Alice Variables: Intro

• A variable is a name that represents a storage location for a value

– Represented by an identifier (name) using the same naming conventions asfor methods

– Corresponds to a data type - the kind of value it can represent

– Has a location in computer memory allocated to it

– The data type and amount of memory limits the range of values a variablecan store

– Stores a value of the specified data type

• Data types:

– Double: Fractional values (include decimal point)

– Integer: Whole numbers (no decimal point)

– Boolean: True/false

– String: Literal values indicated by double quotes

– NOTE: If Java not selected as language, will see slightly different typewindow

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Alice Variables: Intro (2)

• A variable declaration specifies a variable’s name and type

– Alice expects an initial value in addition

– Alice refers to the combination of declaration and initialization as variabledefinition

• A variable’s scope refers to where a variable’s value can be accessed

• There are three types of variables based on where they are declared:

1. Local variables

– These are declared within a method

– Their scope (where they can be used) is within the body (code) of thatmethod; hence the name local

2. Parameters

– These are declared in a method’s header - in the parentheses that followthe name of the method when created

– These are used to hold values that can be sent to the method when it iscalled

– Their scope is the same as for local variables

– The values that are sent to the method are called arguments

– They allow the method to behave differently on each call, providingdifferent arguments are passed to the method

3. Instance variables (properties)

– These are declared within a class

– They specify properties for objects of that class

– Their scope is the whole class (i.e., they can be used by methods of theclass)

• Variables are used to store values that are used in a program

– They make it easy to update values as a program executes

– They make it easy to modify a program’s code

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Alice Variables: Local Variables

• Steps in declaring a local variable:

1. Drag variable into a method from the list of control structures in edit modeA pop-up window appears

2. The radio button variable next to the label Is variable should be selected(see constants below)

3. Select a data type

4. Enter an initial value; Alice insists on this

– You may want something entirely different for an initial value than theoptions provided

– Simply select one - you will be able to change this once the variable hasbeen declared

5. Name the variable

– The name should be a meaningful noun or noun phrase

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Alice Variables: Local Variables (2)

• Expressions

– An expression is a piece of code that produces a value

– There are several forms this can take:

1. A literal (actual) value: 5, 17.2

2. A variable: x, length

3. A function call: this.alice.getWidth(), Math.sqrt(x)

4. A compound expression: expression1 operator expression2, where ex-pression1 and expression2 can be any of these types of expressions

• Variables are initialized with an expression

– Frequently want to initialize a variable with an expression other than asimple value

– Considerx : 5y : 10z : 0.5 ∗ x + 1.5 ∗ y

– When declaring z, Alice won’t allow the above, but you can change it afteryou’ve created the variable z

• To change the expressions for a variable:

1. Click the down arrow to the right of the expression - a set of choices appears

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Alice Variables: Local Variables (3)

These include

– Default values

– Compound expressions (Math)

– Randomly generated values

– User-specified values

– Values of existing variables (listed at bottom)

• Math allows the creation of complicated expressions involving operators

– It also provides math functions like square root, trig functions, etc.

• Function calls can be dragged into the value slots of expressions

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Alice Variables: Local Variables (4)

• Input can be interactively supplied by the user using the getTYPEFromUser()function (under Scene), where TYPE is replaced by one of Alice’s data types

– This function has a String parameter (message )that is printed before re-trieving a typed-in value from the user

– This string is called a prompt and should tell the user exactly what isexpected

– The program will wait until the user enters a value and hits ENTER beforecontinuing

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Alice Variables: Local Variables (5)

• Assignment statements

– These are used to change the value of a variable as a program executes

– Assignment statements are dragged from the control list at the bottom ofthe edit window

– They require an expression in the same way as when a variable is initialedon declaration

– The syntax is var name = expression

– The semantics: The value of the expression is stored in the memory locationassociated with var name

– Generally, only local variables appear on the left of assignment statements

• Constants

– When creating a variable, there radio buttons labeled variable and constantnext to Is variable

– A programming constant is a value that does not change throughout aprogram’s execution

– By declaring a variable constant, you will be prevented from changing it’svalue

– Constant names use all caps, unlike variable names

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Alice Variables: Parameters

• A parameter is a ”variable” that stores a value passed to a method

– The value is sent when the method is called, and the value is called anargument

• Creating a parameter for a method is similar to creating a local variable

– Once a method has been named and created, an add parameter buttonappears in its header

– Selecting this brings up a window just like for variables

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Alice Variables: Parameters (2)

– When selecting the data type, you’ll see a new option: Gallery

∗ Under Gallery are listed all the classes available in your program

∗ You can pass actual objects as arguments into a method

∗ When selecting a type from the gallery, choose the most general typethat you want your method to accept

∗ Classes that start with S (S-types, which stand for story types) areclasses that cannot be modified by the programer

∗ Appendix C lists the complete Alice S-type hierarchy

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Alice Variables: Instance Variables

• Instance variables define the properties of a class

• They are created the same as for methods except by choosing Add Propertymenu of the selected class

• When an instance variable is created, Alice automatically creates two methodsassociated with it:

1. Procedure set var name for assigning a value to var name

2. Function get var name for retrieving the value of var name

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Alice Variables: User-defined Functions

• Just like with methods, you can create custom-designed functions to generatevalues

• They are created in the same way a user-defined procedures are, with thefollowing differences:

– Since a function returns a value, a function has an associated data type,called its return type

– When creating a new function, a pop-up window similar to that for newvariables

– In addition, the body of the function requires a return statement

∗ When executed, a return statement halts the execution of the function,and specifies the value to be returned

∗ Return statements are added by dragging from the control statementlist at the bottom of the edit window

∗ The value to be returned can be any acceptable expression, and theexpression is created in the same way as for variables

• Remember: a function call can appear anywhere a value is expected in a pro-gram

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Alice Variables: Vehicles(Note: This topic has nothing to do with variables)

• Every class has a vehicle property

• Its default value is Scene

• When a vehicle moves, every object with that vehicle property will move withthe vehicle by the same amount

• To change an object’s vehicle, use the setVehicle() method (a procedure)

• The vehicle property makes it easy to have multiple objects move in tandem

– The alternative would be to program each of the objects to move by thesame amount, using a doTogether() control statement

– This requires a lot more code and effort

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