Variables (ActionScript: The Definitive Guide)

2.1. Creating Variables (Declaration)

Creating a variable is called declaration. Declaration is the "open an account" step of our bank metaphor, where we formally bring the variable into existence. When a variable is first declared, it is empty -- a blank page waiting to be written upon. In this state, a variable contains a special value called undefined (indicating the absence of data).

To declare a new variable, we use the var statement. For example:

var speed;
var bookTitle;
var x;

The word var tells the interpreter that we're declaring a variable, and the text that follows, such as speed, bookTitle, or x, becomes our new variable's name. We can create variables anywhere we can attach code: on a keyframe, a button, or a movie clip.

We can also declare several variables with one var statement, like this:

var x, y, z;

However, doing so impairs our ability to add comments next to each variable.

Once a variable has been created, we may assign it a value, but before we learn how to do that, let's consider some of the subtler details of variable declaration.

2.1.1. Automatic Variable Creation

Many programming languages require variables to be declared before data may be deposited into them; failure to do so would cause an error. ActionScript is not that strict. If we assign a value to a variable that does not exist, the interpreter will create a new variable for us. The bank, to continue that analogy, automatically opens an account when you try to make your first deposit.

This convenience comes at a cost, though. If we don't declare our variables ourselves, we have no central inventory to consult when examining our code. Furthermore, explicitly declaring a variable with a var statement can sometimes yield different results than allowing a variable to be declared implicitly (i.e., automatically). It's safest to declare first and use later (i.e., explicit declaration), as shown throughout this book.

2.1.2. Legal Variable Names

Before running off to make any variables, be aware that variable names:

Must be composed exclusively of letters, numbers, and underscores. (No spaces, hyphens, or punctuation allowed.)
Must start with a letter or an underscore.
Must not exceed 255 characters. (Okay, okay, that's a lie, but reevaluate your naming scheme if your variable names exceed 255 characters.)
Are case-insensitive (upper- and lowercase are treated identically but you should be consistent nonetheless).

These are legal variable names:

var first_name;
var counter;
var reallyLongVariableName;

These are illegal variable names that would cause errors:

var 1first_name;                 // Starts with a number
var variable name with spaces;   // Contains spaces
var another-illegal-name;        // Contains a hyphen

2.1.2.1. Creating dynamically named variables

Although you'll rarely, if ever, use dynamically created variable names, it's possible to generate the name of a variable programmatically. To create a variable name from any expression, use the set statement. For example, here we assign the value "bruce" to player1name:

var i = 1;
set ("player" + i + "name", "bruce");

Arrays and objects, discussed in later chapters, provide us with a much more powerful means of tracking dynamically named data and should be used instead of dynamic variable names.

2.1.3. Declare Variables at the Outset

It's good practice to declare your variables at the beginning of every movie's main script space, which is usually the first keyframe that comes after a movie's preloader. Be sure to add a comment explaining each variable's purpose for easy identification later. The beginning of a well-organized script might look like this:

// ^^^^^^^^^^^^^^^^^^^^
// Initialize variables
// ^^^^^^^^^^^^^^^^^^^^
var ballSpeed;   // Velocity of ball, max 10
var score;       // Player's current score
var hiScore;     // High score (not saved between sessions)
var player1;     // Name of player 1, supplied by user

We can give variables an initial value at the same time we create them, as follows:

var ballSpeed = 5;   // Velocity of ball, max 10
var score = 0;       // Player's current score
var hiScore = 0;     // High score (not saved between sessions)

2.1.2.1. Creating dynamically named variables

Chapter 2. Variables

Contents:

2.1. Creating Variables (Declaration)

2.1.1. Automatic Variable Creation

2.1.2. Legal Variable Names

2.1.3. Declare Variables at the Outset