Tài liệu Building Expressions ppt

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Building Expressions
As we mentioned earlier in this lesson, an expression is a phrase—or a collection of
variables, numbers, text, and operators—that evaluates to a value. To understand this
concept, take a look at this example:

var oneDozen:Number = 6 * 2;


To the right of the equals sign, you see 6 * 2—this is an expression. When this script is
executed, the expression 6 * 2 is replaced by the result of 6 multiplied by 2, or 12. Thus,
oneDozen equals 12.
An expression can contain variables, arrays, even function calls—anything that, when
evaluated, returns a value. For example:

var total:Number = subTotal + tax;


The value of total is based on the result of adding the variable subTotal to the variable
tax. The expression is subTotal + tax.

var discount:Number = totalPrice * employeeDiscount[2];


The value of discount is based on the result of multiplying the variable totalPrice by the
value of the third element in the employeeDiscount array; totalPrice *
employeeDiscount[2] is the expression.
Here:

var usDollars:Number = 10;

var japaneseYen:Number = convertToYen(usDollars);


the value of japaneseYen is based on the value returned by a call to the convertToYen()
function—making the function call itself the expression.
NOTE
For more information about functions, see Lesson 5
, "Using Functions."

Expressions are used to do the following:
•
Set variable values
•
Set array element values
•
Determine whether conditions are being met (using the comparison operators)
•
Dynamically name movie clips, variables, and objects
•
Call functions dynamically
•
And more
Expressions enable you to avoid hard-coding values that will remain the same no matter
what. By assigning and manipulating values via expressions, you can make the data used
by your scripts dynamic—resulting in more interactive projects. Many of the scripts in
this book rely on expressions—that's because without them, your project plays back in
exactly the same way each time it's viewed.

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