Computer Science 15-110 (Lecture 4), Spring 2010
Homework 8
Due:  Thu 25-Mar-2009 at 10pm (email copy to ytay) (no late submissions accepted).
Hw8 Submission Coordinator:  ytay

This is non-collaborative, so follow the same instructions as hw1, only replace "hw1" with "hw8" throughout.  There are no restrictions on what Java constructs you may use.

  1. Mystery Methods
  2. Transpose Musical Notes
  3. Scrabble Score
  4. The Television Class
  5. The Coke Machine Class

  1. Mystery Methods
    In the written portion of your submission, answer the following questions in general, and in just a few words of plain English.
    1. In general, what does the following method do?
      public static int mystery1(String[] a) {
        if (a == null) return 0;
        int result = 0;
        for (int i=0; i<a.length; i++)
          if (a[i] == null)
        return result;
    2. In general, what does the following method do?

      // Note:  mystery2 uses the mystery1 method from above
      public static String[] mystery2(String[] a) {
        if (a == null) return null;
        String[] result = new String[a.length - mystery1(a)];
        int j = 0;
        for (int i=0; i<a.length; i++)
          if (a[i] != null) {
            result[j] = a[i];
        return result;
    3. In general, what does the following method do?
      public static int[] mystery3(int[] a, int x) {
        if (a == null) return null;
        int[] result = new int[a.length + 1];
        for (int i=0; i<a.length; i++)
          result[i] = a[i];
        result[a.length] = x;
        return result;
    4. In general, what does the following method do?
      public static boolean mystery4(int x) {
        if (x < 2) return false;
        for (int i=2; i<x; i++)
          if (x % i == 0)
            return false;
        return true;
    5. In general, what does the following method do?

      // Note:  mystery5 uses both the mystery3 and the mystery4 methods from above
      public static int[] mystery5(int n) {
        int[] result = new int[0];
        for (int i=0; i<=n; i++)
          if (mystery4(i))
            result = mystery3(result, i);
        return result;
  2. Transpose Musical Notes
    In order to have an interesting application of arrays, we will use a simple musical note player.  First, download and compile the following program:  Run it, listen to the results.  Now, look inside that file.  You will see a method with this signature:
      public static void playNotes(int[] notes, int[] times) throws Exception {

    Here is a simple explanation of how this works, taken from the header comment in the file:
      // Simple demo of a simple musical note player.
      // It takes two arrays -- one containing the notes, the other containing the times for each note.
      // For the notes, 60 is middle C.  61 is C# (the next note).  62 is D (the note after that).
      // Legal note range:  1 to 88 (inclusive)
      // For the times, 64 is a typical quarter note, 256 a typical whole note.
      // Legal time range:  1 to 256 (inclusive)
      // If the "times" array is null, it will play all quarter notes.

    You do not need to understand music theory in order to complete this exercise.  However, the methods you write here will create arrays of integers which can be used as arguments to the "playNotes" method.  So you can hear your results!  Fun!

    Now, in the file, write the method:
      public static int[] transposeNotes(int[] notes, int step) {

    This method makes a song higher-pitched or lower-pitched.  This method takes a non-null array of notes (as described above), and another int, the "step", and returns a new array of notes where each note in the resulting "transposed" array equals the corresponding note in the original array plus the (possibly-negative) step.  There is this exception, however:  in no case shall you transpose so that any notes fall outside of the legal range of [1,88].  In such a case, you should reduce the step's magnitude by just enough to keep the transposed result in range.  For example, if the highest note in the incoming array is 80, and the step is +10, then you should reduce the step to +8.  Similarly, if the lowest note in the incoming array is 8, and the step is -10, you should change the step to be -7.  Be sure to include an appropriate test method!  Also, for fun, try transposing the sample song in the MusicalNotePlayer, perhaps as such:
      for (int step=0; step < 12; step++)
         playNotes(transpose(songNotes,step), songTimes); // a familiar song, now transposed repeatedly

    You should hear the song played repeatedly, increasing its pitch by a little bit (one-half step) each time it plays.
  3. Scrabble Score
    In the game of Scrabble, players receive points for spelling words, where each letter has an associated point value (1 for A, 3 for B, 3 for C, 2 for D, and so on...).  For example, ignoring special squares (which we will), the word "CAB" is worth 3+1+3, or 7 points.  For more details, here is the Wikipedia page on Scrabble:

    In the file, write the method:
       public static int scrabbleScore(String s, int[] letterValues)
    This method takes a String s and an array of 26 integers representing point values of the letters from A to Z in order, and returns the sum of the point values for each letter in the string.  If the string is null or empty or if it contains any characters besides uppercase letters, or if the letterValues array is null or not of length 26, the result should be -1 to indicate an error.

    If you are interested, here is an array holding the actual letter values for the English-language version of Scrabble:
       private static int[] letterValues = {
       // A, B, C, D, E, F, G, H, I, J, K, L, M
          1, 3, 3, 2, 1, 4, 2, 4, 1, 8, 5, 1, 3,
       // N, O, P, Q, R, S, T, U, V, W, X, Y, Z
          1, 1, 3,10, 1, 1, 1, 1, 4, 4, 8, 4,10

    Note:  The letterValues array may contain point values besides the official point values (say, 25 for 'A', 18 for 'B', -33 for 'C', and so on).  Your method must work for whatever letterValues array is provided.
  4. The Television Class
    Start with this file:
    Do not modify the Hw8Television main class. Make this code work by adding the appropriate classes with the appropriate methods as described by the test methods called by this main method. Note that you do not have to add any code to the test cases, though you do have to solve them with general-purpose solutions (and not just hard-code the example test cases!).

    Hint #1: look carefully at the test code to infer the behavior of the Television class.  It is straightforward (no tricks, really!), but you will not be provided with any description beyond this test code.  "Use the force, read the source!" 

    Hint #2: to solve this incrementally, you may wish to comment out parts of the test code so the parts you have implemented will compile and can be tested as you go. 
  5. The Coke Machine Class
    Start with this file:
    As with the preceding problem, do not modify this problem's main class, and make this code work by adding the appropriate classes with the appropriate methods as described by the test methods called by this main method.  Same hints apply, too.

Carpe diem!