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Class: ExpressionEvaluator in Category General VBA/VB6 : String Handling from Total Visual SourceBook

Class to implement expression evaluation in VBA and VB6.

Expression evaluation is a common need in many programs, especially if you deal with numbers and operators.

Procedure List

Procedure Name	Type	Description
(Declarations)	Declarations	Declarations and private variables for the CExpressionEvaluator class.
Expression	Property	Get the expression to evaluate.
Postfix	Property	Get the postfix representation of the expression string.
Class_Initialize	Initialize	Set initial values to defaults which may be overridden with property settings.
Evaluate	Method	Evaluates the expression. For simplicity the postfix representation of the expression is evaluated.
CharType	Private	Determine the type of the current character.
ConvertToPostfix	Private	Converts the expression to postfix and stores the result in m_strPostfix.
GetNextToken	Private	Get the next token in the string identified by the text property.
HandleSeparator	Private	Handles a separator character.
HandleToken	Private	Handles a token character.
HandleWhiteSpace	Private	Handles a whitespace character.
ParseExpression	Private	Parses an expression.
ParseFactor	Private	Parses a factor.
ParseTerm	Private	Parse a term. A term is something like 2+7, or 1 + sin(1), while a token is the smallest meaningful piece of data, such as 2 or sin.
PopResult	Private	Pops an item off the stack.
PushResult	Private	Pushes an item onto the stack.
SetTokenString	Private	Set the string used by the tokenizer.

Example Code for Using Class: ExpressionEvaluator

' Example of CExpressionEvaluator
'
' To try this example, do the following:
' 1. Add a command button named 'cmdTest'
' 2. Paste all the code from this example to the new form's module.
' 3. Run the form

Private mExpressionEvaluator As CExpressionEvaluator

Private Sub cmdTest_Click()
  Dim dblResult As Double
  Set mExpressionEvaluator = New CExpressionEvaluator

  With mExpressionEvaluator
    .Expression = "5 * 10 / (12.5 * 2)"
    dblResult = .Evaluate
    Debug.Print "5 * 10 / (12.5 * 2) = " & dblResult
  End With

End Sub

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