CS100, Spring 2000, Part 1 of Review questions for Prelim 2

[Part 2 should be available in a day or two.]

0. Do Exercise 3 or any of the other programming assignments (again) 
   if you haven't done them or had trouble with them.  Go over our sample
   solutions -- make sure you understand how they work and how they compare
   to yours.

1. [Maybe a little too vague for a prelim, but useful for you to think about.]

   Each line in the program segment below has
   something wrong with it.  Fix each line.

   int 3 = x;
   if (7 - x = 4)
       System.Out.Println("four is " + 4);
   int x = "4";
   if (3 < 4, 4 < 5)
       System.out.println(4 is between 3 and 5);

2. [Maybe a little too vague for a prelim, but useful for you to think about.]

   Briefly answer each question below.

   (a) What is the difference between "=" and "=="?

   (b) What is the difference among the following three statements?

       i) int x;    ii) int x = 3;    iii) x = 3;

   (c) Is there any difference between $fred$ and $Fred$?

   (d) Assume the code below is correct.   Indent it properly.
       Note that there is more than one correct answer.

while (x<0) { counter *= 10; if (y<z) y++; else z = z*2; x = x+2; w = w/7; }

3. [Having both parts would make this question too long for a prelim.]

   Write code to print "hollow n-triangles".

   Note: the equations involved are *much* simpler than on Project 3
         because the slopes are "simple", e.g. -1.

a) Write code to read a positive integer n from input and print a 
   hollow n-triangle.  Examples for 1, 2, 3, 4, 5 are shown below.
   
   __ Assume $TokenReader in = new TokenReader(System.in);$
      has already been included.
   __ Use good style
   __ Clearly indicate in a comment how many "hollow" spaces you
      intend to print on each line.
   __ Clearly indicate in a comment how many "padding" spaces you
      intend to on each line before the first star.

   *  **   ***    ****     *****
       *    **     * *      *  *
             *      **       * *
                     *        **
                               *

b) Same question as (a), but assume $n$ is odd, 
   and print isosceles triangles, as shown below for 1, 3, 5, 7.

   *   ***   *****   *******
        *     * *     *   *
               *       * *
                        *
 
4. [This was a prelim question from last semester.]

   Write code to read a sequence of positive integers and print out only
   the "perfect" numbers.  The sequence is terminated by 0.

   Definition: a number m>0 is "perfect" if it is equal to the sum of all
   its proper divisors d (proper means 0<d<m), e.g. 6=1+2+3 and
   28=1+2+4+7+14 are perfect, but 5 != 1 and 15 != 1+3+5 are not perfect.
   It turns out 1 is also not perfect: 1 has no proper divisors, so the
   sum of its proper divisors is 0, which is different from 1.

   __ Declare all variables.
   __ Use good style.
   __ Use the pattern for processing input with a stopping value.
   __ Use the pattern for repeating something a given number of times.
   __ No arrays.  No methods.  No error-checking.  No additional prompts.

5. Fill in the box to complete the method below.  (The box is small here
   to save space; we'd give you plenty of space on a prelim.)

   (Assume that $g$ has been properly translated so that the origin (0,0)
   is placed exactly at the upper-left corner of the window, just beneath
   the title bar.)
 
   Recall that $g.clearRect(left,top,width,height)$ erases a rectangle
   and $g.fillOval(left,top,width,height)$ draws a filled-in oval.
 
   Recall that $Math.random()$ returns a random floating point number
   between 0 and 1, exclusive.

   // Draw the contents of a window $w$ pixels wide, $h$ pixels high:
   // erase the contents, and then draw 3 filled-in, randomly placed
   // but fully visible circles:
   //  a Color.yellow circle of radius 10,
   //  a Color.cyan circle of radius 20, and
   //  a Color.magenta circle of radius 30.
   void paint(Graphics g, int w, int h) {
       +----------------------------------------------------------------------+
       |                                                                      |
       |                                                                      |
       |                                                                      |
       |                                                                      |
       +----------------------------------------------------------------------+
   }

6. Consider replacing the line marked "***replace this line***"
   with the lines of code specified in parts (a), (b), (c), below.

   What output is produced in each case by running the resulting program?

a) x=y; y=z;        
b) x=y; y.k=z.k;    
c) x.k=y.k; y.k=z.k; 

class Person{
    int k;
    void print() { System.out.println(k); }
} 

public class Equal_notEqual {
    public static void main(String[] args) {
        Person x = new Person(); 
        Person y = new Person(); 
        Person z = new Person(); 
        x.k=1; y.k=2; z.k=3; 
        // ***replace this line***
        x.print(); 
        y.print(); 
        z.print(); 
    }
}