User Guide

Table Of Contents

72 Chapter 3: Using ColdFusion Variables

There are two ways to use the cfparam tag to test for variable existence, depending on how you

want the validation test to proceed:

• With only the name attribute to test that a required variable exists. If it does not exist, the

ColdFusion server stops processing the page and displays an error message.

• With the name and default attributes to test for the existence of an optional variable. If the

variable exists, processing continues and the value is not changed. If the variable does not exist,

it is created and set to the value of the

default attribute, and processing continues.

The following example shows how to use the

cfparam tag to check for the existence of an

optional variable and to set a default value if the variable does not already exist:

Example: testing for variables

Using the

cfparam tag with the name attribute is one way to clearly define the variables that a

page or a custom tag expects to receive before processing can proceed. This can make your code

more readable, as well as easier to maintain and debug.

For example, the following

cfparam tags indicate that this page expects two form variables named

StartRow and RowsToFetch:

If the page with these tags is called without either one of the form variables, an error occurs and

the page stops processing. By default, ColdFusion displays an error message; you can also handle

the error as described in Chapter 14, “Handling Errors,” on page 307.

Example: setting default values

The following example uses the

cfparam tag to see if optional variables exist. If they do exist,

processing continues. If they do not exist, the ColdFusion server creates them and sets them to

the default values.

You can use the cfparam tag to set default values for URL and Form variables, instead of using

conditional logic. For example, you could include the following code on the action page to ensure

that a SelectedDepts variable exists: