Using VariablesNow that you've had the chance to use some basic functions, it's time to introduce variables. Variables are an important part of just about every programming language, and CFML is no exception. A variable is a container that stores information in memory on the server. Variables are named, and the contents of the container are accessed via that name. Let's look at a simple example. Type the code in Listing 8.4 into a new file (feel free to use your own name instead of mine), save it as hello4.cfm, and browse it. You should see a display similar to the one shown in Figure 8.7. Listing 8.4. hello4.cfm
Figure 8.7. Variables are replaced by their contents when content is generated.<cfset>:<cfset FirstName="Ben"><cfset> is used to set variables. Here, a variable named FirstName is created, and a value of Ben is stored in it. After it's created, that variable will exist until the page has finished processing and can be used, as seen in the next line of code: This line of code was placed in a <cfoutput> block so ColdFusion will know to replace #FirstName# with the contents of FirstName. The generated output is then: Variables can be used as many times as necessary, as long as they exist. Try moving the <cfset> statement after the <cfoutput> block, or delete it altogether. Executing the page now will generate an error, similar to the one seen in Figure 8.8. This error message is telling you that you referred to (tried to access) a variable that doesn't exist. The error message includes the name of the variable that caused the problem, as well as the line and column in your code, to help you find and fix the problem easily. More often than not, this kind of error is caused by typos. Figure 8.8. ColdFusion produces an error if a referenced variable doesn't exist.Chapter 3), go to the Debugging Settings page, and turn on Enable Robust Exception Information.Chapter 10). An exception to this rule does exist. In Chapter 17, "Working with Sessions," you learn how to create and use variables that persist across requests. (Each page access is known as a request.)Here is a new version of the code, this time using the variable FirstName six times. Save Listing 8.5 as hello5.cfm, and then try this listing for yourself (feel free to replace my name with your own). The output is shown in Figure 8.9. Listing 8.5. hello5.cfm
Figure 8.9. There is no limit to the number of functions that can be used in one page, which enables you to render content as you see fit.[View full size image]  UCase() returns FirstName converted to uppercase, but the contents of FirstName itself are intact and are not converted to anything at all. FirstName was not modified; a copy was made and modified instead, and that copy was returned. To save the uppercase FirstName to a variable, you must do something like this: Here a new variable, UpperFirstName, is created. UpperFirstName is assigned the value that is returned by UCase(FirstName), the uppercase FirstName. And this new variable can be used like any other variable, and as often as necessary. Listing 8.6 is a modified version of the Listing 8.5. Try it for yourselfthe output will be exactly the same as in Figure 8.9. Listing 8.6. hello6.cfmThis code deserves a closer look. Six <cfset> tags now exist, and six variables are created. The first creates the firstName variable, just like in the previous examples. The next creates a new variable named upperFirstName, which contains the uppercase version of firstName. And then lowerFirstName, reverseFirstName, lenFirstName, and repeatFirstName are each created with additional <cfset> statements.The <cfoutput> block here contains no functions at all. Rather, it just displays the contents of the variables that were just created. In this particular listing there is actually little value in doing this, aside from the fact that the code is a bit more organized this way. The real benefit in saving function output to variables is realized when a function is used many times in a single page. Then, instead of using the same function over and over, you can use it once, save the output to a variable, and just use that variable instead.One important point to note here is that variables can be overwritten. Look at the following code snippet: Here, firstName is set to Ben and then set again to Nate. Variables can be overwritten as often as necessary, and whatever the current value is when accessed (displayed, or passed to other functions), that's the value that will be used.Knowing that, what do you think the following line of code does? This is an example of variable overwriting, but here the variable being overwritten is the variable itself. I mentioned earlier that functions such as UCase() don't convert text; they return a converted copy. So how could you really convert text? By using code such as the line just shown. <cfset firstName=UCase(firstName)> sets firstName to the uppercase version of firstName, effectively overwriting itself with the converted value. Variable NamingThis would be a good place to discuss variable naming. When you create a variable you get to name it, and the choice of names is up to you. However, you need to know a few rules about variable naming:
TIPAvoid the use of abbreviated variable names, such as fn or c. Although these are valid names, what they stand for is not apparent just by looking at them. Yes, fn is less keystrokes than FirstName, but the first time you (or someone else) must stare at the code trying to figure out what a variable is for, you'll regret saving that little bit of time. As a rule, make variable names descriptive.Using PrefixesColdFusion supports many variable types, and you'll become very familiar with them as you work through this book. For example, local variables (the type you just created) are a variable type. Submitted form fields are a variable type, as are many others.ColdFusion variables can be referenced in two ways:
As for the cons, there is just one:
In other words, if you refer to #firstName# (without specifying a prefix) and that variable exists both as a local variable (VARIABLES.firstName) and as a FORM field (FORM.firstName), VARIABLES.firstName will be used automatically. NOTEAn exception to this does exist. Some ColdFusion variable types must always be accessed with an explicit prefix; these are covered in later chapters. |
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