Alice Workshop Functions, Loops, and Parameters

Copyright 2008Wanda Dann, Steve Cooper, Don Slater

Alice Workshop

Functions, Loops, and Parameters


Where are we? Thus far, we have written object-level methods for the

dragon flapWings

takeOff fly

Do in order

dragon flies to princess

princess climbs on dragon's back

dragon and princess escape

knight shakes his arm (and sword) in protest

dragon takes off


Next

Now, the dragon must fly to the princessBreak it down into simple steps:

Do in order

fly to the princess

dragon turn into a position that will allow princess to climb on board


World-level method

This method clearly involves two different objects: dragon princess

We will write a world-level method rather than a object-level method.

General rule of thumb: If more than one

object is involved in a method, write the

method as a world-level method.


turn to face


Loop The fly method moves the dragon forward 1

meter. We want to call the fly method repeatedly to move

the dragon to the princess. Alice has a Loop statement.


Problem

The problem is: How many times should the fly method be called?

?


Functions Alice has several built-in distance functions, that can be

used to determine the number of meters objects are from one another.

In this example, the Loop should execute once for each meter of this distance.


Behind the scenes

To be truthful, Loop needs a whole number (e.g., 4) The distance in front of function returns a

double number (e.g., 4.37 meters)

Behind the scenes, Alice truncates the distance to a whole number


Complete Methods


Animation:Smooth, Continuous Movement

For most purposes, execution of an instruction works best with a gentle slowdown in speed (toward the end of the duration)Successive repetitions, however, may have a smoother continuous motion if made to end abruptly


Animation:Smooth, Continuous Movement

For most purposes, execution of an instruction works best with a gentle slowdown in speed (toward the end of the duration)Successive repetitions, however, may have a smoother continuous motion if made to end abruptly


Object-level method with parameters

We could also write this method for the dragon object

But it still involves two objects in the world dragon princess

We must use a parameter in order to access the princess from the dragon method


Create a object-level method


Parameters A parameter is a shared element that conveys

information between one method and another A parameter, has a name, a value, and a type

Type describes the kind of information that is shared

Click on create new parameter button


Parameters

Name the object "who” (or something else appropriate)

Select the Object type then click OK


Use the parameter

Turn the dragon to face …


What is a Variable?

A variable is an element in a program to store values that are likely to change as the program executes

Also useful for Naming values that may be abstract Allowing code to be more readable Decomposing a complex process into a

series of simpler processes


Create a variable in a method


Like Parameters

Variables have a name (distanceToWho) Variables have a type (Number)


Parameters vs. Variables

Both parameters and variables Have a name Have a type (which describes the kind of

data being stored) Store values that can change as the

program runsParameters carry values from one

method to anotherVariables belong to one particular

method only (local)


Using a variable

Drag the variable tile into the code editorSelect a value for the variable


Using the function with parameter

To set the value of the variable


Using the variable in the loop


Code


Text

For more information Functions and Expressions

• Chapters 3 and 6 Loop

• Chapters 3 and 7 (Section 1) Methods and Parameters

• Chapter 4 Variables

• 10-1


Your turn… Write code to have the

princess climb onto the dragon's back (not climb, actually – just move to the dragon)

dragon and princess escape from the castle (use vehicle)

Do in order

princess climbs on dragon's back

dragon and princess escape

knight shakes his arm (and shield) in protest

dragon takes off

dragon flies to princess

Documents

Alice Workshop Functions, Loops, and Parameters