MP3: Encryption

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.

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.

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.

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.

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:

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.

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:

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.

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.

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.

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

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.

Please review the MP1 academic integrity guidelines.