WEAP borrows an approach made popular in spreadsheets--the ability for users to enter data and construct models using mathematical expressions. Expressions are standard mathematical formulae used to specify the values of variables in WEAP's Data View. WEAP supports a comprehensive set of functions that you can include in your expressions to create your models. For more information, see Expressions for an introduction to expressions. Functions are divided into three groups:

**Modeling functions**: the major functions used to help you model data.**Mathematical functions**: standard mathematical functions, similar in syntax to the ones used in Microsoft Excel.**Logical functions**: which can be used to create complex conditional modeling expressions.

In addition to WEAP's built-in library of functions, you can easily write your own functions in standard scripting languages, such as VBScript, JavaScript, Perl, Python, Ruby and PHP. These functions can be accessed from your expressions using the CALL function. See Scripting for more information.