MP3: Encryption

Computers are frequently used to transform and analyze data. Rendering web pages, playing music, displaying video, compressing files, and graphing data are all examples that involve either transformation, analysis, and sometimes both.

MP3 comprises two string-based tasks that involve straightforward transformation and analysis tasks. It is due Friday, October 6th, 2017 @ 5PM. To receive full credit, you must submit by this deadline. In addition, 10% of your grade on MP3 is for committing code that earns at least 20 points by Tuesday, October 3rd, 2017 @ 5PM.

1. Learning Objectives

The purpose of MP3 is to begin to introduce you to data processing, including both transformation and analysis. You’ll begin to learn how to:

  1. programmatically transform strings by implementing both simple encryption and decryption

  2. complete a simple (but useful) cryptography-related data analysis task

  3. write Javadoc comments

  4. not start the MP at the last minute

We’ll also continue to reinforce the learning objectives from MP1 and MP2.

2. Assignment Structure

MP3 consists of two short programs:

  1. CaesarCipher.java: encrypts and decrypts text using the so-called Caesar Cipher

  2. CharDistribution.java: parses text and computes the distribution of each of the alphabetic characters

You may find our official MP3 online documentation helpful in understanding what each function is supposed to do.

2.1. Obtaining MP3

Please follow the instructions from MP1. You’ll find MP3 in an MP3 folder in your Subversion home directory. Unless you want to learn a lot more about Subversion, do not delete your MP3 folder once you have it.

2.2. Your Goal

At this point you should be familiar with the requirements from MP1 and MP2. MP3 is similar. The only small difference with MP3 is in the grading, so see below.

3. Fixing Problems

All of the MP1 and MP2 about how to approach the MPs still apply. But as usual, we’ll furnish your continued development as programmers with some additional advice—this time about fixing problems.

We’ve all been there. My stupid code is not working! Why not? I’m angry, I’m frustrated, I want to throw my computer through a window or stomp it to bits. What should you do?

3.1. Calm Down

Well, the first thing to do is calm down. This can be a good time to get a cup of coffee, take a walk, engage in some YouTube browsing, catch up on email, or whatever 1 Get your head out of what you are doing for a few minutes and you’ll be fresher when you get back.

3.2. Accept Responsibility

Second, accept that something is wrong with your code. If it’s not doing what you want, then that’s obvious. And for CS 125, the test suites are the arbiters of correctness. So if you aren’t passing them, then your code is incorrect. Period.

3.3. Do A Slow, Close Read

Now that you’re calmer and ready to find your mistakes, go through your code carefully line by line. This can be a good time to do some cleanup—add comments where needed, improve your variable names, get all of your indentation and formatting worked out, etc. Sometimes looking at your code in a slightly different way causes a bug to become apparent. And you might as well be doing other useful tasks as you do your close read.

During your first slow read it can be helpful to actually read code out loud, or at least in your head so that you don’t bother others around you. For example, if you’re examining the code below:

You might say:

  1. "For every index value of searchArray" (you want to develop a shorthand for loops)

  2. "If the index member of anotherArray is equal to what I’m search for"

  3. "Then break."

  4. "But wait, why am I using an index from searchArray to look in anotherArray? That’s probably wrong."

This is a place where it really helps to use variable names that aren’t i and j, and to pull the current value of arrays into a local variable immediately or use the new enhanced for loop syntax.

3.4. Add Instrumentation

Once you’ve completed one pass, start over and begin adding some print statements. This is sometimes called printf debugging, after the C language printf statement—its equivalent to System.out.println in Java. In Java, of course, you would use System.out.println to do this.

You will find that there are snobs out there that look down on printf debugging. Hook up a real debugger, they’ll say. (Don’t worry: we’ll get there.) Or rewrite your entire project using a functional programming language where bugs are impossible—as if that’s a real possibility.

And then there are the rest of use that just want to build cool and useful things as quickly as possible, and who will happily take the chance to add instrumentation as needed. printf debugging is not powerful enough to solve every problem, and there are times that you will need better tools. But use it when you can. And remind the snobs what none other than Brian Kernighan says:

The most effective debugging tool is still careful thought, coupled with judiciously placed print statements.

Usually the right approach is to start at the top of the block of code that you are examining and use System.out.println to examine things that you are unsure are working properly. This is particularly useful when used in conjunction with test suites.

3.5. Consider A Different Approach

If you are really stuck, it might be time to try a different approach. This is particularly true when you can acknowledge that your current approach has some, shall we say, limitations. Maybe you thought that this particular way of doing things would work out well, but then you had to add one hacky bit, and another. And now it’s rapidly approaching spaghetti code that you’re not even sure that you understand anymore.

In that case, by all means start over! You’re learning, and you’ll learn more by completing a few alternative implementations than you will by clinging to an ugly solution. Feel free to talk to another student—In English—about how they approached the problem. Maybe they found a much cleaner way that you can try.

3.6. Getting Help

The course staff is ready and willing to help you every step of the way! Please come to office hours, or post (in the right place) on the forum when you need help. You should also feel free to help each other, as long as you do not violate the academic integrity requirements.

4. Grading

MP3 is worth 100 points total, broken down as follows:

  1. 40 points: CaesarCipher.java

    • 10 points for submitting code that compiles

    • 30 points for passing the test

  2. 40 points: CharDistribution.java

    • 10 points for submitting code that compiles

    • 40 points for passing the test

  3. 10 points for no checkstyle violations

  4. 10 points for committing code that earns at least 20 points before Tuesday, October 3rd, 2017 @ 5PM.

Programming is a skill, and you learn it with regular consistent practice. If you wait until the day of the deadline, it’s unlikely that you’ll get as much practice and learn as much as if you start early. When you give yourself enough time to complete the MP, you can also slow down, enjoy yourself, and do things right. Take a few extra minutes to learn more about a library that you’re using, or clean up your code so that it really shines. The earlier you start, the more likely you’ll do some of these useful things.

So, we’re going to incentive you starting on time. 10% of MP3 is earned by submitting something that earns 20 points by Tuesday, October 3rd, 2017 @ 5PM. You don’t have to do much to get 20 points, but we hope that it will get you reading and think about the MP before the end of the week, when office hours are packed.

4.1. Test Cases

Like MP1 and MP2, we have provided exhaustive test cases for each part of MP3. Please review the MP1 testing instructions.

4.2. Autograding

Like MP1 and MP2, we have provided you with an autograding script that you can use to estimate your current grade as often as you want. The Eclipse project contains a launcher that will run MP3 autograder. Note, however, that unlike previous assignments, the local autograder can only calculate 90 out of your 100 total points. This is because it can’t tell if you have committed the required code before the deadline to earn the 10 "start the assignment on time" points 2.

Unless you have modified the test cases or autograder configuration files, the autograding output should equal the score that you will earn when you submit. If you modify our test cases or the autograding configuration, all bets are off.

5. Submitting Your Work

Overall you should refer to our instructions for using Subversion. Commit early and often! You only earn credit for the version of your code that is committed to your repository, so ensure that we have your best submission before the deadline.

And remember, you must commit something that earns 20 points before Tuesday, October 3rd, 2017 @ 5PM to earn 10 points on the assignment.

5.1. Academic Integrity

CS 125 is now CS 124

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Created 10/24/2021
Updated 10/24/2021
Commit a44ff35 // History // View
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