The written form of a value in an ActionScript program is known as an expression. For example, the following code shows a new expression—an expression representing a new object (in this case, a Date object):
Likewise, the following code shows a literal expression representing a Number object with the value 2.5:
Individual expressions can be combined together with
operators to create a compound
expression, whose value is calculated when the program
runs. An operator is a built-in command that
combines, manipulates, or transforms values (which are known as the
operator's operands). Each operator is written
using either a symbol, such as
or a keyword, such as instanceof.
For example, the multiplication operator, which multiplies two
numbers, is written using the asterisk symbol (
*). The following code shows a compound
expression that multiplies 4 and 2.5:
4 * 2.5
When the preceding code is executed, ActionScript calculates the
result of multiplying 4 by 2.5, and the entire compound expression
4 * 2.5) is replaced by that
single calculated result (
Calculating the value of an expression is known as
evaluating the expression.
For a complete list of ActionScript operators, see Chapter 10.
To represent values that are not known when a program is compiled (at compile-time), but are supplied or calculated when the program runs (i.e., at runtime), we use variable names. When ActionScript evaluates an expression containing a variable name, it replaces that variable name with the corresponding variable's value. The process of replacing the variable name with the variable's value is known as retrieving, getting, or reading the variable value.
For example, consider the following compound expression, in which two values represented by variable names are multiplied together:
quantity * price
price are placeholders for values
that will be determined at runtime. The value of
quantity might be, say, a number supplied by
the user, while the value of
might be a number retrieved from a database. For the sake of this
example, let's assume that the variable
quantity has the value 2, and the variable
price has the value 4.99.
When ActionScript evaluates the expression
quantity * price, it replaces
quantity with 2 and
price with 4.99. Hence, during evaluation,
the expression reads:
2 * 4.99
And the final value of the expression is:
In formal terms, an expression that contains a variable name
only, such as
quantity, is known
as an identifier expression.
Now let's try using an identifier expression in our virtual pet program.