Problem Solving Assignment 4

Due Monday, February 5 at 11:59pm PST

There are two problems in this PSA.

Notes:

• This IS a pair assignment. You MUST do the assignment with your partner.

• You must complete an interview with a Lab Tutor for this PSA. Interviews are individual (partly to ensure that one partner doesn't do all the work). See instructions here for more details.

• This work should be done in its own directory called 'psa4'. Instructions here.

• This assignment has sample solution code that you can run on ieng6. You won't be able to see the java code itself =) but you will be able to do tests to see what it does, and make sure yours does the same thing. YOUR CODE MUST DO THE EXACT SAME THING. EXACT!! Wording, spaces, everything. Instructions on how to run the sample solution code are here .

• Each file must have a header with the team members' names, logins, and PIDs. The header should also describe the program in 2-3 sentences.

This homework requires skills from Chapter 1 through Chapter 3.

Problem 1

Write a Caesar Cipher secret code system.

Introduction:

• A cipher takes a sequence of characters called the "plaintext" and returns another sequence of characters called the "ciphertext" which is (a) apparently hard to read, but (b) can be converted back into the original plaintext by following a certain algorithm.
• A substitutional cipher works by substituting for each letter of the alphabet another particular letter of the alphabet; recovering the plaintext then just involves doing the reverse substitution. In this assignment, you will implement a kind of substitutional cipher called a rotational cipher or Caesar cipher (check out this for more on codes, ciphers, and codebreaking). There are many better ways of securely encrypting sensitive information these days. But still, substitutional ciphers are useful in some contexts.

Details:

• Your program must be in a file called TopSecret.java
• First, ask the user if they would like to encrypt or decrypt a message. The user should enter "E" to encrypt and "D" to decrypt. Read this as a string. As you practiced in class on Monday, you need to chastise the user if they enter something other than "E" or "D", like "encrypt" or "blahblah". (keep chastising and re-asking until you get "E" or "D")
• Then ask the user for the key to "lock" and "unlock" the message. This should be a whole number between 1 and 25 (inclusive) (plus hitting "ENTER"). To decrypt a message, the user provides the same key value that encrypted it. Again, you need to chastise the user if the key is not between 1 and 25. (keep chastising and re-asking until you get a valid key) (Note: you can assume that the user enters some integer, you are just checking the value. You will need to do an "extra" scanner nextLine() to get rid of the "ENTER" as shown on pg. 81 of your book, line 24 of the code example)
• Then ask the user to input the text to encrypt or decrypt (a string, on one line).
• Finally, output the encrypted or decrypted string. All non-letter characters (punctuation, numbers, spaces, etc,) should remain in place. Also, capitalization should be preserved.

Encrypting and Decrypting:

• This is a rotation cipher. So the key the user entered is how many times to shift the alphabet.
• For example, if the user chooses 3 for the key, you will shift all letters by 3, with wraparound. So A becomes D, B becomes E, and so on up to W, which becomes Z. Then the sequence wraps around: X becomes A, Y becomes B, and Z becomes C. (that's why 25 is the maximum value for they key--26 would just shift it back to A = A, B = B, etc).
• Your program needs to both encrypt and decrypt messages. Luckily, you don't need to write separate code to do the decryption. To decrypt, simply adjust the key the user entered to be a value that will rotate the alphabet back to the original. Then continue with your regular encryption procedure. (Big hint: remember that 26 total rotations takes you back to start.)
• You can see a sample solution by following these instructions.

String and Unicode Hints:

• Remember that a string is an immutable object (it cannot be changed once it has been initialized), so you will be creating another string that contains the cipher. Actually, you will be creating the string character by character, starting from the empty string. So you will be creating a whole sequence of strings along the way.
• Do NOT use the standard Java class "Character" or any of it's static methods in this assignment. Additionally, NEVER use Unicode values directly (or as named constants) in the code (like 97). Your code should work no matter if someone changes Unicode values as used by Java. You can assume that the "ordering" of Unicode values will remain the same (that is 'a' will always be less than 'z'). Big hint: See that last statement-- I compared letters! ('a' < 'z')
• Update 2/1/07 9am: You are allowed to use variables of type char (although additional casting will be needed). And String class methods are ok, in particular, you should be thinking about using the String class method charAt(). The ban on Character class is to prevent you from wasting your time scouring that class looking for a solution to this problem, because you won't find it there.
• More about Unicode: All the uppercase letters have int values in the range 65 (which is the int value of the char 'A') through 90 (which is the int value of 'Z'). Similarly, the lowercase letters have the 26 int values in the range 97 through 122, which are the int values of 'a' and 'z' respectively. When computing a letter substitution, the resulting letter has to have an int value in the same range. Hint, some type casting here will allow you transform one letter into another.
• Hint: I recommend that you think of each letter having an offset from either 'a' or 'A'. Where that offset is in the range of 0 - 25. So the letter 'c' would have an offset of 2 from the letter 'a'. (How can you compute an offset from 'a'/'A'? You want to add and subtract, but you need to do some casting first.)
• There will be some extra problems with wraparound. Though 'z's original offset is 25, it's rotated offset (with for example key=3) is not 28! All offsets should be in the range of 0-25. "Wrapping back around" will involve determining when you have "gone over then end" and then finding out the "left over amount" you have gone over then using that amount to offset from the beginning of the alphabet.

