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Dealing With Empty Variable Names In ColdFusion

By Shannon Eric Peevey, UNT Central Web Support

This is the long awaited ColdFusion article that has kept many of you on your seats for the past few months, as I was either too busy to write this article, or had more important information to get to you as time drifted by. Therefore, to recap the subjects that we have written about in the past, we have covered the topic of how to get started using ColdFusion here at the University of North Texas, Creating your first application using ColdFusion, and Connecting to a database using ColdFusion. We, therefore, have a good basic foundation on which to draw from to continue in our path down web application creation lane. Now it is time to look at an oft overlooked, and very under-documented occurrence of an empty parameter. How do you deal with an empty or non-existent variable that is passed to the evaluating .cfm page?It’s easy!!

In this article, we are going to show you how to deal with offending empty variables, and bring about smooth, elegant applications using ColdFusion.

To Begin…

To start, we must have our beginning html page, named testParam.html, that will hold two forms named: name and phone.

<HTML>

<HEAD>

<TITLE>Welcome to the Test</TITLE>

</HEAD>

<BODY>

<B>Welcome to the ColdFusion Test Application:</B></FONT></P>

<P><FONT SIZE=3><B> Please enter your name here:

<P><FONT SIZE=3><B> Please enter your telephone here:

</DIV>

</FORM>

</BODY>

</HTML>

Many of you will recognize this from the first application article in this series. This is a simple form that accepts input into two text boxes, and sends that input to a file named testParam2.cfm. In testParam2.cfm, we are going to use the existing code that we created in the first app article, which looks like this.

<HTML>

<HEAD>

<TITLE>This page interprets the data from test.cfm </TITLE>

</HEAD>

<BODY>

<H2># Form.phone #</H2>

</CFOUTPUT>

</BODY>

</HTML>

This page outputs the data held in the variables form.name and form.phone to the browser. This is a nice simple script to get started with, but what happens if we add a checkbox to the first page and try to interpret the data on the next page? Well… Let’s try it. Add this code beneath your text boxes on testParam.html.

<p> <FONT SIZE=3><B> Wasn’t this the greatest page? (Check if you like the page) <input type="checkbox" name="likePage" value="1">

This html adds a checkbox to the bottom of our page which asks the user to tell us if they liked the page or not… As you know, if they mark the checkbox, then the page will send forward a variable that can be accessed with #form.likePage# from the testParam2.cfm page. Like this:

<HTML>

<HEAD>

<TITLE>This page interprets the data from test.cfm </TITLE>

</HEAD>

<BODY>

<H2>#Form.phone#</H2>

<H2>#Form.likePage#</H2>

</CFOUTPUT>\

</BODY>

</HTML>

If the checkbox is marked, you will get a returned value of 1, because that is the value that we have set the checkbox equal to if checked. But… What if it’s not checked? Will the testParam.html page send forward a variable value of 0? Of course not. HTML is not intelligent, and will only send forward the variables that hold values. We will then get an error when the user clicks on the submit button at the bottom of the page.

How do we deal with this?

The trick to handling this tricky situation is to introduce the <CFPARAM> tag. The <CFPARAM> tag is ColdFusion’s handy way of “test(ing) for a parameter’s existence and optionally provide a default if it is not found”[1]The way to use the <CFPARAM> tag is to place it at the top of your page, before the <HTML> tag, and create a default for the parameter that you are looking for. Here is the syntax diagram for a <CFPARAM> statement:

Therefore, our <CFPARAM> tag will look like this:

The reason that I am setting a default of zero, is because it is logical to test for 0, no, the user doesn’t like the page, or 1, yes, the user did like the page. (In the world of Boolean datatypes, 1 equals true, and 0 equals false.) Therefore, if the user does not like our wonderful page, they will click submit, sending forward a value for the Form.name and Form.phone variables, and nothing at all for our Form.likePage variable. Our testParam2.cfm page receives this data, and is sent to the ColdFusion server for parsing, where the first statement of the page checks for a non-existing value for Form.likePage, and then, because the value doesn’t exist, creates a value for Form.likePage=”0”. The rest of the page parses, and the values are then output to the browser. Here is the final code:

<HTML>

<HEAD>

<TITLE>This page interprets the data from test.cfm </TITLE>

</HEAD>

<BODY>

<H2>#Form.phone#</H2>

<H2>#Form.likePage#</H2>

</CFOUTPUT>

</BODY>

</HTML>

It’s that easy!

That is the quick and easy answer to a troubling, and oft ignored question of non-existing parameters, which would have stopped our ColdFusion application dead in it’s tracks. With the use of the <CFPARAM> tags, we can check for any number of non-existing parameters from the sending page, and then test that value for flow-of-control purposes. That being the case, please wait with baited breathe until the next installment which will begin our journey down the first true programming concept of ColdFusion. Control Structures! Until then… Live long and prosper!

Shannon Peevey – Central Web Support

[1] CFML Language Reference, Allaire Corporation.