Programming in Alice

Chapter 2

Today’s Agenda

Designing a Program

Writing Methods

Executing Instructions Simultaneously

Comments

Tips for Setting Up an Initial Scene

2-2

Steps in Creating Programs

Four step process

1. What is problem?

2. Design or plan ahead

3. Implement or write the program

4. Test to see if it works

2-3

Scenario

Problem statement or story

o Also called requirement specifications

Scenario in Alice consists of

o World scene

o Objects

o Actions

2-4

Scenario Provides Answers To

What story is to be told?

What objects are needed?

o Primary objects

o Background objects

What actions are to take place?

o Become instructions in program

2-5

Scenario: First Encounter

After traveling through space, a robot-manned craft just landed on a moon

The robot sets up a camera so scientists in Houston can view this historic event

The camera view shows the robot, lunar lander and nearby rocks

An alien surprises the robot when it peeks out from behind a rock

The robot sees the alien and walks over for a closer look

The alien is frightened and hides behind rocks

The robot calls Houston to report alien

2-6

Step 1: What is Problem?

What story will be told?

o A robot’s first encounter with an alien on a distant moon

What objects are used?

o Robot, lunar lander, alien, lunar background

What actions take place?

o Alien peeks out from behind rock

o Robot turns head around & moves toward alien

o Alien hides behind rocks

o Robot sends message back to earth

2-7

Pseudo Code

List of English worded steps to perform a task

Provides algorithmic structure

Breaks problem down into smaller tasks

List all things that will be done in a scene

You can also make basic drawing to better illustrate scene you wish to create

Book refers to pseudo code

Algorithm will be used in this class since it is a more familiar word and has similar meaning

2-8

Example Sketch For Scenario

2-9

Scenario Scene

Not intended to be artistic

o Can use simple circles, squares and lines

o Label color of shapes

Just give an idea of what scene will look like

2-10

Create Initial World

Use Alice scene editor to add objects

Arrange objects in various poses

Capture screen as each successive scene is created for storyboard

2-11

Algorithm for First Encounter Scenario

Do following steps in order

Alien moves up

Alien says "Slithy toves?"

Robot's head turns around

Entire Robot turns to look at alien

Do together

Robot moves toward alien

Robot legs walk

Alien moves down

Robot turns to look at camera

Robot’s head turns red to signal danger

Robot says “Houston, we have a problem!”2-12

Step 3: Algorithm Implementation

Translate algorithm steps into program

Program

o A list of instructions to have objects perform animation actions from algorithm

When programming you need to be concerned with

o Syntax – statement structure and punctuation

Alice takes care of this for you

You drag instruction to method editor

o Semantics – meaning of statements

o Sequence – order of programming instructions

2-13

Translating the Design

Some algorithm steps can be written as a single instruction

o Ex: robot turns to face alien

Other steps are composite actions that require more than one instruction

o To make the robot legs walk you need more than one instruction to cause multiple legs to move forward and bend at a joint

2-14

Writing Methods

• What is a method?

o Set of instructions

o Create methods by dragging tiles from details pane into the Method Editor

o These tiles are the instructions

o Method Editor not available in Scene Editor mode

2-15

Method Editor

my first method

Dragging Method Tile to Editor • To place instruction in method editor drag from details pane

• Pop-up menu appears asking for direction, select one

• Pop-up menu then asks for amount of distance to move

o Example: move instruction

Arguments used in example

Amount to move (use predefined values or other (enter value with keyboard))

Direction

Drag method to editor

2-16

Method with Instruction Tiles

• Methods execute the instructions (tiles) that are dragged into the Method Editor

2-17

Terminology

• Calling a method - executing a method

• Parameter – tells method more information on what to do

• Argument – information sent to method

• For method hare.move below

• What direction to move?

• Possibilities: up, down, left, right, forward and backward

• How far to move is another argument to send move method

• Passing arguments

• A method is called with arguments sent to its parameters

2-18

object

argument argument

editing tagmethod name

Entering Custom Number

• When other is chosen for amount to move

• A number pad pops up

o Can enter number with keyboard

o Click on number pad

Does not always work

o Click Okay or Cancel when done

o ‘/’ is for fractions instead of manually calculating it

2-19

Editing Tag

• Holds optional arguments

• Options available depend on the method

• Ex: Duration - amount of time for action to take place

• Default - 1 second

2-20

Duration Argument

Each instruction takes 1 second to execute by default

Change time with duration option

Increase or decrease duration for realism

o Increase to show movement over long distance and time

o Decrease to speed up action

o Synchronize objects together

Example: Make leg move in sync with lunar robot

o Lunar robot moved 1 sec at time

o Each leg needed to move forward and backward in 1 sec

o Therefore move forward ½ sec and backward ½ sec

2-21

Style Argument

Specifies how smoothly one movement will blend into next movement

o Gently – begins and ends gradually

o Abruptly – begins and ends instantly

Sometimes needed for smooth transition since gently shows what appears to be an abrupt end and beginning

o Begin gently – begins gradually, ends instantly

o End gently – begins instantly, ends gradually

2-22

Instructions (Methods)

