Variables and Functions

Chapter 3

3-1

Variables Named storage location in computer’s

memory Programs may need to store data when

runningo Stored in a variable

Types of data that can be storedo Local variableso Parameter variableso Class-level variableso World-level variables

3-2

Variable Types Local variables

o Belongs to a specific methodo Only used in that methodo Variables cease to exist when method stops

Parameter variableso Holds argument that is passed to a called

method Class-level variables

o Variable that belong to a specific object World-level variables

o Variable that belongs to the world3-3

Creating Local Variables Local variables belong to

a specific method (such as world.my first method)

Create variable by clicking on create new variable in desired method

Variables require 3 thingso Nameo Typeo Initial value

3-4

name

type

value

Names, Types and Initial Values• Variable Names

• Must be unique within the method• Should be meaningful and reflect the variable’s

purpose• Named using camelCase

• Variable Types• Numbers, Boolean, Objects, Other (such as

String, Color, or Sound)

• Initial Value• Value initially stored in the variable

3-5

Variables

Tile appearance shows the type of variable

Numbers, Boolean, Objects, String Variable declaration - creating a variable

3-6

Variable Assignment

• Variables have an initial value• Initial value held until a different value is

assigned• New values can be

assigned while the method is runningo Use Set instructionso Done by dragging variable tile into Methods

Editor

3-7

Checkpoint What is a variable? What are three things that must be specified

when creating a new variable? In what part of the Method Editor do

variable tiles appear? What do the characters 123 appearing on a

variable tile tell you? How do you create a set instruction for a

variable?

3-8

Function Asks a question

o Checks a conditiono Computes a value

Used to get information about…o Properties of objects

Is snowwoman's face red?

o Relationship of one object to another What is distance between lunar robot and rock?

o A current condition What keyboard key was pressed?

World Level Primitive Functions List of functions categories

o Boolean logico Matho Randomo Stringo Ask usero Mouseo Timeo Advanced matho Other

3-10

Value What a function returns after it

o Asks questiono Performs computation

Type of value depends on kind of functiono In lunar example

What is distance of robot to rock?

o Number value is expected to be returned Whole number ex: 3 meters Fractional value, ex: 1.2 meters

Types of Values Number

o As in example Boolean

o true, false String

o “Slithy Toves” Object

o snowman, helicopter Position in world

o (0, 0, 0) – center of an Alice world

Asking User for Input

World has three primitive functions that ask for user input

Each function is displayed in a dialog box

3-13

Is it Yes or No?

Adding Question for Number

Class Primitive Object Functions Objects have different

primitive functions from the worldo Proximityo Sizeo Spatial relationo Point of viewo Other

Object primitive proximity functions

Using Function distance to

Built-in distance to function o Determines distance from center of one object

to center of another objecto Better than guessing distance to object

An Alice function is not a "stand-alone" instructiono It is nested within another instruction

Checkpoint What makes a function different from a

method? How do you see a list of the functions an

object has? What are the three primitive world functions

that ask the user to input a value? What are some class level functions? What does the primitive distance to function

do?

3-17

Creating Math Expressions Math Operators are used to

create math expressions Math expressions perform a

calculation and return a value Simple math expression

o 12 + 2o Value returned is 14

Operands appear to the right and left of an operatoro Example with variables “payRate” and

“hoursWorked” payRate * hoursWorked

Operator Description

+ Addition

- Subtraction

* Multiplication

/ Division

3-18

Determining Distance Camera angle influences our perception of scene We need assistance with determining distance Example: How many meters does robot need to walk to

reach rocks where alien is hiding?o Can use

pointOfView property Primitive function

Avoiding Collisions• When program is run, robot collides with

rocko Distance between two objects is measured

center-to-center

• One way to avoid a collisiono Reduce calculated distance to objecto Use expression to subtract small number (1 or

2) from distance

Expressions in Arguments Math expressions can be used in place of

o Fixed valueo To adjust value returned by function

Click down arrowto get math operation

o Adjusts distance to move to not collide with rock

Adding Math Expression in Alice

3-22

Subtracting Functions Two meters subtracted from distance is arbitrary To be more precise, subtract width of rock

o But still not accurate

Resulting expression subtracts value of one function from value of another function

More Precision

The most accurate way to determine distance and to avoid collisiono Add half width of both objects subtract from

distance Be careful of excessive parentheses

Checkpoint What do the following operators do in most

programming languages?o + - * /

What does a math expression do? When you use the move to instruction to

move one object to another object, why do they collide?o What are multiple methods you can use to avoid

this? When can an object be driven into the

ground or above the ground when using move to or move toward instructions? 3-25

Working with Strings and Text

A string is a sequence of characters Strings represent

o Nameso Addresseso Messageso etc.

3-26

Asking User to Enter a String World’s ask user function can ask user to enter string Function returns the text that the user enters Example displays what user entered in dialog bubble

3-27

Defines string variable called name

Asks user for name

Stores in variable name Puts contents of name variable in bubble

Joining Strings

String Concatenationo Joins string a with string bo Strings a and b are argumentso Does not change the values

of strings a or b

World functiono a joined with b

Drag a joined with b from string section in World function to where you want to join strings

3-28

Example of a joined with b

3-29

Hello joined with was only thing added

Converting Numbers to Strings Sometimes you need to display numbers to

user Number needs to be converted to string first Use world functionWhat as a string

Example of what as a string

3-31

3D Text can be added to Alice worlds Found at end of the local gallery Pop up window appears after clicking

on it Enter text and font style

3-32

Homework Read chapter 3 Do tutorials

o Tutorial 3-1 Creating and using a variable, p 116o Tutorial 3-2, Creating a set instruction, p. 118o Tutorial 3-3, Calling an ask user function, p. 122o Tutorial 3-4, Using a proximity function, p. 127o Tutorial 3-5 Using math to avoid collisions, p 132o Tutorial 3-6, Converting a number variable to a string, p.

141 Answer questions in handout Do lab assignments after handing in answered

questions Due one week after assigned with 1 week grace