Designing Active Server Pages

Using Dynamic Evaluation and Execution

As a programmer, you have performed static evaluation and static execution countless times. For example, the following code snippet performs both static evaluation and static execution:

<% @LANGUAGE="VBSCRIPT" %>
<%
  Dim iAnswer
  iAnswer = 22 / 7   'A rough approximation of pi
%>

The first line, Dim iAnswer, is an example of static execution. When a browser visits the ASP page that the above code snippet resides in, the first line of code is executed, causing a memory location to be set aside to store the value of iAnswer. The second line, iAnswer = 22 / 7, is an example of both static evaluation and static execution. Initially, the expression 22 / 7 is evaluated, returning the result 3.14285714285714. The execution occurs when this value is stored into the variable iAnswer.

This type of code evaluation and execution is referred to as static because the statement being evaluated and the commands being executed are hardcoded into the script. The only way the expression 22 / 7 will change is if a developer edits the actual ASP file by entering a new expression.

The VBScript 5.0 scripting engine offers two functions that facilitate dynamic evaluation and execution. Let us examine each of these issues separately.

Dynamic evaluation

Imagine you wanted to present your users with a form with a single text box, into which they could enter a mathematical expression. Once the form was submitted, the result would be displayed. For example, the user might enter ...