Problem 2

Write a virtual Etch-A-Sketch picture-drawing program.

Introduction:

• Maybe you had one of these as a kid:
• There are two knobs that control moving around a pin that draws on a board. To start over, you just shake it and the drawing erases. You'll be making a virtual Etch-A-Sketch game. Yours will differ from the original in that it will have a typing-based interface to give drawing commands. Also, yours will feature a really awesome turtle graphic. The turtle draws a line wherever it goes, just like the EtchASketch pin. To draw pictures, you move the turtle around the screen.

Details:

• Important: This program uses graphics windows that will probably only display correctly if you are working in the Lab. You will not be able to use putty to work on this assignment. (Actually, it might be possible depending on your software, and I put a couple suggestions on the webboard. But it would be easiest to just work in the lab.)
• Important: To see how to run sample solutions, go here
• Your program must be in a file called EtchASketch.java. We have provided some starter code that you will need. So to begin your assignment, do this (after making your psa4 directory):

  -bash2.05b$ cp ~/../public/EtchASketch.java ~/psa4/

  Your code will all be inside a method called "run" instead of the usual "main". Don't worry about the other code in the file, just leave it alone. It is ok if you don't understand what it is doing. So, inside the "run" method, you should do the following:
• First, ask the user if they want the turtle to go forward, turn right, or turn left, or if they want to quit the program. You should read in a line of input from the user, then just examine the first letter only to see if it is G/L/R/Q (indicating go/left/right/quit). Allow the user to also use lower case, so a first letter of 'l' or 'L' means left, 'r' or 'R' means right, etc. If the user entered something other than a word starting with one of the allowed letters, chastise as usual.
• If the user wants to go forward, ask a follow-up question: how far? The number must be a double. If the user enters a double that is less than zero, chastise as usual. (How far should the user tell the turtle to move per turn? For reference, the turtle's window is 400 by 600. But we won't be checking to see if the turtle goes off the edge.)
• If the user wants to turn towards the left or right, ask a follow-up question: how many degrees? The number must be a double. If the user enters a double that is less than zero or more than 360, chastise as usual.
• Then you need to move the turtle as requested by the user. There is an example in the provided code, but obviously it doesn't do what the user wants. So delete those lines and add back in something similar that does the right thing. Basically, you need to call these methods:
  • turtle.forward(double);
  • turtle.left(double);
  • turtle.right(double);
  You'll notice that the turtle variable is not declared anywhere in your run() method. Don't worry, we took care of that for you already, somewhere else in the provided code.
• Until the user choses to quit, you should present the original choices (go, left, etc) again after moving the turtle so the user can keep drawing.
• Little note: you should add this line as the very LAST line in your run() method: "System.exit(1);" This means that when you get to the end of the run() method, you want to quit the program. Don't use this code anywhere else in your work.
• As always, run the sample solution if you want to see how it all comes together. The pic above is a screenshot.
• Post screenshots of your coolest EtchASketch drawings to the class WebBoard. Prof. Simon might show yours in class!

Details

Start by creating a directory (folder) in your ieng6 cs8fzz account (this is necessary for the bundle-psa4 script). After you log in, create a directory by typing the commands:

-bash-2.05b$ mkdir psa4

Go into that directory (like double clicking to open a folder) with the command:

-bash-2.05b$ cd psa4 

Note: you must name your directories (folders) and files as indicated in the problem descriptions for the turn in program to work.

In order to run the sample solution program you should invoke the following command:

-bash-2.05b$ runsample [classname]

So for this assignment, do:

-bash-2.05b$ runsample TopSecret

or

-bash-2.05b$ runsample EtchASketch

To see a sample solution WITH graphics, you can do:

-bash2.05$ cd ~/../public
-bash2.05$ java EtchASketch

To go back to your own directory after you are done, do

-bash2.05$ cd ~/psa4/

To see a sample solution with NO graphics, you can do:

-bash2.05$ runsample EtchASketchMinimal

This will run a version of the sample solution that is exactly like the real one in terms of all the user interaction and input from the user but has NO GRAPHICS. It just has little placeholder S.o.p() statements like "[moving turtle left 20 degrees]" and so on. This is useful if you want to do SOME work from home, then come to lab to make sure the graphics part is working.

This assignment is worth 20 points.

General (4 pts):
• 1 Point: For proper formatting. Your code is properly and consistently indented.
• 1 Point: For documentation. You must write a 2-4 sentence description of the program (as a comment) that provides insight into what the program does and what it's useful for. In this header comment, you are also required to put the author names, author PIDs and author logins
• 2 points: Interview (done individually) see instructions here.

TopSecret.java (8 pts):

• 2 points: Correctly input encrypt/decrypt (including chastising)
• 2 points: Correctly input key (including chastising)
• 3 points: Correctly output encrypted message
• 1 points: Correctly output decrypted message
• Everything must match the sample solution exactly.

