MACROMEDIA COLDFUSION MX 61-DEVELOPING COLDFUSION MX, Develop Manual

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Dynamic expressions and dynamic variables 97
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
Chapter 5, “Structure
notation,” on page 114 .)
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.
The following example has the same result as the preceding example and is more efficient:
<cfoutput>
ProductName: #Form["product_" & i]#
</cfoutput>
In this code, ColdFusion does the following:
1
Evaluates the expression in the associative array index brackets as the string "product_"
concatenated with the value of the variable i.
2
Determines the value of the variable i; 1.
3
Concatenates the string and the variable value to get product_1.
4
Uses the result as the key value in the Form structure to get Form[product_1]. This associative
array reference accesses the same value as the object.attribute format reference Form.product_1;
in this case, Air popper.
This code format does not use any dynamic evaluation, but it achieves the same effect, of
dynamically creating a structure reference by using a string and a variable.
SetVariable function considerations
You can avoid using the
SetVariable
function by using a format such as the following to set a
dynamically named variable. For example, the following lines are equivalent:
<cfset SetVariable("myVar" & i, myVal)>
<cfset "myVar#i#" = myVal>