1. Alice in Action with Java
Chapter 4
Flow Control

2. Objectives Use the Boolean type and its basic operations
Use the if statement to perform some statements while skipping others
Use the for and while statements to perform (other) statements more than once
Use Boolean variables and functions to control if and while statements
Use the wait()message to temporarily suspend program execution
Alice in Action with Java

3. Flow Control Flow: sequence of steps for performing a user story
Flow control statement: structure for managing flow
Flow control statements used in previous chapters
doInOrder: produces a sequential execution
doTogether: produces a parallel execution
Control statements introduced in the current chapter
if: directs program flow along one of two paths
for: directs flow into a fixed number of loops
while: directs flow into an arbitrary number of loops
Alice in Action with Java

4. Flow Control (continued)
Alice in Action with Java

5. The Boolean Type A basic Alice type used to define Boolean variables
A Boolean variable holds a value of true or false
Other basic types: Number and Object
Condition (Boolean expression)
Produces a value of true or false
Basis for decision-making in programs
Alice in Action with Java

6. Boolean Functions Return a value of true or false
Can act as a condition in an if or while statement
Many refer to an object’s bounding box
Example: obj.isBehind(obj2)
true, if obj’s position is beyond obj2’s rear edge
false, otherwise
Alice in Action with Java

7. Boolean Variables Used to store a value of true or false
Can be used in condition for if or while statement
How to create a Boolean variable
Click create new variable (or parameter) button
Specify Boolean as variable (or parameter) type
Alice in Action with Java

8. Relational Operators Produce true or false values
Six relational operators: ==, !=, <, <=, >, >=
Located in functions pane of world’s details area
Most often used to compare Number values
Example: hoursWorked > 40
hoursWorked is a Number variable
true when more than 40 hours have been worked
Alice in Action with Java

9. Relational Operators (continued)
Alice in Action with Java

10. Boolean Operators Used to modify or combine relational operations
Three Boolean operators: AND, OR, NOT
Located in functions pane of world’s details area
Example: age > 12 && age < 20
age is a Number variable
Teen number compared to condition returns true
Alice in Action with Java

11. Boolean Operators (continued)
Alice in Action with Java

12. Introducing Selective Flow Control
Summary of a scene with a princess and a dragon
Princess meets a mute dragon and asks questions
Dragon shakes its head to respond yes or no
Objective: write a shakeHead()method
Requirements for shakeHead()
Parameter: yesOrNo, a String
If yesOrNo == “yes”, dragon shakes head up and down
If yesOrNo == “no”, dragon shakes head sideways
Use an if statement to produce conditional behavior
The if control structure is at bottom of editing area
Alice in Action with Java

13. Introducing Selective Flow Control (continued)
Alice in Action with Java

14. if Statement Mechanics
Value of a condition determines direction of flow
Structure of an if statement:
if (Condition ) { Statements } else { Statements }
if statement behavior is also called selective flow
If Condition is true, Statements1 are selected
If Condition is false, Statements2 are selected
Alice in Action with Java

15. if Statement Mechanics (continued)
Alice in Action with Java

16. Building if Statement Conditions
Coding the condition of the if statement
Click on the yesOrNo parameter
Drag parameter into the editing area
Drop the parameter onto the condition’s placeholder
Choose other and then type “yes”
Overview for coding the remainder of shakeHead()
Add headMovement variable for amount of turn
Add turn()statements for up and down motion
Add turn()statements for sideways motion
Alice in Action with Java

17. Building if Statement Conditions (continued)
Alice in Action with Java

18. Building if Statement Conditions (continued)
Building a scene method that uses shakeHead()
princess greets dragon using a say()message
princess asks four questions
shakeHead()is called in response to each question
Click the Play button to test the program
Alice in Action with Java

19. Building if Statement Conditions (continued)
Alice in Action with Java

20. Building if Statement Conditions (continued)
Alice in Action with Java

21. The wait()Statement Pauses a program for specified number of seconds
Form of wait()statement: wait(numSecs);
Use of wait()scene with dragon and princess
Inserted between princess’s first and second lines
Alice in Action with Java

