1. Alice in Action with Java
Chapter 3
Variables and Functions

2. Objectives Use variables to store values for use later in a method
Use a variable to store the value of an arithmetic expression
Use a variable to store the value produced by a function
Use parameters to write methods that are more broadly useful
Alice in Action with Java

3. Objectives (continued)
Define and access property variables
Use the vehicle property to synchronize the movements of two objects
Create functions — messages that return a value to their sender
Alice in Action with Java

4. Variables and Functions
Variable: named container for storing/retrieving data
Three types of variables
Method (local): defined for use within a method
Parameter: variable passed to a method
Object (property): used to store an object property
Tools for creating method variables and parameters
Buttons in the upper-right corner of the method box
Tool for creating object variables
Button under the property pane of the details area
Alice in Action with Java

5. Variables and Functions (continued)
Alice in Action with Java

6. Method Variables Defined using create new variable button
Information needed to define a variable
Name: refers to a location in memory
Type: refers to kind of data stored; e.g., a number
Initial value: starting value of specified type; e.g., 1
Method variables are local (valid only in the method)
Common uses of method variables
Computing and storing values for later retrieval
Storing values entered by a user
Alice in Action with Java

7. Example 1: Storing a Computed Value
User story objects: nativeGirl and horse
Objective: move nativeGirl closer to horse
Strategy for moving the girl the right distance
Define variable storing distance from the girl to horse
Ask the girl how far she is from the horse
Store the girl’s reply in the variable
Use variable’s current value in a move()message
Preparing to program with user-defined variables
Position nativeGirl and horse in a new world
Review types: Number, Boolean, Object, Other
Alice in Action with Java

8. Example 1: Storing a Computed Value (continued)
Alice in Action with Java

9. Example 1: Storing a Computed Value (continued)
Creating the first local variable
Click the create new variable button
Define Number type named distanceToHorse
Drag distanceToHorse to the editing area
Set the variable value using a placeholder value of 1
Click on functions pane for nativeGirl object
Drag distanceInFrontOf()onto placeholder
Specify the entire horse as the object argument
Alice in Action with Java

10. Example 1: Storing a Computed Value (continued)
Alice in Action with Java

11. Example 1: Storing a Computed Value (continued)
Alice in Action with Java

12. Example 1: Storing a Computed Value (continued)
Using distanceToHorse in the move() method
Drag the move()message into the editing area
Select Forward->expressions->distanceToHorse
A test of method shows the girl too close to horse
Adjusting the distance that the girl moves
Click the list arrow next to distanceToHorse
Select math->distanceToHorse-->.5
Use functions and variables over trial-and-error
Behavior will adapt to repositioning of objects
Alice in Action with Java

13. Example 1: Storing a Computed Value (continued)
Alice in Action with Java

14. Example 1: Storing a Computed Value (continued)
Alice in Action with Java

15. Example 1: Storing a Computed Value (continued)
Alice in Action with Java

16. Example 2: Storing a User-Entered Value
User story built around the Pythagorean Theorem
skaterGirl requests the lengths of two sides
User passes edge1 and edge2 values to skaterGirl
skaterGirl computes hypotenuse and displays value
Overview of implementation
Create skaterGirl method computeHypotenuse()
Declare three Number variables for triangle sides
Add two say()statements for interactive dialog
Add set() method for edge1
Incorporate NumberDialog into set()parameter list
Alice in Action with Java

17. Example 2: Storing a User-Entered Value (continued)
Alice in Action with Java

18. Example 2: Storing a User-Entered Value (continued)
Alice in Action with Java

19. Example 2: Storing a User-Entered Value (continued)
Overview of implementation (continued)
Add set()method for edge2
Incorporate NumberDialog into set()parameter list
Add set()method for hypotenuse
Drag and drop Math.sqrt()over placeholder value
Pass a mathematical expression to Math.sqrt()
Define String variable named hypotString
Convert hypotenuse to string and store in hypotString
Add say()method to display results
Display concatenates dialog and hypotString value
Alice in Action with Java

20. Example 2: Storing a User-Entered Value (continued)
Alice in Action with Java

21. Example 2: Storing a User-Entered Value (continued)
Alice in Action with Java

22. Parameters Argument: value passed to an object via a message
Parameter: variable that stores an argument
Example using the say() message
The parameter is a single variable of String type
One possible argument is “Oh, hello there!”
You may pass multiple arguments to messages
Example: roll() message requires two arguments
Alice in Action with Java

23. Example 1: Old MacDonald Had a Farm
Objective: display song lyrics for four animals
Use of parameters helps avoid repetitious coding
Two parameters needed: animal and noise
Overview of implementation
Create new world with scarecrow before fence
Create singVerse()method for scarecrow
Add two parameters, animal and noise
Add String types, firstLine and doubleNoise
Incorporate parameters and variables in say()methods
Call singVerse()messages in singOldMacDonald()
Alice in Action with Java

