6.031 — Software Construction
Fall 2020

Problem Set 0: Turtle Graphics

Part I (#0 – #4) due
before class Friday, September 4, 2020, 11:00 am

Alpha due
Tuesday, September 8, 2020, 10:00 pm
Because this deadline is extended due to Labor Day, you may take at most 1 slack day on PS0 alpha.
Code reviews due
Friday, September 11, 2020, 11:00 am
Beta due
Monday, September 14, 2020, 10:00 pm

Welcome to 6.031!

This course is about three essential properties of software:

Safe from bugsEasy to understandReady for change
Correct today and correct in the unknown future. Communicating clearly with future programmers, including future you. Designed to accommodate change without rewriting.

The purpose of this problem set is to:

  • introduce the tools we will use in 6.031, including Java, Eclipse, JUnit, and Git;
  • introduce the process for 6.031 problem sets;
  • and practice basic Java and start using tools from the Java standard library.

You should focus in this problem set on writing code that is safe from bugs and easy to understand using the techniques we discuss in class.

You must install the required tools and complete Part I (problems 0 through 4) before class on Friday, September 4.

Part I

Problem 0: Install and set up

Read and complete the Getting Started guide. The guide will step through:

  • installing the JDK, Eclipse, and Git
  • configuring Eclipse
  • configuring Git
  • learning and practicing the basics of Git

You need to complete all the steps in the guide before you start working on this problem set.

Problem 1: Clone and import

This process will be identical for each problem set.

  1. Create your remote repository with the starting code for the problem set by visiting didit.mit.edu/6.031/fa20. 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-fa20/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.

Problem 2: Collaboration policy

To check your understanding of the policy, answer these questions:

reading exercises

Standard library

I can import and use classes from the Java standard library.

(missing explanation)

Online resources

(missing explanation)

I can read or post to the class Piazza forum while writing some part of my solution.

(missing explanation)

I can publicly post part of my solution online as part of a portfolio of code to show to recruiters.

(missing explanation)

Getting and giving help

I can write my solution while sitting near (or on a video call with) another person writing theirs.

(missing explanation)

I can write some part of my solution in lab or office hours.

(missing explanation)

I can get help from someone else while their own solution is visible to me or them.

(missing explanation)

I can talk about ways to solve the problem set with another student in the class.

(missing explanation)

I can write my solution while sitting near (or on a video call with) another person writing theirs, reading code aloud to each other to make sure we both got it.

(missing explanation)

Use of student course materials

I can show or send my solution to another student in the class.

(missing explanation)

I can look at someone else’s solution to get help writing part of my solution.

(missing explanation)

I can use someone else’s solution to check my own solution, for example by running their tests on my code.

(missing explanation)

My solution can include code that I wrote while taking this class in a previous semester.

(missing explanation)

At the end of every problem set, you will answer a few questions similar to these about how you collaborated on the assignment.

Problem 3: Turtle graphics and drawSquare

Logo is a programming language created at MIT that originally was used to move a robot around in space. Turtle graphics, added to the Logo language, allows programmers to issue a series of commands to an on-screen “turtle” that moves, drawing a line as it goes. Turtle graphics have also been added to many different programming languages, including Python, where it is part of the standard library.

In this problem set, we will be playing with a simple version of turtle graphics for Java that contains a restricted subset of the Logo language:

  • forward(units)
    Moves the turtle in the current direction by units pixels, where units is an integer. Following the original Logo convention, the turtle starts out facing up.
  • turn(degrees)
    Rotates the turtle by angle degrees to the right (clockwise), where degrees is a double precision floating point number.

You can see the definitions of these commands in Turtle.java.

Now look at the source code in TurtleSoup.java, in the src folder in package turtle.

  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.

Problem 4: Commit and push your work so far

After you’ve finished implementing the drawSquare function, let’s do a first commit.

  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 main

    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.

Didit: when you push your commit, the Didit system will run a subset of the autograder on your code. Didit clones its own copy of your code from what you’ve pushed to github.mit.edu, compiles it, and runs some tests.

Visit Didit after you push to see the report. Right now, almost all tests will fail since you’ve only implemented one method. But as the deadline approaches, it is your responsibility to check Didit’s build report and ensure that we can compile and run your code.

github.mit.edu: you can browse the contents of your remote repository by visiting github.mit.edu/​6031-fa20 or following links from Didit to github.mit.edu.

Part II

Problem 5: Drawing circles

For detailed requirements, read the specifications of each function to be implemented above its declaration in TurtleSoup.java.

Hint: be careful when dealing with mixed integer and floating point calculations.


In this problem set, you should only need to edit TurtleSoup.java. You should not change any of the method signatures (what’s a method signature?) in that file, or you risk receiving zero points on the problem set.

You are free to add new helper methods inside TurtleSoup if you want, and you can add tests to TurtleSoupTest if you want, but this isn’t required.

Don’t change any other Java files or add new Java files to this problem set.

  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.

    passing JUnit test for chordLength

    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.

    failing JUnit test for chordLength

    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.

    Javadoc documentation for assertEquals pops up on mouse hover

    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.

Commit to Git. Once you’re happy with your implementations of these functions, commit and push! Committing frequently – whenever you’ve fixed a bug or added a working and tested feature – is a good way to use version control, and will be a good habit to have for your team projects. Pro tip: you can skip the editor popup using git commit -m, e.g.:

git commit -m "implemented chordLength and drawApproximateCircle"

Problem 6: Calculating distances and points

  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!

Problem 7: Personal art

Implement drawPersonalArt.

In this function, you have the freedom to draw a piece of art you wish, with some complexity and aesthetic appeal. Your work will be judged both on aesthetics and on the code used to draw it.

Your art should need more than a few lines of code, so just drawing a few lines or polygons is not sufficient. Use helper methods, loops, etc. rather than simply listing forward and turn commands.

For drawPersonalArt only, you may also use the color method of Turtle to change the pen color. You may only use the provided colors.

Here are some examples of the kinds of images you can generate procedurally with turtle graphics, though note that many of them use more commands than what we’ve provided here.

Modify the main method to see the results of your function.


Make sure you commit AND push your work to your repository on github.mit.edu. We will use the state of your repository on github.mit.edu as of 10:00pm MIT time on the deadline date. When you git push, the continuous build system attempts to compile your code and run the public tests (which are only a subset of the autograder tests). You can always review your build results at didit.mit.edu/6.031/fa20.

Didit feedback is provided on a best-effort basis:

  • There is no guarantee that Didit tests will run within any particular timeframe, or at all. If you push code close to the deadline, the large number of submissions will slow the turnaround time before your code is examined.
  • If you commit and push right before the deadline, the Didit build does not have to complete in order for that commit to be graded.
  • Passing some or all of the public tests on Didit is no guarantee that you will pass the full battery of autograding tests — but failing them is almost sure to mean lost points on the problem set.


Your overall ps0 grade will be computed as approximately:
~50% alpha autograde + ~5% alpha manual grade + ~35% beta autograde + ~10% beta manual grade

The autograder tests will not change from alpha to beta.

Manual grading of the alpha may examine any part of the problem set. Manual grading of the beta may also examine any part, including how you addressed code review feedback.