User Guide

Table Of Contents

Dynamic expressions and dynamic variables 85

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 97.

Selecting how to create variable names

The following two examples describes cases when you need dynamic variable names:

• Form applications where the number and names of fields on the form vary dynamically. In this

case, the form posts only the names and values of its fields to the action page. The action page

does not know all the names of the fields, although it does know how the field names (that is,

the variable names) are constructed.

• If the following are true:

■ ColdFusion calls a custom tag multiple times.

■ The custom tag result must be returned to different variables each time.

■ The calling code can specify the variable in which to return the custom tag result.

In this case, the custom tag does not know the return variable name in advance, and gets it as

an attribute value.

In both cases, it might appear that dynamic expressions using the

Evaluate function are needed

to construct the variable names. However, you can achieve the same ends more efficiently by

using dynamic variable naming, as shown in “Example: a dynamic shopping cart” on page 92.

This does not mean that you must always avoid dynamic evaluation. However, given the

substantial performance costs of dynamic evaluation, you should first ensure that one of the

following techniques cannot serve your purpose:

• An array (using index variables)

• Associative array references containing expressions to access structure elements

• Dynamically generated variable names

Dynamic variable naming without dynamic evaluation

While ColdFusion does not always allow you to construct a variable name in-line from variable

pieces, it does let you to do so in the most common uses, as described in the following sections.

Using number signs to construct a variable name in assignments

You can combine text and variable names to construct a variable name on the left side of a

cfset

assignment. For example, the following code sets the value of the variable Product12 to the string

"Widget":