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A simple expression evaluator
[ All Languages » VB »  Math]

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While any Windows user could pop up the Calculator accessory to perform any type of math calculations, it would be great if you could offer him or her the capability to do simple math from within your application. This is a very simple expression evaluator function that does it:

This evaluator is very simple, and has a number of limitations: it does not account for negative numbers nor for parenthesis and sub-expressions, and operands are evaluated from left to right without any priority rule (e.g. "10+2*3" returns 36, and not 16). Nevertheless it is very handy and lets you make you program friendlier to your customer by adding the capability to perform calculation from within a textbox control. This routine shows how to evaluate the expression in a textbox control when the user presses the F2 key:

Click here to copy the following block
Sub Text1_KeyDown (KeyCode As Integer, Shift As Integer)
  If KeyCode = vbKeyF2 And Shift = 0 Then
    Text1.Text = EvalExpression(Text1.Text)
    Text1.SelStart = Len(Text1.Text)
  End If
End Sub

If you want to evaluate more complex expressions you can use the MS Script Control library, which offers a simple way to do it. This control has an Eval method that evaluates any expression, with parenthesis, sub-expressions, math functions such as sqr, power, etc., and evaluates the operators in the right order. Add the "Microsoft Script Control" library from the References dialog window, and execute the following code to calculate the result of the given sample expression:

Click here to copy the following block
Dim oScriptCtl As New ScriptControl Dim sExpr As String
oScriptCtl.Language = "VBScript"
sExpr = "sqr(25)+2^2+((3+7)/5+1)"
MsgBox sExpr & " = " & oScriptCtl.Eval(sExpr)

Click here to copy the following block
Function EvalExpression(ByVal expression As String) As Double
  Dim result As Double
  Dim operand As Double
  Dim opcode As String
  Dim index As Integer
  Dim lastIndex As Integer
  ' the null character mark the end of the string
  expression = expression & vbNullChar
  For index = 1 To Len(expression) + 1
    If InStr("+-*/" & vbNullChar, Mid$(expression, index, 1)) Then
      If lastIndex = 0 Then
        ' this is the first operand in the expression
        result = Val(Left$(expression, index - 1))
        ' extract the new operand
        operand = Val(Mid$(expression, lastIndex, index - lastIndex))
        ' execute the pending operation
        Select Case opcode
        Case "+"
          result = result + operand
        Case "-"
          result = result - operand
        Case "*"
          result = result * operand
        Case "/"
          result = result / operand
        End Select
      End If
      opcode = Mid$(expression, index, 1)
      lastIndex = index + 1
    End If
  EvalExpression = LTrim$(result)
End Function

Submitted By : Nayan Patel  (Member Since : 5/26/2004 12:23:06 PM)

Job Description : He is the moderator of this site and currently working as an independent consultant. He works with, SQL Server and other MS technologies. He is, MCDBA and MCSE. In his free time he likes to watch funny movies and doing oil painting.
View all (893) submissions by this author  (Birth Date : 7/14/1981 )

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