6.031 — Software Construction
Spring 2017

Problem Set 0: Turtle Graphics

Part I (#’s 0—4) due
before class Friday, February 10, 2017, 11:00 am

Beta due
Thursday, February 16, 2017, 10:00 pm
Code reviews due
Tuesday, February 21, 2017, 11:00 am
Final due
Thursday, February 23, 2017, 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, February 10, 2017.

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. Obtain a remote repository with the starting code for the problem set by visiting didit.csail.mit.edu. Find psets/ps0 under New assignments and create a repository. You will only be able to create a repo after the course signup forms are processed and the problem set is announced.

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

    • If you are 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.

    Getting Started step 4 introduces the command line.

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

    Note: Getting Started step 5 has setup you must perform before using Git.

    In the terminal, run (all on one line):

    git clone ssh://[username]@athena.dialup.mit.edu/mit/6.031/git/sp17/psets/ps0/[username].git ps0

    The result should be a directory named ps0 with the starting code for the problem set. Keep the terminal 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.

    • If git clone reports a “protocol error” or asks for your password but then does nothing, you should review any changes you made to your Athena dotfiles, especially .bashrc.mine and .bash_environment. Ask a TA for help if you are not familiar with Athena.

    • If you are a listener or not taking 6.031 for credit, you can clone a problem set repository using a modified command: replace the last instance of [username] with didit/starting. 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 2 has setup you must perform before using Eclipse in 6.031.

    To import a project:

    • In Eclipse, go to File → Import… → Git → Projects from Git
    • On the “Select Repository Source” page, select “Existing local repository”
    • On “Select a Git Repository,” click Add…, and Browse… to the directory of your clone
    • The “Search results” list should show your clone, with “.git” at the end; click “Finish”
    • On “Select a wizard” for importing, choose “Import existing projects”
    • Finally, on “Import projects,” make sure ps0 is checked, and click “Finish”

Problem 2: Collaboration policy

Follow the link on the General Information page to read the course policy on collaboration and public sharing.

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

collaboration check

Online resources

I can search the web while writing my solution.

(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 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 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 package turtle.

  1. Your first task is to implement drawSquare(Turtle turtle, int sideLength), using the two methods introduced above: forward and turn.

  2. Once you’ve implemented the method, run the main method in TurtleSoup.java. To 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.

    This main method creates a new turtle, calls your drawSquare method, and instructs the turtle to draw on screen. 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.” If you run

    git diff

    those changes are no longer shown! 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 5 has setup you must perform before using Git.

    Now run

    git status

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

  6. You can use the command

    git log

    to see the history of commits in your project. Right now, you should see two of them: the initial commit to create your problem set repository with the starting code, 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.

  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 Athena 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 Athena; if you forget to push, we won’t see your commits.

Didit: when you push your commit, you should see the Didit system wake up and run a subset of the autograder on your code. Didit clones its own copy of your code from what you’ve pushed to Athena, compiles it, and runs some tests. Look for output right in your terminal like this:

remote: Requesting Didit build for psets/ps0/bitdiddle rev a1b2c3d
remote: Started build
remote: Checked out rev a1b2c3d
remote: Compilation succeeded
remote: Public tests FAILED
remote: Details: < https://didit.csail.mit.edu/psets/ps0/bitdiddle >

Visit Didit to see the full 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.

gitweb: you can see the contents of your Athena repository with your web browser on gitweb.

Part II

Problem 5: Drawing polygons

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.

You should not change any of the method declarations (what’s a declaration?) on this problem set. If you do, you risk receiving zero points on the problem set.

  1. Implement calculateRegularPolygonAngle.

    There’s a simple formula for what the inside angles of a regular polygon should be; try to derive it before googling/binging/duckduckgoing.

  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, right-click on TurtleSoup­Test.java in the Package Explorer, and go to the Run As option. Click on JUnit Test, and you should see the JUnit view appear.

    If your implementation of calculateRegularPolygonAngle is correct, you should see a green check mark next to calculateRegular­Polygon­AngleTest 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 2 has setup you must perform before using Eclipse.

  3. Try breaking your implementation and running TurtleSoupTest again.

    You should see a red X or blue X next to calculateRegular­Polygon­AngleTest in the JUnit view, and if you click on calculateRegular­Polygon­AngleTest, 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 calculateRegular­Polygon­AngleTest(), and look at the code for the test. On that line, you should see a call to your calculateRegular­Polygon­Angle 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.

  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 calculatePolygonSidesFromAngle.

    This does the inverse of the last function; again, use the simple formula. However, make sure you correctly round to the nearest integer. Instead of implementing your own rounding, look at Java’s java.lang.Math class for the proper function to use.

  6. Implement drawRegularPolygon.

    Use your implementation of calculateRegularPolygonAngle. To test this function, change the main method to call drawRegularPolygon 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.

Problem 6: Calculating headings

  1. Implement calculateHeadingToPoint.

    This function calculates the parameter to turn required to get from a current point to a target point, with the current direction as an additional parameter. For example, if the turtle is at (0,1) facing 30 degrees, and must get to (0,0), it must turn an additional 150 degrees, so calculateHeadingToPoint(30, 0, 1, 0, 0) would return 150.0.

  2. Implement calculateHeadings.

    Make sure to use your calculateHeadingToPoint implementation here. For information on how to use Java’s List interface and classes implementing it, look up java.util.List in the Java library documentation. Note that for a list of n points, you will return n-1 heading adjustments; this list of adjustments could be used to guide the turtle to each point in the list. For example, if the input lists consisted of xCoords=[0,0,1,1] and yCoords=[1,0,0,1] (representing points (0,1), (0,0), (1,0), and (1,1)), the returned list would consist of [180.0, 270.0, 270.0].

  3. At this point, you should have a green check for every test when you run JUnit, and a satisfying   big green bar  .

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 any piece of art you wish. Your work will be judged both on aesthetics and on the code used to draw it. Your art doesn’t need to be complex, but it should be more than a few lines. 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 Athena. We will use the state of your repository on Athena as of 10:00pm on the deadline date. When you git push, the continuous build system attempts to compile your code and run a subset of the autograder tests. You can always review your build results at didit.csail.mit.edu.

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.