Computer Science 15-110, Lecture 9 (Sections M-Q), Fall 2009
Homework 4
Due:  Thu 1-Oct-2009 at 11:59pm (email copy to your CA)
(no late submissions accepted).

Read these instructions first!

• The hw instructions from hw1 all apply here.  In particular:
   
  • This is not a lab and this is not collaborative!
     
  • You must work alone on this assignment (except for help from CA's or the course instructor, who you are always welcome to consult).
     
• Submit all your source (.java) file(s) all in a single zipped folder, hw4-<andrewId>.zip, sent by email as an attachment to your CA.  For example, I would submit hw4-koz.zip (you should replace "koz" with your andrewId).
   
  • Do not submit .java~ files (with the tilde (~) at the end).  These are DrJava backup files.  They are not useful.
     
  • Do not submit .class files.  These are compiled bytecode files.  They also are not useful.
     
  • Do not submit project files or any other files.  Just .java files and your one free-response file.

• Style counts!!!  Use well-named variables, proper indenting, reasonable commenting, etc.
   
• Write test methods for every non-graphical method you write!!!
   
• Note:  You may use conditionals ("if" statements or ternary operators (?:)) and loops.  However, you may not use Java concepts we have not yet covered, including arrays, or methods from any classes in java.util.* (besides Scanner or others we explicitly use) to solve these problems (which isn't a problem for most of you, seeing as we have not yet covered these!). While they may be helpful, every problem here is solvable without them, and they are not permitted for now.

1. nthEmirpPrime
2. mostCommonVowel
3. "Loopy" Animation
4. Bonus/Optional: longestCommonSubstringLength
5. Bonus/Optional:  Simple Invaders Game

1. nthEmirpPrime
   In the file Hw4.java, write the method:
      public static int nthEmirpPrime(int n)
   This method takes an int n and returns the nth "Emirp prime", where an Emirp prime is a prime number which becomes a different prime number when its decimal digits are reversed.  For example, 13 is an Emirp prime because when we reverse the digits we get 31, which is also prime.  2, 3, 5, 7, and 11 are all examples of primes that are not Emirp primes because they are the same prime when the digits are reversed.  Here are the first several Emirp primes:
     13, 17, 31, 37, 71, 73, 79, 97, 107, 113, 149, 157, 167, 179, 199,...
   So if n==1, your method should return 13.  If n==2, it should return 17.  And so on.  If n is less than 1, your method should return -1.
   
   Aside:  in case you missed it, emirp is prime spelled backwards.  Also, we didn't make this up (look up "emirp" on Wikipedia)!
    
2. mostCommonVowel
   In the file Hw4.java, write the method:
      public static char mostCommonVowel(String s)
   This method takes one string and returns the lowercase vowel ('a', 'e', 'i', 'o', or 'u') that occurs most often in the string, regardless of case (so "Aha!" contains two a's).  In the case of a tie, the first vowel alphabetically is returned (so "Yippee, I say!" would return 'e', even though it also has 2 i's).  If the string is null or does not contain a vowel, the method should return ((char)0), which is the character with unicode value 0 (which no character has!).
    
3. "Loopy" Animation
   First, take a close look at the (updated) class notes on nested loops and then, especially, the class notes on nested loops in graphics.  Compile that code, run it, be sure you thoroughly understand it, because you're going to need it!   While you may not discuss your solutions to the homework, you may discuss the class notes with each other, with your CA's (of course), with anyone else.  Do not start this problem until you understand those notes very well!
   
   Then, in the file Hw4Animation.java, write this "loopy" animation:
        Hw4AnimationDemo.jar   (also available in Hw4AnimationDemo.zip)
   Download and run the demo program (Hw4AnimationDemo.jar), which shows how this program should work once it is completed.  Carefully review the program so you do not omit any details (such as pausing, or resizing with arrows, or clicking to move the selection).
   
   Hint:  divide-and-conquer!  That is, solve this problem in small, independent sub-problems.  Also, use top-down design, which basically means to solve each sub-problem using plenty of well-chosen helper methods.
    
4. Bonus/Optional:  longestCommonSubstringLength
   In the file Hw4.java, write the method:
      public static int longestCommonSubstringLength(String s1, String s2)
   This method takes two strings and returns the length of the longest string s3 such that both s1 and s2 contain s3.  If no such string exists (say, if either s1 or s2 is null), then your method should return 0.
   
   Note:  your method must not create any new strings (because that would be very inefficient!).  The string "s3" in the description is just to help you understand the problem.  You may not create such an "s3" in your solution, and in fact you should not have any variables of type String except s1 and s2, which you should not modify.
   
   Also:  the only methods in the String class you may use are charAt() and length().
    
5. Bonus/Optional: Simple Invaders Game
   In the file Hw4BonusInvaders.java, write this simple version of a SpaceInvaders-like game:
        Hw4InvadersDemo.jar  (also available in Hw4InvadersDemo.zip)
   Download and run the demo program (Hw4SimpleInvadersGame.jar), which shows how this program should work once it is completed.
   Hint:  you may want to use Math.random() here.  Or you can read about using the Random class.  Your choice.

Carpe diem!