  EtchASketch.java (8 pts):

• 2 Points: Correctly check user's main menu choice (including chastising)
• 2 Points: Correctly check user's distance and degree choices (including chastising)
• 1 Point: Correctly move turtle left
• 1 Point: Correctly move turtle right
• 1 Point: Correctly move turtle forward
• 1 Point: Correctly end game
• Everything must match the sample solution exactly.

How to turn in your homework electronically

When you are ready to turn in your homework, run bundle-psa4 from your psa4 directory. The session should look like this:

-bash-2.05b$ cd ~/psa4
-bash-2.05b$ bundle-psa4
Good; all required files are present:

        TopSecret.java
        EtchASketch.java

Do you want to go ahead and turnin these files? [y/n]y
OK.  Proceeding.


Performing turnin of approx. XXXX bytes 
Copying to /home/linux/ieng6/cs8w/turnin.dest/cs8wzz.psa2
.
Done.
Total bytes written: XXXX 
Please check to be sure that's reasonable.
Turnin successful.

If you want to make changes after you've turned in your homework, make the changes and run bundle-psa4 again. The program will warn you that a "previously turned-in file exists" and ask if "you wish to over-write this existing file" (delete the old version and replace it with this new one). Respond in the affirmative by typing y and pressing the Enter key.

10% of your grade on this assignment will come from an interview you will have with one of the tutors during Open Lab Hours. You have 48 weekday hours after the electronic turnin deadline to get your interview done. The deadline for the interview for PSA 3 is Wednesday, Jan 31, theoretically at midnight, but actually by the end of the last tutor hour scheduled. You get an interview by going into B260 during a scheduled tutor hour. Find the tutor, ask them if you can interview for PSA 3. If they have others who are waiting for interviews, your name will go on the end of the list of people waiting. Interviews take about 5 minutes each. It is your responsibility to go in early enough that you can get your interview completed. If everyone waits until the last hour on Wed, this will not work. The tutor will ask you to log in, change to the directory where your work is (psa4) and then to follow their directions. They will ask you to run your program to show that it works, open files in your favorite editor, and ask you questions about how your code works. Additionally, they may question you as to how you would (theoretically) make small changes to the program to have it do something slightly different. Learning to be able to explain your problem solving process and discuss code with another human being is a very important professional skill. We hope that this process will help you develop this skill.

Copying the assignment directory to partner's home directory

You can copy the psa4 directory and its files to your partner's home directory by:

bash% scp -r psa4 cs8wxx@ieng6.ucsd.edu:

(don't forget the colon at the end of the command) where cs8wxx is your partner's username. You must be in the parent directory of the psa4 directory. So, you if you are currently in the psa4 directory you should execute "cd ..". Similarly, if the psa4 direcory is in your home directory then you have to be in your home directory. If you aren't there just execute "cd ~" and you'll be there. Your partner is required to enter his password when you enter the scp command. To check if the copy worked, your partner must login and ckeck if the psa4 directory exists in his home directory and if the proper files are located in it. This can be done by executing: "ls ~/psa4". The files in the psa4 directory should be listed.

Redirecting the input and the output

You can redirect the input af a program by using the "<" symbol in the command line. Instead of reading the input from the standar input device (which is the keyboard):

bash% java MyProgram < sampleEmail.txt

This command will redirect the input of the MyProgram program to the file sampleEmail.txt. It will read the contents of the file as if they were given by pressing the buttons on keyboard. Similarly you can redirect the output af a program by using the ">" symbol in the command line. Instead of displaying the output on the standar output device (which is the computer monitor):

bash% java MyProgram > outFile.txt

This command will redirect the output of the MyProgram program to the file sampleEmail.txt. It will write the output in the "outFile.txt" file (all contents of the will be erased if file exists, if it doesn't exist, it will be created) as if they would be printed on the computer screen (so, you won't get any output on the screen). You can combine both redirections. For example you may have an email.txt file with a sample email, and also you want the results of the scan to be put on the file output.txt. This can be done by:

bash% java MyProgram < email.txt > output.txt

Finding the differences between files

You can find the differences between two text files by using the diff command:

bash% diff filename1 filename2

The output will be the differences between the files. If there is no output at all that means that the files are identical.

Example: Testing the output of your program, can be completed in four steps, using the above tips.

Create a file with sample input (lets name it "input.txt" for your program (as if you would write it if the program was running)

Run your program and redirect the exit to a file: bash% java Statistics < input.txt > output1.txt

Run the provided program and redirect the exit to a second file: bash% runsample Statistics < input.txt > output2.txt

Check if there are differences between the two output files: bash% diff output1.txt output2.txt

Learning to use text editors

Vim

Run the command vimtutor, a separate program that will teach you to use Vim.

Emacs

Use the built-in tutorial by running emacs and, within the editor, typing CTRL-h t.

Pico

You can reach Pico's built-in help by typing CTRL-g. If you want further instruction, look at this Pico tutorial from NC State.