Handling Expressions in ColdFusion MX

Expressions involve operators and operands. Operands can be of various types, which can be in the form of literals, variables, or results of a function call. There are four types of operators that can be used in expressions: arithmetic, string, comparison, and Boolean.

When evaluating expressions, ColdFusion uses standard operator precedence. The order of evaluation of operators is given in Table 2.3.

Table 2.3: Operators in ColdFusion

Operator	Type	Description
Unary +, Unary -	Arithmetic	Sets the sign of the number
^	Arithmetic	Raises a power to a number
*, /	Arithmetic	Used for multiplication and division, respectively
\	Arithmetic	Used for integer division
MOD	Arithmetic	Returns the remainder of integer division
+,-	Arithmetic	Used for addition and subtraction, respectively
&	String	Used for string concatenation
EQ, NEQ, LT, LTE, GTE, CONTAINS, DOES NOT CONTAIN	Comparison	Returns a Boolean value after the respective comparison operations
NOT	Boolean	Returns TRUE for a FALSE value and vice versa
AND	Boolean	Returns TRUE when both the comparison expressions return TRUE; otherwise, it returns FALSE
OR	Boolean	Returns TRUE when one of the comparison expression returns TRUE
XOR	Boolean	Represents Boolean exclusive OR
EQV	Boolean	Represents Boolean equivalence
IMP	Boolean	Represents Boolean implication

Typeless Expressions

Unlike traditional programming languages, ColdFusion can evaluate an expression even if the operands and operators are of different types. The operands are converted to the datatypes as required by the operator. For example, the concatenation operator requires operands to be strings. If one of the operands isn't a string, it's converted to a string for concatenation.

Arithmetic Expressions

Arithmetic expressions involve numeric data or variables as operands and arithmetic operators. The <CFSET> tag can be used to perform computations.

The <CFOUTPUT> tag is used to display the results of variable calculations on a Web page. Variable names must be enclosed within pound signs (#) when used with the <CFOUTPUT> tag. <CFOUTPUT> replaces the variable with its current value when the Web page is displayed in the browser. The following example illustrates this:

 <CFEST num=20> <CFSET numtimes2=(num * 2)> <CFOUTPUT>num is: #num#</CFOUPUT><br> <CFOUTPUT>numtimes2 is: #numtimes2# </CFOUTPUT><br> 

The preceding code generates the following output:

 num is: 20 numtimes2 is: 40 

String Expressions

String expressions involve string values or variables that are operated using string operators. The <CFSET> tag is used to assign a value to the string variable, as follows:

 <CFSET City="Washington"> 

Here, "Washington" is a string literal.

To assign the result of a string expression, the <CFSET> tag is used as follows:

 <CFSET FirstName="William"> <CFSET LastName="Trueman"> <CFSET FullName = FirstName & LastName> 

The preceding code stores the string WilliamTrueman in the FullName variable.

Unlike other programming languages, ColdFusion lets you create string literals that span multiple lines, as follows:

 " a b c " 

Similar to numeric variables, string variables can also be displayed by using the <CFOUTPUT> tag. Variables that are surrounded by pound signs are replaced with their actual values.

If you want to include a quotation mark in a string, you need to type both quotation marks together, as follows:

 <CFSET Name="William ""Buck"" Trueman"> 

The preceding code stores the value William "Buck" Trueman in the Name variable.

Boolean Expressions

Boolean expressions are used when a program must take an action depending on a condition.

You can use Boolean constants in expressions directly, as follows:

 <CFSET ValueIsCorrect = True> 

In Boolean expressions, TRUE, non-zero numbers, and the string "Yes" are equivalent, and FALSE, 0, and the string "No" are equivalent.

Note

The pound or hash sign (#) has a special meaning in ColdFusion. It's used to separate expressions from plain text. When expressions surrounded by # signs are evaluated, the result is substituted for the expression.