|By Jeffry Houser||
|November 4, 2005 01:00 AM EST||
This month I will examine the cfif tag and discuss how we can use that tag to make decisions in our code. Along the way I'll talk about Boolean logic, decision operators, and Boolean operators in CFML.
Understanding Conditional Logic
Before you can implement conditional logic in your code you have to understand what conditional logic is. I have no doubt that every one of you knows what conditional logic is, although you may never have heard it referred to as such. Conditional logic is just a fancy way of saying that you are going to make a decision. Usually the decision is based upon some sort of condition or the result of an operation.
You and I make decisions every day, so let's delve into some real-world examples. Suppose I'm driving a car and come to an intersection with another road. I need to make a decision whether to turn left, turn right, or continue straight on the main road. Each decision may have different consequences. If I turn right, I go to the bank. If I turn left, I go to the grocery store. Going straight might take me somewhere else, home, for example. Since it has been a long day, I want to go home. I'm going to drive straight. This is an example of conditional logic in practice. I made a choice to go straight, based on a condition, I want to go home, and something happened as a result of that condition.
Making decisions about groceries and the bank is all well and good, but how does this apply to Web development? I'm glad you asked. Suppose you have a sign-up form on your Web site. Your users enter some account information, such as an e-mail address, their name, a username, and password. You take that information and create a user account in your database.
What happens if the user does not enter a username? Can you still create the account? How do you handle that situation? You can use conditional logic to make sure that the username is not left blank. Suppose a user looks at the shopping cart before adding items to it? You can use conditional logic to display an "empty cart" message. Suppose a user enters a birth date as November 15, 2050? You get the idea - we can use conditional logic in our code to solve these types of problems.
The Format of the cfif Tag
In CFML, you can use the cfif tag to perform conditional logic statements. It comes in this format:
The cfif tag is an oddball tag in the CFML language. Unlike most tags, it doesn't take any explicit parameter name and value arguments; it is simply followed by an expression after the tag name. A review of expressions can be found in last month's article; or you can go straight to the source: http://livedocs.macromedia.com/coldfusion/6.1/htmldocs/cfml_b14.htm. The CFML interpreter uses an expression to determine whether or not to process data. Expressions are literal values, variables, or functions (which return a Boolean value).
The expression inside the cfif tag must evaluate to a Boolean value (a true/false). Boolean values are represented as either "true" or "false", "yes" or "no", or "0" or any number. If the expression evaluates to true, then the action immediately following the <cfif> is performed. If the expression evaluates to false, then the action is not performed and template execution will continue after the end cfif tag. There are two types of operators that are used with cfif tags: decision operators and Boolean operators. Both evaluate to Boolean values.
Decision and Boolean Operators
The two types of operators used in a cfif expression are decision operators and Boolean operators. Boolean operators refer to specific operators from Boolean algebra. Decision operators are used for comparisons. This is a list of the Boolean and decision operators supported in CFML:
- NOT: A Boolean operator that returns the opposite of the specified expressions. If you want the NOT of true, then you get false. The NOT of false is true.
- AND: A Boolean operator that returns true if both operands are true, but false otherwise.
- OR: A Boolean operator that returns true if at least one of the operands is true, but returns false otherwise.
- XOR: A Boolean operator that performs an exclusive or. If only one of the values is true, then the result is true. Otherwise the result is false.
- EQV: A Boolean operator that performs equivalence. Returns true if both operands are true or both operands are false. Returns false if the operands are different.
- IMP: A Boolean operator that performs implication. It is akin to the logical statement "If condition 1, then condition 2." This returns false if condition 1 is true and condition 2 is false. True is returned in all other cases.
- IS, EQUAL, EQ: Decision operators that test for two values being equal. If the first value is equal to the second value, the final result is true.
- IS NOT, NOT EQUAL, NEQ: Decision operators that test for two values not being equal. If the two values are equal, then the final result will be false.
- GREATER THAN, GT: Decision operators that are used to check if the first value is greater than the second value.
- GREATER THAN OR EQUAL TO, GTE, GE: Decision operators that check to see if the first value is larger than or equal to the second value.
- LESS THAN, LT: Decision operators that are used to check if the first value is smaller than the second value.
- LESS THAN OR EQUAL TO, LTE, LE: Decision operators that check to see if the first value is smaller than or equal to the second value.
- CONTAINS: A decision operator that returns true if the value on the left contains the value on the right.
- DOES NOT CONTAIN: A decision operator that returns true if the value on the left does not contain the value on the right.
