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Alice 3, CS1, and Quaternions

Posted by cayhorstmann on September 28, 2009 at 10:49 AM PDT

This semester, I am teaching the CS1 course again. If you just teach plain Java, it isn't easy to come up with interesting lab assignments. Some of the students have built exciting animations with Alice in the CS0 course. Somehow, they aren't as excited about printing prime numbers or digits of π in CS1. But the latest version of Alice, now in beta, can be programmed in Java. This is very cool. Students can write Java code that directs the Alice models. For example, one assignment asks students to make a Car class whose drive method moves the car and reduces the gas in the tank. (I couldn't find a gas gauge, so I used the cat clock. The tail moves to the left as the tank gets emptier.) I provide the code for moving the car and rotating the tail, and the students compute the gas consumption.

Here is my procedure for making a lab assignment.

  • Arrange the objects that students should manipulate in the Alice environment and save the world.
  • Open the world in Netbeans (with the Alice 3 plugin)
  • Clean up the generated code and make subclasses of the Alice classes, adding fields and methods used for the assignment
  • Replace nbproject/private/ with an empty file
  • Zip up the Netbeans project directory and post for students

The CS1 students use NetBeans, not the Alice IDE. (The Alice IDE is wonderful for pre-CS1 because students drag and drop statements instead of typing them in. They can never make a syntax error! But in CS1, we want to use actual Java, and students need to learn how to deal with syntax errors.) Autocompletion works nicely (and is pretty much a requirement since there is no API documentation available yet). Students learn how to implement methods, branches, and loops in Java. The visual feedback with the Alice models really helps with learning the material.

There are some things that are easier to do with Alice-in-Netbeans than with the Alice IDE, in particular constructing lots of objects. It is a simple matter for the car to drop toasters to mark the path, or to make a whole bunch of wall segments in a maze:

There are just a few things you need to know:

  1. In the Alice coordinate system, the x-axis runs from left to right, y from bottom to top, and z from front to back. You can hide that from your students, but you need to know yourself when you implement the classes that they extend.
  2. To insert another object into the same scene in its default orientation, use
    Transformable obj = new Toaster(); // or some other model
    obj.setLocalPointOfView(new PointOfView(
       new Quaternion(0, 0, 0, 1),
       new Position(x, y, z)));
  3. If you want different orientations (such as the walls above), you can call the turn method:
    obj.turn(TurnDirection.LEFT, angle, 0);

    Here, 0 means that the turning happens instantaneously, and angle is between 0 and 1. (A quarter turn would have an angle of 0.25).

    Or, you can use quaternions. In Alice, the quaternion a + b i + c j + d k is a new Quaternion(b, c,     d, a). That is, the real coefficient comes last! In other words, new Quaternion(0, 0, 0, 1) is the unit quaternion. Rotation around the vector (x, y, z) with angle α is sin(α/2) (x i + y j + z k) + cos(α/2). For example, to rotate a wall 90 degrees around the y-axis (as with the walls facing east/west in the image above), you use a new Quaternion(0,     Math.sqrt(0.5), 0, Math.sqrt(0.5)).

  4. If you want to write code for object interactions, use
    for (Composite match : getScene().findAllMatches()) {
      if (match instanceof Wall) ...

Of course, you want to hide those quaternions from your students in a suitable superclass, or they will beg you to go back to computing prime numbers or the digits of π.

If you want to give all this a try, download Alice 3 and the Netbeans plugins from the Kenai site and check out the labs that are posted here.

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To estimulate professors to

To estimulate professors to keep updating their homework assignments, we created the PUJ in Brazil. Several JUGs are already adopting it and it proves great results in the academic community + local market.

Real Java + Alice sounds like a good combo

I know a girl who took a class using Alice at San Jose State and I was trying to help her with her homework. I have to say that the drag and drop programming was confusing for me and definitely baffled her, but it clearly addresses some of the major problems students come across with syntax. The problem is that some things are just very unintuitive to drag and drop. That's pretty cool if Netbeans can cross the divide. The cat's tail for the gas left is pretty hilarious too. I miss your classes ;). --Glenn

I know exactly what you mean

I know exactly what you mean about confusion with drag-and-drop programming. There were times when I tore my hair out trying to figure out just what to drag and drop so that I could get to where I wanted to be. I definitely prefer using NetBeans and the help that you get from a modern IDE. But on the flip side, I taught that class that your friend was in, using Alice. And after a few weeks of using Alice, we moved on to Greenfoot, a very nice environment that uses Java. And Java syntax. And the students hated it with a passion because they kept wrestling with semicolons, parentheses, and braces, when they wanted to get cool stuff done. If they hadn't had Alice and the confidence that they can get cool stuff done, I think some of them would have just dropped out. Instead, they stuck with it and got cool stuff done in Java.

Good teaching style

That's exactly the correct way to teach, make sure the students are interested in the work. When I used to teach intro C++ and intro Java, I would always make sure there was SDL usage in C++ and Swing in Java. The functions would HAVE to make cool things move on the screen for the attention to stay. I used to have them do Tic tac toe screens using system.outs, but just couldn't get them to stay interested with that.