Alice in Action with Java

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

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

4
Flow Control (continued)
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

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 objects bounding box
• Example obj.isBehind(obj2)
• true, if objs position is beyond obj2s rear
  edge
• false, otherwise

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

8
Relational Operators

• Produce true or false values
• Six relational operators , !, lt, lt, gt, gt
• Located in functions pane of worlds details area
• Most often used to compare Number values
• Example hoursWorked gt 40
• hoursWorked is a Number variable
• true when more than 40 hours have been worked

9
Relational Operators (continued)
10
Boolean Operators

• Used to modify or combine relational operations
• Three Boolean operators AND, OR, NOT
• Located in functions pane of worlds details area
• Example age gt 12 age lt 20
• age is a Number variable
• Teen number compared to condition returns true

11
Boolean Operators (continued)
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

13
Introducing Selective Flow Control (continued)
14
if Statement Mechanics

• Value of a condition determines direction of flow
  
• Structure of an if statement
• if (Condition )
  Statements1
  else
  Statements2
• if statement behavior is also called selective
  flow
• If Condition is true, Statements1 are selected
• If Condition is false, Statements2 are selected

15
if Statement Mechanics (continued)
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 conditions
  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

17
Building if Statement Conditions (continued)
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

19
Building if Statement Conditions (continued)
20
Building if Statement Conditions (continued)
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 princesss first and second
  lines

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
  gt 0
• Check jump length with distance lt MAX_DISTANCE
• Combine two conditions with the AND () operator
  
• distance gt 0 distance lt 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)

23
Validating Parameter Values (continued)
24
Validating Parameter Values (continued)
25
Validating Parameter Values (continued)
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

27
Introducing Repetition (continued)
28
Introducing Repetition (continued)
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 0index lt limitindex) Stateme
  nts
• 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

30
Mechanics of the for Statement (continued)
31
Mechanics of the for Statement (continued)

• To test a for loop, trace the behavior with
  values
• Statements are executed while index lt 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 index2
• Note neither version of for allows you to count
  down

32
Mechanics of the for Statement (continued)
33
Mechanics of the for Statement (continued)
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 castles drawbridge
• Move dragon up until it is even with the castles
  tower
• Drop a dummy and then drag the dragon off-screen
• Use setPointOfView()to properly position dragon

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

36
Nested Loops (continued)
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

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

39
Introducing the while Statement (continued)
40
Introducing the while Statement (continued)

• Building the third shot (continued)
• Drag dragons 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
  

41
Introducing the while Statement (continued)
42
Introducing the while Statement (continued)
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

44
while Statement Mechanics (continued)
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

46
A Second Example

• Setting up the scene
• Use shebuilder to create a soccer player (Jane)
• Place soccerBall object in Janes 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 gt 0
• Writing a janeDropsBall()method
• Send roll()messages to forearms
• Send dropAndBounce()message to soccerBall

47
A Second Example (continued)
48
A Second Example (continued)
49
A Second Example (continued)
50
Flow-Control Functions

• Functions can be defined to answer questions
• Complex questions are asked by flow-control
  functions

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 gt 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

52
Spirals and the Fibonacci Function (continued)
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

54
Spirals and the Fibonacci Function (continued)
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 gt 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

56
The Fibonacci Function (continued)
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 , !, lt, lt, gt, gt

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