Avoiding the Evaluate function

Example 1

You might be inclined to use the Evaluate function in code such as the following:

<cfoutput>1 + 1 is #Evaluate(1 + 1)#</cfoutput>

Although this code works, it is not as efficient as the following code:

<cfset Result = 1 + 1> 
<cfoutput>1 + 1 is #Result#</cfoutput>

Example 2

This example shows how you can use an associative array reference in place of an Evaluate function. This technique is powerful because:

• Most ColdFusion scopes are accessible as structures.

• You can use ColdFusion expressions in the indexes of associative array structure references. For more information on using associative array references for structures, see Structure notation.

The following example uses the Evaluate function to construct a variable name:

<cfoutput> 
    Product Name: #Evaluate("Form.product_#i#")# 
</cfoutput>

This code comes from an example where a form has entries for an indeterminate number of items in a shopping cart. A product name field exists for each item in the shopping cart. The field name is of the form product_1, product_2, and so on, where the number corresponds to the product entry in the shopping cart. In this example, ColdFusion does the following:

1. Replaces the variable i with its value, for example 1.

2. concatenates the variable value with "Form.product_", and passes the result (for Form.product_1) to the Evaluate function, which does the remaining steps.

3. Parses the variable product_1 and generates an executable representation of the variable. Because ColdFusion must run its parser, this step requires substantial processing, even for a simple variable.

4. Evaluates the representation of the variable, for example as "Air popper".

5. Returns the value of the variable.

The following example has the same result as the preceding example and is more efficient:

<cfoutput> 
    ProductName: #Form["product_" & i]# 
</cfoutput>

In this code, ColdFusion does the following:

1. Evaluates the expression in the associative array index brackets as the string "product_" concatenated with the value of the variable i.

2. Determines the value of the variable i; 1.

3. Concatenates the string and the variable value to get product_1.

4. Uses the result as the key value in the Form structure to get Form[product_1]. This associative array reference accesses the same value as the object.attribute format reference Form.product_1; in this case, Air popper.

This code format does not use any dynamic evaluation, but it achieves the same effect, of dynamically creating a structure reference by using a string and a variable.