CS100, Spring 2000, Solutions to Prelim 2

1.  // read non-ascending sequence of even #s;
    // for each n, print a centered row of n stars
    int biggest = in.readInt(); // max # stars to print in any row
    for (int n = biggest; n > 0; n = in.readInt()) {
        // print (biggest-n)/2 spaces
            for (int i = 0; i < (biggest-n)/2; i++) System.out.print(" ");
        // print n stars
            for (int i = 0; i < n; i++) System.out.print("*");
        // advance to next line
            System.out.println();
    }

    Thought question: Suppose we wanted to somewhat relax the conditions
    "non-ascending" and "all even".  What changes to the code are required
    and/or what weaker conditions suffice?

2. "Crossed-out objects" are simply omitted below.
    "nu*l" stands for "crossed out $null$".
    "0/1/2" stands for "crossed out 0, crossed out 1, 2".
    We ignored $args$ of method main both below and in your solutions.
    Do not confuse $null$ with "undetermined/nonsense/garbage value".

              +--------------------------------+
              |                                |
class String  |         class Glorph           |            
+-------------+-+       +----------------------+-----+
|             | |       |                      |     |
| "sputty"<---+ |       | +----------+  +------+---+ |
|               |       | |   +----+ |  |   +--+-+ | |
+---------------+       | | s |null| |  | s |nu*l| | |
                        | |   +----+ |  |   +----+ | |
class Problem2          | |          |  |          | |
+---------------+       | |   +----+ |  |   +----+ | |
|               |       | | g |null| |  | g |null| | |
| main (args)   |       | |   +----+ |  |   +----+ | |
| +-----------+ |       | |          |  |          | |
| |    +----+ | |       | |   +----+ |  |   +----+ | |
| | G1 | *--+-+-+-------+>| w |nu*l| |  | w |nu*l| |<+--------+
| |    +----+ | |       | |   +--+-+ |  |   +--+-+ | |        | 
| |           | |       | +------+---+  +------+---+ |        | 
| |    +----+ | |       |        |             |     |        | 
| | W1 | *--+-+-+----+  +--------+-------------+-----+        | 
| |    +----+ | |    |           |             |              | 
| |           | |    |           +---+  +------+              | 
| |    +----+ | |    |               |  |                     | 
| | W2 | ?  | | |    |  class Whappy |  |                     |  
| |    +----+ | |    |  +------------|--+----------------+    | 
| +-----------+ |    |  |            V  |                |    | 
|               |    |  | +----------+  |   +----------+ |    | 
+---------------+    |  | |   +-----+|  |   |   +----+ | |    | 
                     +--+>| x |0/1/2||<-+   | x | 0  | | |    | 
                        | |   +-----+|      |   +----+ | |    | 
                        | |          |      |          | |    | 
                        | |   +----+ |      |   +----+ | |    | 
                        | | g |nu*l| |      | g |null| | |    | 
                        | |   +--+-+ |      |   +----+ | |    | 
                        | +------+---+      +----------+ |    | 
                        |        |                       |    | 
                        +--------+-----------------------+    | 
                                 |                            | 
                                 +----------------------------+

3.  class Worker {
        int id;
        Worker boss;
        
        // return company owner
        Worker owner() {
            Worker topboss = this;
            while (topboss.boss != null) topboss = topboss.boss;
            return topboss;
        }
        
        // return description: ID, boss's ID (if any), owner's ID
        String describe() {
            String s = "ID: " + id + ", ID for boss: ";
            if (boss == null) s += "NO BOSS"; else s+= boss.id;
            s += s + ", ID for owner: " + owner().id;
            return s;
        }
    }

another solution to describe:

  String s = "NO BOSS";
  if (boss != null) s = "" + boss.id;
  return "ID: " + id + ", ID for boss: " + s + ", ID for owner: " + owner().id;

4.  Worker thirty = new Worker ();
    thirty.id = 30;
    thirty.boss = sixty;
    five.owner().boss = thirty;
    System.out.println(x.owner().describe());
    if (x.owner() != y.owner())
        System.out.println(y.owner().describe());

B1. 70 total possible core project pts; 55 required for perfect project score.

B2. "loop storyline" or "loop invariant": tank contains 2*time liters of water.

B3. Worker owner() { return boss == null ? this : boss.owner(); }

B4. void dtd() { 
        if (13 <= age && age <=19) describe(); 
        if (youngestChild != null) youngestChild.dtd(); 
        if (olderSibling != null) olderSibling.dtd();
    }

B5.    4      or    4     
      / \          / \
     2   3        3   2
     |                |
     1                1