MIT 6.096:
Introduction to Interactive Programming

Laboratory 1:
Expressions, Statements, and Interactions


This lab will allow you to get some practice writing simple expressions and statements in Java. It will also give you an opportunity to experiment with interactions among entities and how these generate a variety of basic behaviors. Portions of this lab assignment are also designed to familiarize you with the 6.096 lab environment and to begin to build a backkground in thinking the 6.096 way.

Be sure to read through the 6.096 general information handout, and in particular the collaboration policy. For this assignment, you may discuss the project in as much detail as you like with your classmates, but you should do the writeup on your own. You may also get comments from other students on all portions of your writeup before turning it in, if you wish. Please include the names of anyone with whom you collaborate, in any way, on this assignment, and indicte the nature of the collaboration. [Failure to include this information is a violation of the collaboration policy.]

This assignment emphasizes the following topics

You should read through this entire assignment and complete the Lab preparation sectionbefore you come to lab. Some portions of the PostLab writeup also require your thinking about them before and during the laboratory portion of the assignment.

This assignment is due at the beginning of class on Monday, 15 September.



This week's pre-lab has three parts. Future labs will have more extensive explicit lab preparation but less additional preparatory work.

A. Written Exercise

Pretend that you are corresponding with a Martian pen pal. (Alternately, pretend you're corresponding with your 6.096 professor, which can often feel the same.) Pick something that you encounter in every day life and describe/explain it to your pen pal. You may choose (for example) an artifact, an institution, a process or activity. Include both descriptions of how it behaves/how you use it, of how it fits into context, and also of how it works.

Remember that Martians (and Professors) know very little about life on Earth, so you should make your description fairly detailed and specific. The description should run approximately two paragraphs, and in no circumstances should it exceed one page. (Martians have notoriously short attention spans.)

Bring this writeup with you to lab. While you may revise it after lab (and before the problem set is due), the course assistant will want to see this writeup as a part of lab check-in.

B. Finger exercises

A writeup of the finger exercises shouldalways be turned in along with your completed laboratory. This week, you should also bring them to check-in.

1. In Java, every expression has a type. Assume that the following declarations apply:

For each expression below, if it is syntactically legal Java, indicate its type (not its value). If it is not syntactically valid, indicate why.
  1. 6
  2. 24L
  3. +3.5
  4. 3.5f
  5. 2e-16
  6. -25b
  7. i
  8. i+3
  9. i+3.0
  10. i+s
  11. l+d
  12. f+s
  13. i / 0
  14. 4 * 3.2
  15. i = 0
  16. i == 0
  17. 'c'
  18. "An expression in double-quotes"
  19. "An expression in double-quotes" + "another one"
  20. "6" + 3
  21. !b
  22. !i
  23. b || true
  24. i += s
  25. s += i
  26. i += f
  27. l = i = s
  28. i = l += s
  29. l++
  30. (long) s
  31. s
  32. (short) l
  33. l
2. Assume that the following lines of code are executed in the order shown.Each numbered line is a comment; on that line, give the value of each of the variables indicated at that point in the execution.
    int i = 4;
    int j = 5;
    int k = j;
    k = i;
    i = 3;
    int m = j;
    i = j + k;

    String s = "hello";
    String t = "g'day";
    String u = t;

  1. // s: t: u:

  2. t=s;
    s = "salutations";
    String v = t;
    s = t + u;

C. Lab Preparation

This week's lab work has two goals. One is to teach you about Java and to get you started designing programs. The other is to acquaint you with the particular laboratory facilities that we'll be using this term. Your lab preparation focuses exclusively on the first of these goals. We will provide you with additional materials about the 6.096 lab when you arrive in lab. (Those materials make much more sense when you're sitting at a lab machine.) There is no preparation for the second -- lab-familiarity -- goal.

The spirograph application that you'll be playing with this week is a simple drawing application. There is a blank screen with a dot on it. When the dot moves, it leaves behind a trail sort-of like the children's drawing toy Spirograph.

The motion of the dot is controlled by two entities, one for each axis (horizontal and vertical).These entities can potentially sense certain properties of the dot -- e.g., its position or acceleration -- relative to their own axis. Each of these entities follows a particular control rule that tells it how to behave. The control rule is automatically invoked by the application system; your job is simply to write down appropriate control rules.

The form of a control rule is a sequence of Java statements ending in a

(where double is some Java expression with type double). The value returned by your control rule will be used as the new position of the dot. (Motion will be smoother if you move the dot only a small amount at a time.)When the spirograph window first opens, the screen will be approximately 400x400; the origin (0,0) is in the center of the screen. You should avoid using these numbers explicitly, though; the name maxPos will be provided (i.e., pre-declared) for you and the coordinates of your screen will run from -maxPos to +maxPos.

