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92 Chapter 4: Using Expressions and Pound Signs
Dynamic expressions and dynamic variables
This section discusses the advanced topics of dynamic expressions, dynamic evaluation, and
dynamic variable naming. Many ColdFusion programmers never encounter or need to use
dynamic expressions. However, dynamic variable naming is important in situations where the
variable names are not known in advance, such as in shopping cart applications.
This section also discusses the use of the
IIf
function, which is most often used without dynamic
expressions. This function dynamically evaluates its arguments, and you must often use the
DE
function to prevent the evaluation. For more information on using the
IIF
function, see “Using
the IIF function” on page 98
.
Note: This section uses several tools and techniques that are documented in later chapters. If you are
unfamiliar with using ColdFusion forms, structures, and arrays, you should learn about these tools
before reading this section.
About dynamic variables
Dynamic variables are variables that are named dynamically, typically by creating a variable name
from a static part and a variable part. For example, the following example dynamically constructs
the variable name from a variable prefix and a static suffix:
<cfset "#flavor#_availability" = "out of stock">
Using dynamic variables in this manner does not require dynamic evaluation.
About dynamic expressions and dynamic evaluation
In a dynamic expression , the actual expression, not just its variable values, is determined at
execution time. In other words, in a dynamic expression the structure of the expression, such as
the names of the variables, not just the values of the variables, gets built at runtime.
You create dynamic expressions using string expressions , which are expressions contained in strings,
(that is, surrounded with quotation marks). Dynamic evaluation is the process of evaluating a
string expression. The
Evaluate
and
IIf
functions, and only these functions, perform dynamic
evaluation.
When ColdFusion performs dynamic evaluation it does the following:
1
Takes a string expression and treats it as a standard expression, as if the expression was not a
string.
2
Parses the expression to determine the elements of the expression and validate the expression
syntax.
3
Evaluates the expression, looking up any variables and replacing them with their values, calling
any functions, and performing any required operations.
This process enables ColdFusion to interpret dynamic expressions with variable parts. However, it
incurs a substantial processing overhead.
Dynamic expressions were important in early versions of ColdFusion, before it supported arrays
and structures, and they still can be useful in limited circumstances. However, the ability to use
structures and the ability to use associative array notation to access structure elements provide
more efficient and easier methods for dynamically managing data. For information on using
arrays and structures, see
Chapter 5, “Using Arrays and Structures,” on page 103 .