You can read more about decision operators in the Macromedia Livedocs at http://livedocs.macromedia.com/coldfusion/6.1/htmldocs/expresa6.htm and about Boolean operators at http://livedocs.macromedia.com/coldfusion/6.1/htmldocs/expresa5.htm.
It is worth noting the order in which these operations are applied. This builds right off the order of operations I discussed in last month's article. First, the arithmetic operators are applied, and then the string operator is applied. Following that, the decision operators are applied: EQ, NEQ, LT, LTE, GT, GTE, CONTAINS, DOES NOT CONTAIN from left to right in the order that they appear. Then come the Boolean operators. First the NOT operator is applied, then the AND operator. Then the OR operator, followed by the XOR operator. Next comes EQV, and finally, IMP. You can always use parentheses to change the order of operations. In fact, when in doubt, use parentheses to force the order of operations! In most cases you won't be mixing arithmetic or string operands with decision or Boolean ones; however, decision and Boolean operands are often used together. Let me show you how to put this into practice.
Take Some Code for a Test Drive
Suppose you want to make a simple interface for users to choose which way to turn their car. This sample uses an HTML form to collect the user input, as shown below. If you are unfamiliar with HTML forms, I found some great Web-based tutorials located at www.weballey.net/forms and www.wdvl.com/Authoring/Scripting/Tutorial/html_forms_intro.html.
<form action="Go.cfm" method="post">
<input type="Radio" name="Route" value="Right">Right
<input type="Radio" name="Route" value="Left">Left
<input type="Radio" name="Route" value="Straight">Straight<Br>
The form uses radio buttons to let the user select a direction and then click Submit to see the result page:
<cfif Route is "Right">
Go to Bank<br>
<cfif Route is "Left">
Go to Store<br>
<cfif Route is "Straight">
If you copy this code to see the example in action, make sure that you put it in a file named go.cfm and put both files in the same directory. This code uses the decision operator IS to determine the value of Route. It uses multiple cfif tags to figure out the value of the Route variable and what it should be.
Specifying Additional Conditions with cfelse and cfelseif
Up to this point the cfif tags have performed actions only if an expression resulted to true. What happens when you need to perform some actions when a condition is false? Well, you could use the NOT operator to create a second cfif statement, but there is a better way. CFML offers us the <cfelse> tag. This is the syntax:
Perform Other Actions
The cfif tag remains the same, as do the actions that we perform if the condition is true. What changes is that we have a cfelse tag. The cfelse tag does not take any parameters or expressions - it simply offers the alternative code to run in case a cfif evaluates to false.
If you go back to the driving example, you will see that we have three separate conditions. Turning left directs us to the grocery store, turning right takes us to the bank, and going straight sends us home. In this case, it won't be possible to perform all of the actions at once. Our first example is inefficient because we evaluate all conditions even after finding the correct one.
There is a better way to handle this situation - through the use of a tag called cfelseif. The cfelseif tag allows us to add multiple conditions to our cfif statement. If the first condition does not evaluate to true, then the second cfelseif condition is evaluated. If the second is not true, then the third is evaluated. Use of the cfelseif is optional, and there is no limit to the number of cfelseif conditions you can have. By contrast, the cfelse tag can be used only once. This is the syntax for cfelseif:
Perform Other Actions
The code is very similar to the previous sample. The cfelseif tag contains an additional expression, whereas the cfelse did not. It will be evaluated only if the cfif expression is false. When there are cfelseif tags and a cfelse tag, the cfelse tag must always come last (after all cfelseif tags). The previous example used multiple cfif tags. The cfelseif tag lets us combine the separate cfif tags into a single login. This is the code:
<cfif Route is "Right">
Go to Bank<br>
<cfelseif Route is "Left">
Go to Store<br>
If the route chosen is right, then we go to the bank. If the route is left we go to the store. If nothing else, we continue to go straight on home.
Where Do I Go from Here?
The cfif statement can get very complicated if you start using one cfif statement inside another cfif statement. If you want to do some independent study, I would suggest reading up on cfswitch at http://livedocs.macromedia.com/coldfusion/ 6.1/htmldocs/tags-c10.htm#wp1103819 and cfcase at http://livedocs.macromedia.com/coldfusion/ 6.1/htmldocs/tags-pa9.htm#wp2664410. They are used together to form case statements, which are a different form of conditionals that you can use in your code. The use of cfswitch and cfcase statements is often a great way to simplify complex cfif.
It has been a pleasure writing these articles for you, and I'm pleased to announce that I've received my first topic request. Next month I will be writing about structures and arrays. Keep the ideas coming!
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