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 FunctionThis 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 IfEnd SubIf 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 ScriptControlDim sExpr As String
oScriptCtl.Language = "VBScript"sExpr = "sqr(25)+2^2+((3+7)/5+1)"MsgBox sExpr & " = " & oScriptCtl.Eval(sExpr)
