The Quest for ColdFusion: Control Structures
By Shannon Eric Peevey, UNT Central Web Support
What if...
What if I wanted to make a decision right now? Do I go to Albertson's, or not? If I don't go to Albertson's, what will I do. This type of decision-making is constantly happening in our minds everyday. What will I eat for breakfast? What will I wear? What will I type next...? These decisions are going on, and on, in a rapid-fire succession that becomes almost unnoticed by our conscience mind. These decisions are made quickly, and efficiently by our brains, but what about our web applications? They do not have an innate sense of intelligence. What do I do if this is true, or if that is true? Well, the truth is, your web app doesn't know what to do, unless you tell it. These decision-making elements of our web applications are called "control structures". They are called "control structures", because they control the decision-making aspects of our programs.
To begin...
In this article, we are going to take a look at the <CFIF>-<CFELSE> control statement. The <CFIF>-<CFELSE> statement is considered the easiest control structure to understand by many, and therefore, we will begin our discussion with it.
To begin, we need to create two html/cfm files that allow us to demonstrate the use of the <CFIF>-<CFELSE> statements. The first, testParam.html:
<HTML>
<HEAD>
<TITLE>Welcome to the Test</TITLE>
</HEAD>
<BODY>
<P><FONT SIZE=4 STYLE="font-size: 16pt">
<B>Welcome to the ColdFusion Test Application:</B></FONT></P>
<FORM ACTION="testParam2.cfm" METHOD="post">
<P><FONT SIZE=3><B> Please enter your name here:
<INPUT TYPE=TEXT NAME="name" SIZE=25></B></FONT></P>
<P><FONT SIZE=3><B> Please enter your telephone here:
<INPUT TYPE=TEXT NAME="phone" SIZE=25></B></FONT></P> <DIV ALIGN=LEFT>
<P><FONT SIZE=3><B> <INPUT TYPE=BUTTON NAME="submit" VALUE="Submit"></B></FONT></P>
</DIV></FORM></BODY></HTML>
And, the second, testParam2.cfm:
<CFPARAM NAME="Form.likePage" default="0">
<HTML>
<HEAD>
<TITLE>This page interprets the data from test.cfm </TITLE>
</HEAD>
<BODY>
<CFOUTPUT>
<H2>#Form.name#</H2>
<H2>#Form.phone#</H2>
<H2>#Form.likePage#</H2>
</CFOUTPUT>
</BODY>
</HTML>
For those of you who have been following the ColdFusion Quest articles, you will recognize this code from last month's article, "Dealing With Empty Variable Names In ColdFusion". That is because we are steadily building upon our knowledge from one article to the next, and also, because the <CFIF>-<CFELSE> statement allows us to manipulate our parameter values, (etc, etc) in a easy 1-2-<CFIF>-<CFELSE>.
As you may remember, we have created a form, testParam.html, with two text boxes, name and phone, and then added a checkbox, which asks our user if they like the page. The results of this form are then sent forward to a page called testParam2.cfm, which checked for the value held in a variable, called Form.likePage. If this variable did not exist, it created a variable Form.likePage with a default value of "0".
Here is the question...
What if I wanted to make my application take a specific course of action based upon the value of the variable Form.likePage? Suppose I wanted to display an error message in the case of a zero value on the previous page. (Much like validation code.) With your <CFIF>-<CFELSE> control statement, it is very easy. Take a look at this:
<CFPARAM NAME="Form.likePage" default="0">
<!--- This is the <CFIF> statement. It checks for a value of "0", then reacts accordingly --->
<CFIF Form.likePage IS "0">
<meta http-equiv="refresh" content="3; url=testParam .html">
<h2><center>You have not answered our nice aesthetics question. You will be returned back to the previous page in 3 seconds</h2>
</CFIF>
<HTML>
<HEAD>
<TITLE>This page interprets the data from test.cfm </TITLE>
</HEAD>
<BODY>
<CFOUTPUT>
<H2>#Form.name#</H2>
<H2>#Form.phone#</H2>
<H2>#Form.likePage#</H2>
</CFOUTPUT>
</BODY>
</HTML>
The if statement by itself, will check a condition statement to find if something is true, and then react. If the value of Form.likePage is equal to "0", then a blank page with the message, "You have not answered our nice aesthetics question. You will be returned back to the previous page in 3 seconds". This will then return the user to the previous page within 3 seconds. If the value of Form.likePage is something other than "0", then the <CFIF> statement is ignored, and then the rest of the html page is printed to the screen with the ColdFusion variables embedded into the page. Another way to right this same action is to add a <CFELSE> to the code.
<CFPARAM NAME="Form.likePage" default="0">
<!--- This is a <CFIF> statement with the inclusion of the <CFELSE> tag.
It tests for a value of "0". If this is not true, it returns the HTML
page with the ColdFusion variables embedded. --->
<CFIF Form.likePage IS "0">
<meta http-equiv="refresh" content="3; url=testParam .html">
<h2><center>You have not answered our nice aesthetics question. You will be returned back to the previous page in 3 seconds</h2>
<CFELSE>
<HTML>
<HEAD>
<TITLE>This page interprets the data from test.cfm </TITLE>
</HEAD>
<BODY>
<CFOUTPUT>
<H2>#Form.name#</H2>
<H2>#Form.phone#</H2>
<H2>#Form.likePage#</H2>
</CFOUTPUT>
</BODY>
</HTML>
</CFIF>
This code performs the same action as the previous page, except that it is perhaps easier to follow the flow of control through the script. We see that we are testing to see if Form.likePage="0", if it is not true, then perform the appropriate default action, found after the <CFELSE> tag. In this case, display the html, with the ColdFusion variables, to the client browser. An easy way to remember this is to say, "If this is true, do this action, or else do that action."
To expand on this...
The abilities of the <CFIF>-<CFELSE> statement, can be further expanded, by using <CFELSEIF>. If you need to check for two different possibilities, then you would write:
<CFPARAM NAME="Form.likePage" default="0">
<!--- This introduces the <CFELSEIF> tag. It allows us to check for
multiple possibilities --->
<CFIF Form.likePage IS "0">
<meta http-equiv="refresh" content="3; url=testParam .html">
<CFELSEIF Form.likePage IS "2">
<h2><center>There is no way have a value of 2 for this checkbox. You must be
modifying my code to do that ;-( </h2>
<CFELSE>
<HTML>
<HEAD>
<TITLE>This page interprets the data from test.cfm </TITLE>
</HEAD>
<BODY>
<CFOUTPUT>
<H2>#Form.name#</H2>
<H2>#Form.phone#</H2>
<H2>#Form.likePage#</H2>
</CFOUTPUT>
</BODY>
</HTML>
</CFIF>
As you can see, we are testing Form.likePage for a value of "0", which would mean there was no checkmark in the checkbox on the previous page, plus, we are also testing for a value of "2", (which is impossible with the code that we have). Therefore, if our control statement does find a value of "2", we output the following message to the screen: "There is no way have a value of 2 for this checkbox. You must be modifying my code to do that ;-( ". If neither of these conditions are true, then output the html page with the ColdFusion variables embedded.
There you have it...
There you have it! :) Your introduction to the possibilities of control structures. These examples are just the beginning. Next month, we will begin looping over our ColdFusion code until our conditions are satisfied :) Until then, Adios from your Dynamic, Car-Dodging Web Administrator.
For more information, you can contact me at: speeves@unt.edu