MCS-170 The Nature of Computer Science

Alice Lecture 4

The Loop statement (tile) is used to carry out a series of actions a fixed numebr of times. But, what do we do if we don't know how many times we want to repeat an action?
Consider our Eskimo girl and (cold) snake example from last class. If the distance from the snake to the girl is 20 meters, the Loop will repeat the move method exactly 20 times and then stop. But, suppose we want to be able to move the girl around and have the snake follow, for an indefinite time, until the snake eventually catches the girl. The While statement will allow us to create this action.
The way the While statement works is as follows:

Example: EskimoGirl and snake (version 2) [ Girl is movable through the use of an Event - more on Events later]

Recall that in Javascript, a procedure like prompt took parameters, values or variables that were passed into the function. Ex:

We can likewise create methods that accept parameters in Alice, but now the parameters can be Objects in our scene.

Example: The Funky Chickens Alice world. We will demo this and discuss how the parameters are passed into the world.doADance method. Change a chicken head, for example, and see what happens.

Note: We use the distance behind method and the as seen by other chicken measuring system to move the chicken back after it has danced. (distance will be negative, relative to the other chicken)
Here is a helicopter example (heli.a2w). When we first run the example, the helicopter does not move as we expect. Change the move up method so that as seen by refers to the ground object and play the scene again.

Class Project: Create a new world method in the Funky Chickens World that will take a chicken parameter and carry out some actions with that chicken.

Review how to create a parameter. Note how parameters can be Numbers, Boolean, Objects, or other types (such as Strings).
Difference between Parameter and Variable:

Parameter is an outside Object that gets passed into the method (exists outside method)
Variable is a value only used inside a method (doesn't exist outside method).

So far our Alice worlds have been fun to watch, but there has been no user interaction, we are just passive observers of what is happening on the screen. To add user interaction to our Alice worlds, we will need to have a way for Alice to understand when a user moves a mouse or types on the keyboard. These actions are called events.

An event is something that happens due to user activity and to which the computer responds.

Event Handling: How do we link events of the user to program action? We handle an event as follows:

1) The event must be captured by the program, i.e. must be linked to some method.
2) Once captured, the method should carry out some kind of response to the event.

Example: Suppose we want to create a flight simulator program. We need a way to control movement of a plane. In the world biplane.a2w we create event handlers for keyboard control of a biplane's movement.

Note how events are created and then linked to a method.
Note that the starting of the animation of an Alice world is itself an event, which is handled by the method world.my first method.
Note how we set the camera to track the plane's movements. Another option would be to set the camera's point of view to the biplane.

camera -> set point of view to biplane
Try this - camera doesn't continuously set point of view, need to embed this in a while statement.