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Alice Variables: Intro • a Variable Is a Name That Represents a Storage Location for a Value – Represented by an Identifier

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Alice Variables: Intro • a Variable Is a Name That Represents a Storage Location for a Value – Represented by an Identifier 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 as for 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 variable can 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 type window 1 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 variable definition • 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 that method; hence the name local 2. Parameters { These are declared in a method's header - in the parentheses that follow the name of the method when created { These are used to hold values that can be sent to the method when it is called { 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, providing different 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 the class) • 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 2 Alice Variables: Local Variables • Steps in declaring a local variable: 1. Drag variable into a method from the list of control structures in edit mode A 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 the options provided { Simply select one - you will be able to change this once the variable has been declared 5. Name the variable { The name should be a meaningful noun or noun phrase 3 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 a simple value { Consider x : 5 y : 10 z : 0:5 ∗ x + 1:5 ∗ y { When declaring z, Alice won't allow the above, but you can change it after you'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 4 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 5 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 is expected { The program will wait until the user enters a value and hits ENTER before continuing 6 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 of the edit window { They require an expression in the same way as when a variable is initialed on declaration { The syntax is var name = expression { The semantics: The value of the expression is stored in the memory location associated 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 constant next to Is variable { A programming constant is a value that does not change throughout a program's execution { By declaring a variable constant, you will be prevented from changing it's value { Constant names use all caps, unlike variable names 7 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 an argument • Creating a parameter for a method is similar to creating a local variable { Once a method has been named and created, an add parameter button appears in its header { Selecting this brings up a window just like for variables 8 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 type that you want your method to accept ∗ Classes that start with S (S-types, which stand for story types) are classes that cannot be modified by the programer ∗ Appendix C lists the complete Alice S-type hierarchy 9 Alice Variables: Instance Variables • Instance variables define the properties of a class • They are created the same as for methods except by choosing Add Property menu of the selected class • When an instance variable is created, Alice automatically creates two methods associated 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 10 Alice Variables: User-defined Functions • Just like with methods, you can create custom-designed functions to generate values • They are created in the same way a user-defined procedures are, with the following 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 new variables { 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 statement list at the bottom of the edit window ∗ The value to be returned can be any acceptable expression, and the expression is created in the same way as for variables • Remember: a function call can appear anywhere a value is expected in a pro- gram 11 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 with the 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 the same amount, using a doTogether() control statement { This requires a lot more code and effort 12.
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