CS 302 sect. 310
Quiz 5 (3% - 30 points)

Instructor: Dan Shiovitz

Name: _____________________________________________________________

1. Terms and Concepts (11 points)
   1. [2 points] What is null and how is it used?
      null is a Java reserved word referring to the empty reference. When a reference is created (on its own or in an array) it is initialized to null if no explicit value is given. If you store null into an array location, it replaces whatever was there previously, which generally means that object will be garbage collected (ie, deleted). But this occurs no matter what you assign into the array location: the old object will always be removed.
      
   2. [3 points] Put a checkmark in front of each phrase that is true of Java Vector objects.
      
      1. _XXX_like arrays, know their length
      2. _____can directly store both primitives and objects
         (Although you can store objects of type Integer, Boolean, Character, etc in a vector)
      3. _XXX_can dynamically change size as needed
      4. _____must be declared to store a single type
      5. _XXX_mask inefficient operations such as when an element is inserted into the middle of the vector

   3. [6 points] Draw the memory diagram produced by the following code fragment. Assume the Card class is the same as the one discussed in lecture (i.e., a card has a rank and a suit). Make sure to show the value of each memory location.

      Card[] gin = new Card[7];
      gin[1]     = new Card('A', "Clubs");
      gin[2]     = new Card('2', "Diamonds");
      gin[5]     = new Card('5', "Hearts");
      

      Ok, hmm. Every single person lost a point for forgetting that the length of an array is part of its memoryspace. Also, it's important to remember that empty spots in an array are initialized to null, and that names are references, and only refer to the actual object indirectly. The actual diagram might look something like this:

              gin [ reference ] --------------+
                                              |
                                              v
         -------------------------------------------------------------------
         |  length:  --------------------------------------------------    |
         |    7      |      | refer| refer|      |      | refer|      |    | 
         |           | null | ence | ence | null | null | ence | null |    |
         |           |      |  |   |  |   |      |      |  |   |      |    |
         |           ----------|------|--------------------|-----------    |
         ----------------------|------|--------------------|----------------
                               |      |                    |              
                               v      +--v                 +-----------v
                             ------   ------                         ------
                             |Card|   |Card|                         |Card|
                             | 'A'|   | '2'|                         | '5'|
                   "Clubs"<-----  |   |   ---->"Diamonds"  "Hearts"<---   |        
                             ------   ------                         ------
      

2. Tracing Arrays [9 points]

   Show the output for the code fragment below in the box provided. Show a trace of your execution for partial credit (the more information given, the more likely we can follow your output).

   char[][] grid = {{'Z', 'Y', 'X'}, {'W', 'V', 'U'}, {'T', 'S', 'R'}};
   
   for (int i = 0; i < 3; i++) {
       for (int j = 2; j >= 0; --j) 
              System.out.print(grid[j][i]);
       System.out.println();
   }
   

   
   
   I hope this is fairly easy if you remember that arrays are indexed row/column. Thus:

   RUX
   SVY
   TWZ
       

   
   

3. Using Arrays [10 points]

   Write a static method that returns the number of values in an array which are below the mean value of the elements of the array. So, for instance, given the array [1 5 2 3 4], the method would return 2, since 1 and 2 are below the average (3) of the array. The method is passed one parameter, an array of integers, and you may assume all elements in the array have been initialized with positive values, and that the array has at least one element.

   Also pretty straightforward, I hope.

   public static int belowMean(int[] array)
   {
     int total = 0, mean = 0, count = 0;
   
     for (int i = 0; i < array.length; i++)
       total += array[i];
   
     mean = total / array.length; // int division is ok here. why?
   
     for (int i = 0; i < array.length; i++)
       if (array[i] < mean) count++;
     
     return count;
   }