User Guide

Table Of Contents

Ensuring variable existence 71

For example, if you submit a form with an unsettled check box, the action page does not get a

variable for the check box. The following example from a form action page makes sure the

Contractor check box Form variable exists before using it:

<cfoutput>Contractor: #Form.Contractor#</cfoutput>

</cfif>

You must always enclose the argument passed to the IsDefined function in double-quotation

marks. For more information on the

IsDefined function, see CFML Reference.

If you attempt to evaluate a variable that you did not define, ColdFusion cannot process the page

and displays an error message. To help diagnose such problems, turn on debugging in the

ColdFusion MX Administrator or use the debugger in your editor. The Administrator debugging

information shows which variables are being passed to your application pages.

Variable existence considerations

If a variable is part of a scope that is available as a structure, you might get a minor performance

increase by testing the variable’s existence using the

StructKeyExists function instead of the

IsDefined function.

You can also determine which Form variables exist by inspecting the contents of the

Form.fieldnames built-in variable. This variable contains a list of all the fields submitted by the

form. Remember, however, that form text fields are always submitted to the action page, and

might contain an empty string if the user did not enter data.

The

IsDefined function always returns False if you specify an array or structure element using

bracket notation. For example,

IsDefined("myArray[3]") always returns False, even if the array

element

myArray[3] has a value. To check for the existence of an array element, use cftry, as in

the following example:

<cftry>

< cfoutput>Items[1][2] does not exist</cfoutput>

</cfcatch>

</cftry>

Using the cfparam tag

You can ensure that a variable exists by using the

cfparam tag, which tests for the variable’s

existence and optionally supplies a default value if the variable does not exist. The

cfparam tag

has the following syntax:

<cfparam name="VariableName"

type="data_type"

default="DefaultValue">

Note: For information on using the

type attribute to validate the parameter data type, see CFML

Reference.