copyright 2008 wanda dann, steve cooper, don slater alice workshop variables & conditions

Copyright 2008Wanda Dann, Steve Cooper, Don Slater

Alice Workshop

Variables & Conditions


Recall: Variable

A variable is an item or value that is likely to change in value.

Variables can belong to: a specific method (local to that method) a class (instance, available to all objects of

that class) the world (global, available anywhere in the

program)


Instance variables

In Alice, an object’s properties are instance variables


Changing a property

A property can be changed, using set


Condition

The set of of variable values describe the condition of objects in an animation program.

As an animation program is running, each statement is likely to change the value of at least one property of an object.

Therefore, conditions change as a program runs.


New variables

Properties are built-in variables, but we can create our own variables to track other values


Example Problem

A penguin (joe) is watching his son (junior) slide across the ice. A hole in the ice is nearby where junior could fall in. Create an animation where junior slides a random distance toward the water. If junior lands in the water, have him sink under the water and then joe dives in after him.


Textual Storyboard

Do in order junior faces the water junior slides forward a random distance if junior is in the water junior goes down under the surface joe goes in after him Do together

joe and junior pop back up to the surface


Random distance

World function to generate a random number in a range of 0.25 to 3


Checking a Condition

How do we know whether junior is in the water?


Is junior in the water?


Problem Solving

Variables can be used to represent values Let d be the distance of junior to the water Let r be the radius of the water


Creating a variable

A variable has a name and a type


Set variable value

Call a function to set the value of d


Use Math to compute radius

Cascading menus for binary operations


Conditional Execution

Alice provides an If/Else control structure to check a condition


Boolean Condition

Is junior’s distance to the center of the water less than the radius of the water?

Must be either true or false


If/Else Statement

Create parameter, targetObjPull if/else control into editor


Relational operations are defined in World’s functions

Alice operators◦== means “is equal to”◦ != means “is not equal to”

Operators


Select placeholder operands


Placeholders

In this example, the operands selected from the cascading menus are just placeholders. Need to replace with calls to functions to obtain

distance of penguin to center of water


Demo

Implementation of if/else


Completed Code


Text

Chapters 3 & 6 Conditions and If/Else

Chapter 10 Variables

copyright 2008 wanda dann, steve cooper, don slater alice workshop variables & conditions

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