Variables and Functions

1
Variables and Functions

• Alice

2
Naming is Important

• If you get a new pet one of the first things you
  do is name it
• Gives you a way to refer to the new pet without
  saying
• Please take that dog we got yesterday for a walk.
• Please take Fifi for a walk.
• In programming we name things we want to refer to
  again
• Gives us a way to work with them
• Like the World object
• This is called declaring a variable

3
Declaring a Variable

• To be able to refer to an object again we need to
  specify what type of thing it is and give it a
  name
• This is also called declaring a variable
• Type name OR
• Type name new Class(value, value, )
• The equal sign doesnt mean equal
• But assign the value of the variable on the left
  to the result of the stuff on the right
• The following creates a variable named earth
  which refers to a World object created on the
  right
• World earth new World()

4
Declaring Variables

• Variables are names associated with values
• If the type of the variable is null
• It doesnt refer to an object yet
• Variables can be reused
• World earth null
• earth new World()
• earth new World()

null
earth
World Object 1
earth
earth
World Object 2
5
A Variable Associates a Name with Space

• A variable is like a box with a label on it
• You can put something in a box
• You can take something out of a box
• You can even change what is in the box
• The size of the box restricts what you can put in
  it

Hat Box
6
Limits on Declaring Variables

• You can't declare two variables with the same
  name!
• gt World earth new World()
• gt World earth new World()
• Error Redefinition of 'earth'
• You can change what an object variable refers to
• gt World earth new World()
• gt earth new World()

7
Declaring Variables and Creating Objects

• You can declare a variable and assign it to refer
  to a new object in one statement
• World earth1 new World()
• Turtle tommy new Turtle(earth1)

Declaration of variables
Creating the objects
8
Storing a Value in Variable x

• Alice
• x.set( value, something )?
• Java
• x something

• object
• number
• string
• some computation

9
Functions in Alice

• A method in Alice is called a void function in
  other languages, because it returns nothing.
• Returns? What's returns?
• A non-void function returns information
• In Alice, this is a function

10
How a function works

• A function receives value(s), performs some
  computation on the value(s), and returns (sends
  back) a value.

11
Types of Functions

• The type of a function depends on the type of
  value it returns.
• a number
• a specific object
• a color
• a Boolean (true or false)?
• other property values

12
SumOfSquares Function

• The sumOfSquares function receives two value(s) x
  and y, performs the computation x2 y2 on these
  value(s), and returns (sends back) this result.

X 3, y 5
x 1, y 4
x2 y2
34
17
13
SumFrom1To Function

• The sumFrom1To function receives a value N, and
  performs the computation 1 2 N, and
  returns this sum.

N 3
N 100
1 2 N
6
5050
14
Using a Built-in Function

• The distanceTo( ) dest_object function receives
  an object and determines the distance from the
  dest_object and the object and sends the value
  back to the caller.

cowboy
distanceTosheriff (this)?
1.3
15
(No Transcript)
16
Give Function Demo
17
A new example

• A common task in sports game programs is to
  bounce a ball. To illustrate how this is done,
  let's bounce the ball over the net. (The ball has
  been positioned 1 meter from the net.)?
• The ball should move up and forward and then down
  and forward in a pattern that looks something
  like this
• Note This looks easy but do not be
  deceived!

18
Design Storyboard

• A design for a World-level method

World.ballOverNet Do in order toyball turn
to face the net Do together toyball
move up toyball move forward Do
together toyball move down toyball
move forward
19
Demo

• Concepts illustrated in this example
• Movement of an object is sometimes relative to
  another object. In this example,
• a built-in height function is used to determine
  the height of the net. Then the ball moves up
  enough to clear the net.
• the toyball's up and down movements are relative
  to the ground. An orient To method
  (alternatively stand Up) is used because it is
  not easy to tell "which way is up."

20
Rolling the ball

• Another sports game action involves rolling a
  ball along the ground.
• We want a realistic motion rather than a slide.
• The ball must simultaneously move and roll.

realisticRoll Do together move ball forward 1
meter turn ball forward 1 revolution
21
Demo

• Our design assumed that the ball's motion is
  relative to the ground. But the ball turns
  relative to it's own sense of direction.
• AsSeenBy ground can be used to make the ball
  turn relative to the ground.

22
Revising the approach

• The ball is made to roll 1 revolution. Suppose
  we want the ball to roll a certain distance (say
  3 or 4 or even 10 meters) along the ground.
• How many times should the ball turn 1 complete
  revolution?

23
(No Transcript)
24
Parameters

• We want to return the value computed as
• distance/( ? diameter)?
• Obviously, what is needed is
• the balls diameter
• the ball object has a built-in width function
• the distance the ball is to travel
• can be sent as a parameter to the function

25
Demo

• Concepts illustrated in this example
• A function must have a return statement to send
  back a computed value.
• In this example, a math expression is created to
  compute a number value.
• The order of evaluation is indicated by the use
  of parentheses, as in traditional math
  expressions.

26
Calling the function
test values We should run the
animation with several test values to be sure it
works as expected. What happens if you use a
negative value?
27
Levels of Functions

• As with methods, functions can be either
  class-level or world-level. (The function just
  presented was class-level.)?
• The guidelines for class-level methods also apply
  to class-level functions
• No references to other objects.
• No references to world-level functions you have
  written (built-in world-level functions are fine
  to use).