Nevron .NET Vision Documentation
Formula Engine

Formulas are strings, which can be parsed and evaluated at runtime. A valid formula is such a string, which is lexically and grammatically correct. A valid formula string is called formula expression or simply expression. Expressions are parsed and evaluated by an instance the NFormulaEngine class. The following code example evaluates formula expressions:
C#
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NVariant res;
// create the formula engine
NFormulaEngine engine = new NFormulaEngine();
// define the a and b variables
engine.Variables.Add("a", new NVariant(10));
engine.Variables.Add("b", new NVariant(20));
// evaluate an expression (res is 50)
res = engine.ParseAndEvaluate("a+b*2");
// evaluate another expression (res is 40.5)
res = engine.ParseAndEvaluate("20.5+(10*2)*COS(0)");
Visual Basic
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Dim res As NVariant
' create the formula engine
Dim engine As New NFormulaEngine
' define the a and b variables
engine.Variables.Add("a", New NVariant(10))
engine.Variables.Add("b", New NVariant(20))
' evaluate an expression (res is 50)
res = engine.ParseAndEvaluate("a+b*2")
' evaluate another expression (res is 40.5)
res = engine.ParseAndEvaluate("20.5+(10*2)*COS(0)")

Following is some background information about the way in which the formula engine works.

How it Works

Expressions are constructed from tokens. Tokens are such substrings from the initial expression, which cannot be broken into other tokens. A lexically correct expression consists of correct tokens. Currently recognized are the following types of tokens:

  • Parenthesis - the '(' and ')' characters, which can be used to change the operators privilege, and enclose the functions arguments.
  • Operator - the predefined operators, which you can use (+,-,*,\,<,> etc.)
  • Comma - the ',' character
  • Constant - this is a constant value embedded in the expression. Currently it can be a number, boolean value or string (discussed later in this topic).
  • Function - this is an identifier, which is resolved as a function
  • Variable - this is an identifier, which is resolved as a variable
  • Reference - this is an identifier, which is resolved as a reference

An identifier is such a substring from the expression, which starts with a letter or the '_' char and is followed by a sequence of letters, digits or the '_', '.' and '!' characters.

The process of cracking the expression to a list of tokens is called tokenization (lexical analysis). The formula engine performs the tokenization internally and uses its output to build a formula elements tree (grammatical analysis). You can use the Parse method of the NFormulaEngine to obtain the root of the formula elements tree (the elements of this tree are discussed later in this topic).

The evaluation of the formula elements tree uses variants. Variants are represented by the NVariant class. In essence this a value-type pair, which facilitates the type conversion of values, as well as arithmetic and comparison operations with values of different type. The result of a formula evaluation is a variant (e.g. a value of certain type - see Variants for more information).

Formula Elements

Each grammatically correct expression can be represented as a tree of formula elements. All types of formula elements derive from the base NFormulaElement class. A formula expression can contain one of the following formula elements:

See Also

 

 


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