1. Create your remote repository with the starting code for the problem set by visiting didit.csail.mit.edu/6.031/sp20. Go to psets/ps0 and click the button to create your repository. You will only be able to create a repo after the course signup forms are processed and the problem set is announced, which happens after the first class meeting.

   • If you can’t create a repo after the ps0 announcement, you are not registered for 6.031. Contact the staff (how?) to check your status.

   • If you are a listener or not taking 6.031 for credit, you can’t create a repo. See instructions below to clone from a read-only remote.

2. Open the terminal (Git Bash on Windows) and go to the directory (using cd) where you would like to store your code. For example, that might be a directory named Source in your home directory.

   It’s better not to use the eclipse-workspace directory for this purpose, because you may need to delete and start a fresh workspace when Eclipse gets into a bad state. Leave eclipse-workspace to Eclipse itself, and put your source code somewhere else.

   If you are unfamiliar with using the command line, Getting Started step 5 introduces key commands like cd and mkdir.

3. Clone your repo. You will access the remote repository over SSH from your laptop.

   Note: Getting Started step 6 has setup you must perform before using Git.

   Find your git clone command at the top of your psets/ps0 page on Didit. It is a long command that looks something like this:

   git clone ... git@github.mit.edu:6031-sp20/ps0-«username».git

   Copy this entire command, paste it into your terminal, and run it. The result should be a directory named ps0-«username» with the starting code for the problem set. Keep the terminal window open — you will also use it to commit your code.

   • If git clone says “does not appear to be a git repository” or “could not read from remote repository”, check your repository URL for typos. Make sure your repository is listed on Didit.

     Check that you can authenticate to github.mit.edu in the last part of Getting Started step 6.

   • If you are a listener or not taking 6.031 for credit, you can clone problem set repositories using a modified URL: after ps0, omit the dash-username. You will not be able to push to this repository, it’s a read-only remote.

4. After cloning your repository, add the project to Eclipse so you can work on it.

   Note: Getting Started step 4 has setup you must perform before using Eclipse in 6.031.

   To import a project:

   • In Eclipse, go to File → Import… → General → Existing Projects into Workspace.
   • Select the root directory of your clone (ps0-«username») by clicking “Browse…” and navigating to it.
   • Make sure the project is checked, make sure “Copy projects into workspace” is not checked, and click Finish.

1. Your first task is to implement drawSquare(Turtle turtle, int sideLength), using the two methods introduced above: forward and turn. The specification comment above drawSquare leaves several things unspecified, like which direction to turn, so those decisions are up to you as the implementer.

2. Once you’ve implemented the method, go to the main method in TurtleSoup.java. This main method creates a new turtle and instructs the turtle to draw on screen. There should be a call to drawSquare already there, but commented out. Uncomment this call so that your drawSquare will be called when the code is run.

   Then run main: right-click on the file TurtleSoup.java in the Package Explorer on the left side of Eclipse, go to Run As, and select Java Application.

   A window will pop up, and, once you click the “Run!” button, you should see a square drawn on the canvas.

1. First, in the terminal, change directory (cd) to your clone, and take a look around with

   git status

   which shows you files that have been created, deleted, and modified in the project directory. You should see TurtleSoup.java listed under “Changes not staged for commit.” This means Git sees the change, but you have not (yet) asked Git to include the change as part of your next commit.

2. You can run the command

   git diff

   to see your changes. (Note: when the diff is more than one page long, use the arrow keys. Press q to quit the diff.)

3. Before committing, files must be staged for commit. Staging a file is as simple as

   git add <filename>

   so use

   git add src/turtle/TurtleSoup.java

   to stage the file. You should also stage the test file if you’ve added more tests.

4. In addition, it’s always a good idea to review your commits before committing to them. Run

   git status

   again to see that your changes are now listed under “Changes to be committed.”

   Now try running

   git diff

   and notice that those changes are no longer shown in the diff output!

   So instead use

   git diff --staged

   to see exactly what Git will record if you commit now.

5. Ready? To perform the commit,

   git commit

   will actually commit the changes locally, after opening your default editor to allow you to write a commit message. Your message should be formatted according to the Git standard: a short summary that fits on one line, followed by a blank line and a longer description if necessary.

   If Git warns you about configuring your default identity or you can’t edit your commit message, you did not follow the instructions in the Getting Started guide. Getting Started step 6 has setup you must perform before using Git.