Your job, in lab, is to write a series of behaviors that cause the spirograph to display certain kinds of pictures. We will suggest a few to begin, but we hope that you will find the environment interesting enough to try a few of your own. You should read through the exercises below and come up with preliminary designs for the code that will solve them. There are also several places where you are asked to predict what your code will do. Be sure to write up your predictions as well as your designs. Bring these notes with you to lab. [There is far more in the lab section than you should expect to do in lab. Do not worry about designing solutions to all of them!]

The spirograph application has many advanced features that you will use. For example, you can move the dot around (with your mouse) so that it begins from a different position, or (using an advanced features dialog) you can give the dot initial velocity. Some of these features are described below; some are mentioned in the lab handout; and others are left for you to discover for yourself. One specific feature involves a distinction that you will need to make in lab: You can declare two different kinds of names in your code. One is a temporary name that can be used during a single application of your rule. These names can be declared anywhere in your code. They are called variables. The other kind of name sticks around from one use of your rule to another. These names must be declared in a special box, separate from your rule code, but can be used freely in your rule code. These names are called fields. (You can also use name that have been pre-defined for you, like maxPos; these are called parameters.)

Things to Try

This section walks you through a series of exercises of increasing complexity. In future labs, you will have increasing responsiblity for designing the progression from simpler cases to more complex ones. It is always a good idea to build and test a simple version before going on to add many features. Testing should be thorough, and designing good test suites (sets of test cases) is a significant skill. Each time that you add a feature, you should test your code again.

There is more listed here than you can reasonably get through in one three-hour lab.

Static Positioning

  1. Write Horizontal and Vertical rules that will place the ball in position (10, 20). Try other coordinates as well, including negative ones.
  2. Use the Horizontal rule for both rules. What do you expect would happen?
  3. Use the Horizontal and Vertical rules separately again. Use the mouse to manually move the ball to another position (see Advanced Environment Options). Do this several times. What happens? Explain. [In lab: Can you "fix" this behavior?]
Implementing Velocity
We have pre-defined the name pos to hold the current position of the dot (along the relevant dimension). Each time your rule is used, pos will have the value at that time. (What value will pos have if you assign to it?) Using this name, solve the following problems.
  1. Write a pair of rules to make the ball move horizontally from left to right. Can you control how fast the ball moves (i.e., velocity) by changing your code?
  2. Use the horizontal rule for both rules. What do you expect would happen? Explain. (When you get to lab, try it out and see if you're right.)
  3. What happens if the horizontal and vertical rules have different effective velocities? (How do you make this happen?)
  4. Use the mouse to move the dot to a different position. What happens? Explain.
[Note: although the spirograph application seems to indicate that there are velocity names, these names are not used in position-control mode.]

Implementing Acceleration

You should try to prepare the lab up to this point. You should also read through the exercises below, and if you are familiar with programming, you should prepare at last some of them.

Wraparound and other boundary conditions

Try running the code you have so far in wrap-around mode and no-wrap-around mode (using the advanced features), and observe its behavior.

  1. Modify your code to make it emulate the behavior of wrap-around mode while using no-wrap-around mode.
  2. Can you make the dot bounce when it hits the end?
Other cool stuff Using Velocity and Acceleration Controls

Although you can implement velocity and acceleration using position controls alone, Spirograph is capable of doing this for you, and makes it easier for you to play around with the effects of different code. In the case of acceleration controls, you can think of the ball as a robot with independent horizontal and vertical motors, and your rules as the controls for its motors.

  1. Play around with the velocity and acceleration-control mode. Play around with bounce and no-bounce modes too.
  2. Try the different position rules you wrote above (in particular, the static positioning, velocity, and acceleration)
  3. Write code that will draw a parabola.

  4. Challenge: Write code that will draw a circle (given an appropriate initial position and velocity). (Hint: remember a = v^2/r from Physics.)


What to Bring to Lab

You should bring the letter to your pen pal, your finger exercises, and a plan of action for the laboratory (including some thoughts on how to solve the various problems described below). You should have read the entire problem set before you arrive. Your notes from lab will form the basis for your post-lab writeup.

Getting Started

Instructions on how to start and use the Spirograph problem set will be available in lab and on-line. In addition, the lab handouts will include information about some simple exercises designed to familiarize you with the lab machines and the Java environment that we will be using. You should allow time for these in lab.

During the lab session, you should work through the lab set-up exercises and as much of the spirograph as you have time for. Details of the spirograph are described in the Things to Try section, above; a supplementary handout (describing how to run the spirograph) will be available in lab.

Before you leave

Before you leave lab, you will need to have your code checked off by a course staff member. You should allow time for an adequate demonstration and discussion of what you have done. Please do not wait until the last minute to be checked off.

Post-Lab, AKA What To Turn In

Your completed assignment should include: Lab assignments are due on Mondays at the beginning of class. They may, of course, be turned in earlier.

This course is a part of Lynn Andrea Stein's Rethinking CS101 project at the MIT AI Lab and the Department of Electrical Engineering and Computer Science at the Massachusetts Institute of Technology.

Questions or comments:


Last modified: Thu Jul 10 13:01:20 1997