How To Evaluate Numeric Expressions Stored as a String

Given a string expression, the Val function returns the number contained in that string. However, if it encounters a character that is not a number or part of a number, it ignores the rest of the string. For example, Val("2") returns the number 2, but Val("2+2") also returns the number 2, and Val("(2+2)") returns the number 0.

This article describes how to obtain the actual value of a numeric expression that is stored as a String by using the existing features in the Excel Object Model.

In some instances, you might want to allow users to type in their own numeric expression in a TextBox and then evaluate it. In other cases, it might be necessary to read an ASCII file that contains a series of numeric expressions and then produce another ASCII file containing the results of those expressions. If this is the only purpose of your Visual Basic application, it is probably best, for speed and efficiency, to write a parser. However, for a simple and quick solution, you can use OLE Automation and the Excel Object Library to accomplish these tasks.

Step-by-Step Example

1. Start a new project in Visual Basic. Form1 is created by default.
2. Add a TextBox and a CommandButton to Form1.
3. Set the Text property of the TextBox to "".
4. Set the Caption property of the CommandButton to "Evaluate."
5. Add the following code to the Click event of the CommandButton:

         Private Sub Command1_Click()
             MsgBox MyVal(Text1.Text)
         End Sub
   						

6. Add the following code to the General Declarations section of Form1:

         Private Function MyVal(s As String) As Double
         Dim xl As Object
   
             Set xl = CreateObject("excel.application")
             xl.Workbooks.Add
   
             xl.ActiveCell.Formula = "=" & s
             MyVal = xl.ActiveCell.Value
   
             xl.ActiveWorkbook.Close savechanges:=False
             xl.Quit
             Set xl = Nothing
   
         End Function
   						

7. Run the project by pressing the F5 key. Type a valid numeric expression in the TextBox and click the CommandButton to evaluate the expression.