   Now run

   git status

   once more, and see that your changes are no longer listed.

6. Use the command

   git lol

   to see the history of commits in your project. (git lol is not a misspelling; it is an alias for the git log command that we configured in the Getting Started step 6.)

   Right now, the log should show three commits: an initial commit with the starting code, a commit when you created your problem set repo, and the commit you made just now.

   Important: only the local history has the new commit at this point; it is not stored in your remote repository. This is one important aspect where Git is different from centralized systems such as Subversion and CVS.

   Find the ID of the commit you just made (git lol shows it as a hexadecimal number like 3d60b25), and use it to view your changes again:

   git show <commit>

   You should see the changes you just made.

7. In order to share the changes with your remote repository (which is the one we will be using for grading), you need to push to the remote repository, with

   git push origin master

   At this point, the remote repository on github.mit.edu now has the same history as your local repository. It is important to remember that we will be grading the history in the repository on github.mit.edu; if you forget to push, we won’t see your commits.

1. Implement chordLength, reading the specification comment above it carefully to understand what it should do. (You can also click this link to see the same specification on a web page.)

   There’s a simple formula for the length of a chord of a circle; try to derive it before searching the web.

2. Run the public tests that we provided. In 6.031, we use JUnit, a widely-adopted Java testing library. We’ll see more about JUnit, and about testing in general, in an upcoming class. For now, let’s focus on running the provided tests against your implementation. By convention, JUnit tests are in a class whose name ends with Test, so the public tests for this problem set are in TurtleSoupTest.java.

   

   To run the JUnit tests in TurtleSoupTest, find TurtleSoup­Test.java under the test folder in the Package Explorer, right-click it, and go to the Run As option. Click on JUnit Test, and you should see the JUnit view appear.

   If your implementation of chord­Length is correct, you should see a green check mark next to chord­LengthTest in the list of results.

   The   big red bar   indicates that at least one test has failed. That shouldn’t surprise us, since the other tests are exercising methods we haven’t implemented yet. When all the tests pass, the bar will turn green!

   If testAssertionsEnabled fails, you did not follow the instructions in the Getting Started guide. Getting Started step 4 has setup you must perform before using Eclipse.

3. Try breaking your implementation (make it return a wrong answer) and run TurtleSoupTest again.

   

   You should see a red X or blue X next to chord­LengthTest in the JUnit view, and if you click on chord­LengthTest, you will see a stack trace in the bottom box, which provides a brief explanation of what went wrong. Double-clicking on a line in the failure stack trace will bring up the code for that frame in the trace. This is most useful for lines that correspond to your code; this stack trace will also contain lines for Java libraries or JUnit itself.

   Try double-clicking on the line in the stack trace for chord­LengthTest(), and look at the code for the test. On that line, you should see a call to your chord­Length implementation with particular values for its arguments, within a call to assertEquals() that checks whether your implementation’s return value was correct. A JUnit test generally has this form: it calls your implementation, then makes one or more assertions about the value that your implementation returned.

   

   Hover your mouse over assertEquals() to learn more about what this method does. Hovering over a class or method name is a good way to get a compact overview of its documentation in Eclipse.

4. Enough breaking: fix your implementation so it’s correct again. Make sure the test passes with a green check .

   Passing the public JUnit tests we provide does not necessarily mean that your code is perfect. You need to review the function specifications carefully. In future problem sets, we’ll also expect you to write your own JUnit tests to verify your code.

5. Implement drawApproximateCircle.

   Use your implementation of chordLength. To test this function, change the main method to call drawApproximateCircle and verify that you see what you expect.

1. Implement distance.

   This function calculates the distance between two points, where the points are represented by the Point class defined in Point.java.

   Remember to run the JUnit tests.

2. Implement findTriangleIncenter.

   This function finds a certain central point of a triangle. There are several ways to solve this problem, which you can learn about on the web. You should use your distance method as appropriate.

3. Implement findShortestPath.

   You should use your distance method as appropriate.

   The performance of your solution is not important, as long as it runs fast enough for testing purposes (less than a second on up to 5 points). Try a brute-force approach first, then optionally think about how to make it faster.

   For information on how to use Java’s List and Set interfaces, and classes that implement them, look up java.util.List and java.util.Set in the Java library documentation.

4. At this point, JUnit should now show a green check for distanceTest, findTriangleIncenterTest and findShortestPathTest.

Commit to Git. Once you’re happy with your solutions for this problem, commit and push!