22. Validating Parameter Values
if statement can be used to guard set of statements
Flow enters only if parameter values are valid
Example: check distance value passed to jump()
Check for positive value with condition distance > 0
Check jump length with distance < MAX_DISTANCE
Combine two conditions with the AND (&&) operator
distance > 0 && distance <= MAX_DISTANCE
How to incorporate validating logic using if structures
Place original jump()logic onto true path (outer if)
Place validating logic in the false path (nested if)
Alice in Action with Java

23. Validating Parameter Values (continued)
Alice in Action with Java

24. Validating Parameter Values (continued)
Alice in Action with Java

25. Validating Parameter Values (continued)
Alice in Action with Java

26. Introducing Repetition
Refer to flapWings()method from Figure 2-16
Enhancement: use for loop to flap wings numTimes
Overview for implementing the enhancement
Open the flapWings()method
Adjust the duration values for the wing movements
Drag loop control to the top of the method and drop
Select numTimes for number of iterations
Drag the doInOrder statement into the for statement
Alice in Action with Java

27. Introducing Repetition (continued)
Alice in Action with Java

28. Introducing Repetition (continued)
Alice in Action with Java

29. Mechanics of the for Statement
Repeat statement execution a fixed number of times
Example: pass 3 to flapWings()for 3 flaps
Structure of the simple for statement
for(int index = 0;index < limit;index++){ Statements }
The for statement is also known as a counting loop
First statement in ( ) initializes the index
Second statement in ( ) checks index against limit
Third statement in ( ) increments the index
Alice in Action with Java

30. Mechanics of the for Statement (continued)
Alice in Action with Java

31. Mechanics of the for Statement (continued)
To test a for loop, trace the behavior with values
Statements are executed while index < numTimes
Example: send flapWings(3)to the dragon object
Simple version of for lets you modify limit value
Purpose of show complicated version button
Change initial value of index and/or update to index
Example: change update to index+=2
Note: neither version of for allows you to count down
Alice in Action with Java

32. Mechanics of the for Statement (continued)
Alice in Action with Java

33. Mechanics of the for Statement (continued)
Alice in Action with Java

34. Nested Loops Three shots enhancing Scene 1 of dragon animation
Dragon flies toward a castle in the countryside
As dragon nears castle, it circles the tower three times
Dragon then descends and lands on the drawbridge
One way to build the first shot
Go to go into the Add Objects window
Position the dragon above the castle’s drawbridge
Move dragon up until it is even with the castle’s tower
Drop a dummy and then drag the dragon off-screen
Use setPointOfView()to properly position dragon
Alice in Action with Java

35. Nested Loops (continued)
One way to build the second shot
Use for statement to control other flying statements
Understanding the mechanics of Shot 2
Outer for controls inner (nested) for in flapWings()
AsSeenBy()attribute revolves dragon around castle
Increase duration of turn()to synchronize moves
Set style to smooth the animation
The third shot is discussed in Section 4.4
Alice in Action with Java

36. Nested Loops (continued)
Alice in Action with Java

37. The while Statement
limit value in for loop must be set to a fixed value
Circumstance when the for loop is appropriate
Statements are to be executed a fixed number of times
Problem: looping when the limit value is unknown
Solution: use a while statement
Alice in Action with Java

38. Introducing the while Statement
Strategy for building third shot of dragon animation
Repeatedly have dragon flap its wings
Move dragon downward while it is above drawbridge
Overview for building the third shot
Place doTogether statement in doInOrder statement
Use setPointOfView()to move camera closer
Send flappingWings()message to the dragon
Drag the while statement to the editing area
Drop the while statement below doInOrder
Insert placeholder value into the while condition
Alice in Action with Java

