MACROMEDIA COLDFUSION MX 61-DEVELOPING COLDFUSION MX, Develop Manual

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Ensuring variable existence 79
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You can use a
cfform input
tag with a
hidden
attribute to tell ColdFusion to display a helpful
message to any user who does not enter data in a required field. For more information on this
technique, see
Chapter 26, “Requiring users to enter values in form fields,” on page 566 .
Testing for a variable’s existence
Before relying on a variable’s existence in an application page, you can test to see if it exists by
using the
IsDefined
function.
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:
<cfif IsDefined("Form.Contractor")>
<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, copy the element
to a simple variable and use the
IsDefined
function to test whether the simple variable exists.
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 .
There are two ways to use the
cfparam
tag to test for variable existence, depending on how you
want the validation test to proceed: