of each line that contained");
   System.out.println("that word.  This is called a \"concordance\" for the file.");
   System.out.println("   After reading the input file, the program asks you to");
   System.out.println("select an output file.  If you select a file, the list of");
   System.out.println("words will be written to that file; if you cancel, the list");
   System.out.println("be written to standard output.  All words are converted to");
   System.out.println("lower case.\n\n");
   System.out.print("Press return to begin.");
   TextIO.getln();  // Wait for user to press return.
   try {
      // Let user select the input file.  If the user cancels,
      // the program ends immediately.
      if (TextIO.readUserSelectedFile() == false) {
         System.out.println("No input file selected.  Exiting.");
      // Create the data structure that will hold the concordance.
      concordance = new TreeMap<String, TreeSet<Integer>>();
      int lineNum = 1;  // The number of the line in the input
                        // file that is currently being processed.
      // Read words from the file until end of file is reached,
      // and add each word to the data.

      while (true) {
         char ch = TextIO.peek(); // Look ahead at next character
         while ( ch != TextIO.EOF && ! Character.isLetter(ch) ) {
                   // Skip over non-letter characters, stopping when 
                   // end-of-file (TextIO.EOF) or a letter is seen.  If the
                   // character is an end-of-line character, add 1
                   // to lineNum to reflect the fact that we are moving
                   // on to the next line in the file.
            TextIO.getAnyChar();  // Reads the next character, which is junk.
            if (ch == '\n') {
               lineNum++;  // Start of a new line.
            ch = TextIO.peek();  // Look at the next character.
         if (ch == TextIO.EOF) {
                 // The end-of-file has been reached, so exit from the loop.
         String word = readWord();  // The next word from the file.
         word = word.toLowerCase();
         if (word.length() > 2  && !word.equalsIgnoreCase("the")) {
                 // Add the reference to word to the concordance, unless
                 // the word is "the" or the word has length <= 2.
      // Write the data to a user-selected file, or to standard
      // output if the user does not select an output file.

      System.out.println(concordance.size() + " distinct words were found in the file.\n");
      if (concordance.size() == 0) {
         System.out.println("No words found in file.");
         System.out.println("Exiting without saving data.");

      TextIO.writeUserSelectedFile(); // If user cancels, output automatically
                                      // goes to standard output.
      printConcordance();  // Print the data to the output file.
   catch (IllegalArgumentException e) {
      System.out.println( "Sorry, some error occurred:  " + e.getMessage() );

} // end main()

 * Writes the data in the concordance to TextIO.  (The output will go
 * to the output file, if one has been selected; otherwise, it will go
 * to standard output.)  Each line of output contains one word from the
 * file and a list of lines on which that word occurred.  The words
 * are in alphabetical order.
private static void printConcordance() {
   for ( Map.Entry<String, TreeSet<Integer>>  entry :  concordance.entrySet() ) {
      String term = entry.getKey();
      TreeSet<Integer> pageSet = entry.getValue();

      TextIO.put( term + " " );
      for ( int page : pageSet ) {
         TextIO.put( page + " " );

 * Add a word reference to the concordance.
private static void addReference(String word, int lineNum) {
   TreeSet<Integer> references; // The set of lines where we have
                                //    previously found the word.
   references = concordance.get(word);
   if (references == null){
          // This is the first reference that we have
          // found for the word.  Make a new set containing
          // the line number and add it to the concordance, with
          // the word as the key.
       TreeSet<Integer> firstRef = new TreeSet<Integer>();
       firstRef.add( lineNum );  // lineNum is "autoboxed" to give an Integer!
   else {
         // The variable references is the set of line references
         // that we have found previously for the word.
         // Add the new line number to that set.  This
         // set is already associated to word in the concordance.
      references.add( lineNum ); // pageNum is "autoboxed" to give an Integer!

 * Read the next word from TextIO.  It is assumed that the next character
 * in input is a letter.
 *    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 readWord() {
   char ch = TextIO.peek(); // Look at next character in input.
   assert Character.isLetter(ch);
   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 break 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.

} // end class Concordance

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