User Guide

Ensuring that Variables Exist 89

For example, the following series of cfparam tags indicates 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 11, “Preventing and

Handling Errors” on page 191.

Example: setting default values

This example uses cfparam to see if optional variables exist. If they do exist,

processing continues. If they do not exist, they are created and set to the

default

value.

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

using conditional logic. For example, the action page could have the following code

to ensure that a SelectDepts variable exists:

Requiring users to enter values in form fields

One of the limitations of HTML forms is the inability to define input fields as

required. Because this is a particularly important requirement for database

applications, ColdFusion provides a server-side mechanism for requiring users to

enter data in fields.

To require entry in an input field, use a hidden field that has a

name attribute

composed of the field name and the suffix "_required." For example, to require that

the user enter a value in the FirstName field, use the syntax:

If the user leaves the FirstName field empty, ColdFusion rejects the form submittal

and returns a message informing the user that the field is required. You can

customize the contents of this error message using the

value attribute of the hidden

field. For example, if you want the error message to read “You must enter your first

name,” use the following syntax:

<input type="hidden"

name="FirstName_required"

value="You must enter your first name.">