Variables and Functions
Chapter 3
3-1
Variables
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Named storage location in computer’s memory
Programs may need to store data when running
Types of data that can be stored
o
o
o
o
Local variables
Parameter variables
Class-level variables
World-level variables
3-2
Variable Types
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Local variables
o
o
o
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Parameter variables
o

Holds argument that is passed to a called method
Class-level variables
o
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Belongs to a specific method
Only used in that method
Variables cease to exist when method stops
Variable that belongs to a specific object
World-level variables
o
Variable that belongs to the world
3-3
Creating Local Variables
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Local variables belong to a
specific method (such as
world.my first method)
Create variable by clicking
on create new variable in
desired method
Variables require 3 things
o
o
o
Name
Type
Initial value
name
type
value
3-4
Variable Definition
 Variable Names
o Must be unique within the method
o Should be meaningful and reflect the variable’s purpose
o Named using camelCase
 Variable Types
o Numbers, Boolean, Objects, Other (such as String,
Color, Sound, etc.)
 Initial Value
o Value initially stored in the variable
3-5
Variables

Tile appearance shows the type of variable
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Numbers
Boolean
Objects
String
Variable declaration - creating a variable
variable
area
3-6
Variable Assignment
 Variables have an initial value
 Initial value held until a different value is
assigned
 New values can be
assigned while the
method is running
o
o
Use Set instructions
Done by dragging variable tile into Methods Editor
3-7
Checkpoint
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What is a variable?
What are three things that must be specified when
creating a new variable?
Where in the Method Editor do variable tiles
appear?
What do the characters 123 appearing on a
variable tile tell you?
What do characters abc on a variable tile tell you?
How do you create a set instruction for a variable?
3-8
Function
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Asks a question to
o
o
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Check a condition
Compute a value
Used to get information about…
o
Size of objects

o
Relationship of one object to another


What is snowwoman's height?
What is distance between lunar robot and rock?
Used to get other information
o
o
Time
Ask user for information
Value

What a function returns after it
o
o

Type of value depends on kind of function
o
o

Asks question
Or performs a computation
distance to returns a number
is to the left of returns a Boolean value
Number value
o
o
o
Whole number ex: 3 meters
Fractional value, ex: 1.2 meters
In lunar example

What is distance of robot to rock?
Function vs. Method

Functions
o
o
o
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Perform calculations
Determine state of an object or system
Return a value
Methods perform an action
3-11
Types of Values
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Number
o
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Boolean
o
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“Slithy Toves?”
Object
o
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true, false
String
o
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As in example
snowman, helicopter
Position in world
o
(0, 0, 0) – center of an Alice world
World Level Primitive Functions
 List of world level functions categories
o
o
o
o
o
o
o
o
o
Boolean logic
Math
Random
String
Ask user
Mouse
Time
Advanced math
Other
3-13
Asking User for Input
 World has three primitive

functions that ask for user
input
Each question is shown in
a dialog box
 Ask user for a number
 User enters a number
 Ask user for yes or no
 User clicks on yes or no
 Ask user for a string
 User answers with words or a
sentence
3-14
Adding Question for Number
Adding Question for Number
3-16
Class Primitive Object Functions
 Objects have different primitive
functions from the world level
 Class level functions
o
o
o
o
o
Proximity
Size
Spatial relation
Point of view
Other
Object primitive proximity functions
Using Function distance to

Built-in distance to function
o
o

Determines distance from center of one object to
center of another object
Better than guessing distance to object
An Alice function is not a "stand-alone"
instruction
o
It is nested within another instruction
Checkpoint
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What makes a function different from a method?
How do you see a list of functions an object has?
What are the three primitive world functions that
ask the user to input a value?
What are some class level functions?
What does the primitive distance to function do?
3-19
Homework
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Read chapter 3
Do tutorials
o
o
o
o
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Tutorial
Tutorial
Tutorial
Tutorial
3-1 Creating and using a variable, p 116
3-2, Creating a set instruction, p. 118
3-3, Calling an ask user function, p. 122
3-4, Using a proximity function, p. 127
Due one week after assigned with 1 week grace