Alice in Action with Java

Alice in Action with Java Chapter 4 Flow Control

Objectives • Use the Booleantype and its basic operations • Use the ifstatement to perform some statements while skipping others • Use the forand whilestatements to perform (other) statements more than once • Use Booleanvariables and functions to control if and whilestatements • Use the wait()message to temporarily suspend program execution Alice in Action with Java

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

Flow Control (continued) Alice in Action with Java

The Boolean Type • A basic Alice type used to define Boolean variables • ABooleanvariableholds a value of trueorfalse • Other basic types: Number and Object • Condition (Boolean expression) • Produces a value of trueorfalse • Basis for decision-making in programs Alice in Action with Java

Boolean Functions • Return a value of true or false • Can act as a condition in an ifor 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

BooleanVariables • Used to store a value of trueorfalse • Can be used in condition for if or while statement • How to create a Booleanvariable • Click create new variable(or parameter) button • Specify Booleanas variable (or parameter) type Alice in Action with Java

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

Relational Operators (continued) Alice in Action with Java

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 • ageis aNumbervariable • Teen number compared to condition returns true Alice in Action with Java

Boolean Operators (continued) Alice in Action with Java

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

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

ifStatement 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 Alice in Action with Java

ifStatement Mechanics (continued) Alice in Action with Java

Building if Statement Conditions • Coding the condition of the if statement • Click on the yesOrNoparameter • Drag parameter into the editing area • Drop the parameter onto the condition’s placeholder • Choose otherand 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

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

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

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

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

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

Validating Parameter Values (continued) Alice in Action with Java

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 loopcontrol 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

Introducing Repetition (continued) Alice in Action with Java

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 } • Thefor 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

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

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 limitvalue • 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

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

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 Objectswindow • 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

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

Nested Loops (continued) Alice in Action with Java

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 limitvalue is unknown • Solution: use a while statement Alice in Action with Java

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 cameracloser • 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

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

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

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

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 Conditionistrue • One statement must eventually falsify Condition Alice in Action with Java

while Statement Mechanics (continued) Alice in Action with Java

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

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 downdistanceToGroundmeters • 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

A Second Example (continued) Alice in Action with Java

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

Alice in Action with Java