24. Example 1: Old MacDonald Had a Farm (continued)
Alice in Action with Java

25. Example 1: Old MacDonald Had a Farm (continued)
Alice in Action with Java

26. Example 1: Old MacDonald Had a Farm (continued)
Alice in Action with Java

27. Example 2: Jumping Fish!
User story: fish jumps, traces an arc, returns to water
Fish object used is an instance of Pinkminnow
Basic elements in the design of jump()method
One parameter: distance
Local var: height, halfDist, startStopDist, angle
Approximate height of jump as 1/3 of the distance moved
Assume starting/stopping point is 1/2 distance of jump
Set the pitch angle to .1
The jump()method is implemented in Figure 3-35
Program should be tested with multiple distance values
Alice in Action with Java

28. Example 2: Jumping Fish! (continued)
Alice in Action with Java

29. Example 2: Jumping Fish! (continued)
Alice in Action with Java

30. Example 2: Jumping Fish! (continued)
Alice in Action with Java

31. Property Variables Also called instance variables or object variables
Property variables are defined within an object
Use the properties pane of the object’s details area
Values in an object variable are not shared
How to create and use a property variable
Open a new world and add a wizard object
Click on wizard properties
Click create new variable
Define String type (in Other list) called myName
Initialize myName to Jim
Alice in Action with Java

32. Property Variables (continued)
Alice in Action with Java

33. Property Variables (continued)
How to create and use a property variable (continued)
Duplicate the wizard using copy button
Close the Add Objects window
Click on the second wizard in the object tree
Click the properties tab in the details area
Change the second wizard’s name from Jim to Tim
Testing the identity of the wizards
Use say()messages to display each wizard’s name
Argument used: “My name is “ + wizard.myName
Check object for predefined properties; e.g., opacity
Alice in Action with Java

34. Property Variables (continued)
Alice in Action with Java

35. Property Variables (continued)
Alice in Action with Java

36. Alice Tip: Using the Vehicle Property
The thing on which an object “rides”
The default vehicle for an object is the world
Vehicle property is used to synchronize movements
Using vehicle property in program with girl and horse
Scene 4 initially shows the nativeGirl on a horse
Add set()method for nativeGirl’s vehicle property
Set the value of the vehicle to the entire horse
Include a move()method below the set()method
Send playScene4()to my_first_method() and test
Alice in Action with Java

37. Alice Tip: Using the Vehicle Property (continued)
Alice in Action with Java

38. Alice Tip: Using the Vehicle Property (continued)
Alice in Action with Java

39. Functions Messages used to retrieve information
May be defined by the user
Alice in Action with Java

40. Example: Retrieving an Attribute from an Object
User story involves program with twin wizards
Objective: enhance program by using titles with names
Jim uses The Enchanter, and Tim uses The Magus
Creating the title property
Define property variable myTitle with type String
Assign title values to each wizard’s myTitle variable
Simplest way to access name and title
Pass myTitle concatenated with myName to say()
Problem: extra time needed to build the full name
Alternative: use a function to produce the full name
Alice in Action with Java

41. Example: Retrieving an Attribute from an Object (continued)
Alice in Action with Java

42. Example: Retrieving an Attribute from an Object (continued)
Defining a function to produce a full name
Select wizard object and then click functions pane
Click the create new function button
Declare function called getFullName as String type
Create variable called fullName
Set value of fullName to concatenated name and tile
Include fullName in the return statement
Defining a simpler function for the second wizard
Return myTitle concatenated with myName
Send both functions to my_first_method
Alice in Action with Java

43. Example: Retrieving an Attribute from an Object (continued)
Alice in Action with Java

44. Example: Retrieving an Attribute from an Object (continued)
Alice in Action with Java

45. Example: Retrieving an Attribute from an Object (continued)
Alice in Action with Java

46. Functions with Parameters
Functions can use parameters to store arguments
Adapting skaterGirl program to use functions
Create Number type called calculateHypotenuse()
Add parameters Number, leg1, and leg2
Create local variable named hypot
Set value of hypot to mathematical expression
Incorporate new function into computeHypotenuse()
Comparing functions to methods
Functions return a value and appear in parameter list
Methods do not return a value and appear as statements
Alice in Action with Java

47. Functions with Parameters (continued)
Alice in Action with Java

48. Functions with Parameters (continued)
Alice in Action with Java

49. Summary Variable: named container for storing and retrieving values
Types of variables: method variables, parameters, object variables
Data types: Number, Boolean, Object, String
Parameter: container for an argument
Argument: data passed to a method or function
Alice in Action with Java

50. Summary (continued) Object vehicle: thing on which an object “rides”
A method does not return a value to its sender
A method is sent from a place where a statement can appear
A function does return a value to its sender
A function is sent from a place where a value can appear
Alice in Action with Java