39. Introducing the while Statement (continued)
Alice in Action with Java

40. Introducing the while Statement (continued)
Building the third shot (continued)
Drag dragon’s isAbove()over condition and drop
Add castle.bridge argument to isAbove()
Insert doTogether statement in the while loop
Add move() method to cause dragon to descend
Send another flapWings()message to dragon
Use setPointOfView()to zoom in for a close-up
Infinite loop occurs if loop lacks falsifying condition
In third shot, move()eventually terminates loop
Alice in Action with Java

41. Introducing the while Statement (continued)
Alice in Action with Java

42. Introducing the while Statement (continued)
Alice in Action with Java

43. while Statement Mechanics
Provides for both definite and indefinite looping
Structure of a while loop
while ( Condition ) { Statements }
The while loop is more general than the for loop
Flow enters while structure if Condition is true
One statement must eventually falsify Condition
Alice in Action with Java

44. while Statement Mechanics (continued)
Alice in Action with Java

45. Comparing the for and while Statements
while statement can produce any type of repetition
for statement is used for fixed number of repetitions
Loop selection question to ask: “Am I counting?”
If yes, use a for statement; otherwise, use while
Both loops test conditions before flow enters structure
Both loops are bypassed if initial condition is false
Alice in Action with Java

46. A Second Example Setting up the scene
Use shebuilder to create a soccer player (Jane)
Place soccerBall object in Jane’s hands
Writing a dropBounce()method for soccerBall
Move the ball down distanceToGround meters
Change distanceToGround by bounce factor (2/3)
Move ball up reduced distanceToGround meters
Bouncing continues while distanceToGround > 0
Writing a janeDropsBall()method
Send roll()messages to forearms
Send dropAndBounce()message to soccerBall
Alice in Action with Java

47. A Second Example (continued)
Alice in Action with Java

48. A Second Example (continued)
Alice in Action with Java

49. A Second Example (continued)
Alice in Action with Java

50. Flow-Control Functions
Functions can be defined to answer questions
Complex questions are asked by flow-control functions
Alice in Action with Java

51. Spirals and the Fibonacci Function
User story
Scene 1: girl finds an old book and reads contents
Scene 2: girl uses the map to locate a palm tree
Scene 3: girl follows spiral from tree to treasure site
Coding spiral motion in Scene 3 is greatest challenge
Many natural spirals are based on Fibonacci numbers
Fibonacci numbers (sequence)
Number > 1 is found by adding two preceding numbers
Example: 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ...
Spiral inscribed in rectangles built from the sequence
Alice in Action with Java

52. Spirals and the Fibonacci Function (continued)
Alice in Action with Java

53. Spirals and the Fibonacci Function (continued)
Approximating Fibonacci spiral in playScene3()
Have girl move 6 times
Distance of each move equals next Fibonacci number
While moving forward, girl also turns left 1/4 revolution
playScene3()requires a fibonacci()function
To be designed and implemented in the next section
Save girl as fibonacciGirl for possible reuse
Alice in Action with Java

54. Spirals and the Fibonacci Function (continued)
Alice in Action with Java

55. The Fibonacci Function
Defining the outline of fibonacci()function
Select girl and create a function named fibonacci
Create a Number parameter named n
Formula: if n > 1, f(n) = sum of two preceding numbers
Designing an algorithm to generate the nth number
Create local variables: result, nextToLast, last
Add if statement to the function
If n == 1 or n == 2, result = 1
Otherwise calculate nth value using formula in for loop
fibonacci()calls in playScene3() specify spiral
Alice in Action with Java

56. The Fibonacci Function (continued)
Alice in Action with Java

57. Summary
Flow control statement: controls the flow of statement execution
Condition: Boolean expression producing a true or false value
Boolean function: returns true or false value
Boolean variable: holds value of true or false
Relational operators: ==, !=, <, <=, >, >=
Alice in Action with Java

58. Summary (continued) Boolean operators: AND, OR, NOT
if statement: directs flow along one of two paths based on evaluation of a condition
for statement: repeats execution of a group of statements a fixed number of times
while statement: repeats execution of a group of statements an arbitrary number of times
wait()statement: a statement that pauses program flow for a specified number of seconds
Alice in Action with Java