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Tip of the Day
Language: VB4,VB5,VB6
Expertise: Intermediate
Oct 6, 2001

A simple expression evaluator

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:

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))
            Else
                ' 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
    Next
    EvalExpression = LTrim$(result)
    
End Function
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:

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: 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)
Francesco Balena
 
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