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Chapter 2. Variables

In a typical scripted movie, we have to track and manipulate everything from frame numbers to a user's password to the velocity of a photon torpedo fired from a spaceship. In order to manage and retrieve all that information, we need to store it in variables, the primary information-storage containers of ActionScript.

A variable is a like a bank account that, instead of holding money, holds information (data). Creating a new variable is like setting up a new account; we establish a place to store something we'll need in the future. And just as every bank account has an account number, every variable has a name associated with it that is used to access the data in the variable.

Once a variable is created, we can put new data into it as often as we want -- much like depositing money into an account. Or we can find out what's in a variable using the variable's name -- much like checking an account balance. If we no longer need our variable, we can "close the account" by deleting the variable.

The key feature to note is that variables let us refer to data that either changes or is calculated when a movie plays. Just as a bank account's number remains the same even though the account balance varies, a variable's name remains fixed even though the data it contains may change. Using that fixed reference to access changing content, we can perform complex calculations, keep track of cards in a card game, save guest book entries, or send the playhead to different locations based on changing conditions.

Is that a gleam of excitement I see in your eye? Good, I thought I might have lost you with all that talk about banks. Let's start our exploration of variables by seeing how to create them.

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.



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