User Guide

Dynamic expressions and dynamic variables 89
As you can see, using dynamic expressions can result in substantial expression evaluation
overhead, and the code can be confusing. Therefore, you should avoid using dynamic expressions
wherever a simpler technique, such as using indexed arrays or structures can serve your purposes.
Avoiding the Evaluate function
Using the
Evaluate function increases processing overhead, and in most cases it is not necessary.
The following sections provide examples of cases where you might consider using the
Evaluate
function.
Example 1
You might be inclined to use the Evaluate function in code such as the following:
<cfoutput>1 + 1 is #Evaluate(1 + 1)#</cfoutput>
Although this code works, it is not as efficient as the following code:
<cfset Result = 1 + 1>
<cfoutput>1 + 1 is #Result#</cfoutput>
Example 2
This example shows how you can use an associative array reference in place of an Evaluate
function. This technique is powerful because:
• Most ColdFusion scopes are accessible as structures.
• You can use ColdFusion expressions in the indexes of associative array structure references.
(For more information on using associative array references for structures, see “Structure
notation” on page 109.)
The following example uses the
Evaluate function to construct a variable name:
<cfoutput>
Product Name: #Evaluate("Form.product_#i#")#
</cfoutput>
This code comes from an example where a form has entries for an indeterminate number of items
in a shopping cart. For each item in the shopping cart there is a product name field. The field
name is of the form product_1, product_2, and so on, where the number corresponds to the
product’s entry in the shopping cart. In this example, ColdFusion does the following:
1. Replaces the variable i with its value, for example 1.
2. concatenates the variable value with "Form.product_", and passes the result (for
Form.product_1) to the
Evaluate function, which does the remaining steps.
3. Parses the variable product_1 and generates an executable representation of the variable.
Because ColdFusion must invoke its parser, this step requires substantial processing, even for a
simple variable.
4. Evaluates the representation of the variable, for example as "Air popper".
5. Returns the value of the variable.