Methods available to use are:

o Move – forward, backward, up, down, left, right

In meters

o Turn – left, right, forward, backward (revolutions)

o Roll – left, right (revolutions)

o Resize – increase or decrease size

o Say – put thought bubble over object

o Turn to face – select who to turn to face

2-23

Primitive Methods

All objects have common set of built-in methods for performing actions

These “primitive methods” get objects to move, turn, change size, etc

Seen in methods tab of details pane

See list in Table 2-1 (pp. 65-66) with explanation of each one

2-24

dot notation: hare.move

Control Statements

Determines the sequence instructions are executed

Drag instruction to code editor

Sequential Action Block

o Actions occur one after

another

Simultaneous Action Block

o Actions occur at same time

Drag block into editor

Do in order

2-25

Do in Order

By default, instructions are executed in the order they appear in the Method Editor one after the other.

It is best to place instructions in “Do in order” block

o Will be easier to move group of instructions if needed later

2-26

2-26

Do Together

• For simultaneous actions, drag“Do together” into the MethodEditor

• Place instructions to be executed simultaneously in “Do together”block

• Make sure the duration for allitems in “Do together” are same

• Otherwise some will finish quicker than others

2-27

2-27

Nesting

• Sometimes two groups of instructions need to be executed simultaneously

• Done by placing a “Do in order”block within a “Do together” block

• Example has brother penguin turn head right and left at same timeas sister penguin does

• If one peguin’s head was turned left and right within one Do together nothing would happen

• Head would turn right and left at exact same time

2-28

Create Program

Choose in what sequence to execute instructions

o At same time

o Consecutively

Refer to algorithm to find instruction order

2-29

First Encounter Program

2-30

Demonstrate Program

Demonstrate creating FirstEncounter

2-31

Step 4: Testing

Important step in creating a program is to run it

o To be sure it does what you expect it to do

Use an incremental development process

o Write a few lines of code then run it

o Write a few more lines and run it

o Write a few more lines and run it

o Allows finding many problems and fix them as you go

Called debugging – find bugs or errors in program

2-32

Comments

Make the code understandable

Not all combination of instructions are immediately obvious

Explains purpose of segment of program to human reader

Helps

o You read program later

o Others when determining what program does

Alice ignores comments when executing program

2-33

Comment Example

2-34

//comment tile

Comment

Tips for Setting Up Initial Scene

2-35

Can set initial position of objects in scene by

o Using mouse mode buttons from chapter 1

o Right clicking object in object tree and move mouse to methods

o Then move mouse to method/action you would like to perform

o This has immediate effect

When methods are placed in method editor will not execute until Play is clicked on

Moving Object to Center of World

Use move to method and choose entire world as method’s argument

Places object’s center point at world’s center point of (0, 0, 0)

o This drives hare’s lower body into ground

o Center of gravity of hare is in abdomen

Useful when lose object in scene or to set starting point

2-36

Positioning Character’s Arms

2-37

Tips for Setting Up Initial Scene

Positioning objects a specified distance apart

Ex: position teapot’s center point ½ meter to right of plate’s center point

o Move teapot object so its center point is same as plate’s center point

o Set pointOfView in properties of both objects world’s center point of (0, 0, 0)

o Use move method by right clicking on teapot then method then choose direction of rightthen ½ meter under amount

2-38

Moving Camera To An Object

• Right-click the object and choose Camera get a good look at this

• Camera moves to a position so the object is in plain view

2-39

Homework

Read chapter 2

Do tutorials

o Tutorials 2-1, p 62

o Tutorials 2-2, p. 67

o Tutorials 2-3, p. 82

o Tutorials 2-4, p. 89

o Tutorials 2-5, p. 98

Do exercises in handout

Due one week after it is assigned

1-40

Lab

Lab exercises require you to demonstrate your program to the professor

When you write code you must explain to professor what each line of code does and why.

Professor will sign lab sheet after you do that

You only get full credit for lab exercise if professor signs lab sheet

If you hand in lab without explaining code then you will lose half credit