A Replacement for VB Evaluate(string)

To graph a function we need to evaluate a string into its math equivalent. Excel VBA has the Application.Evaluate method which takes a variant for an argument and will evaluate a wide range of math symbols, with or without quotations – whatever is between the parentheses. In Visual Studio that doesn’t exist. We want to type a function (the right side of Y = some expression of X) into a textbox on a form, save it to a string variable, then calculate the value to a double for a range of X values. For instance,

strfunc = “X^3 – 3*X^2 + 2*x + 2”

A loop calculates X and Y values starting at X minimum, incrementing by a small X increment, ending at X maximum, and saving all the values to an array to be an argument for AddLightWeightPolyline.

the number of points to be stitched into a graph is calculated –
numpts = CInt((Xmax – Xmin) / Xinc) ‘number of line segments

there is always one more point than line segments
numpts = numpts + 1

the array of doubles is sized –
ReDim Pt(0 To numpts * 2 – 1) ‘store x and y for one pt

after the number of points has been calculated, and the array is ready, the loop is

For i = 1 To numpts
x = Xmin + ((i - 1) * Xinc)
y = eval_func(strfunc, x)
pt(i * 2 - 2) = x
pt(i * 2 - 1) = y
Next i

X is recalulated each time through the loop. Rather than adding the increment to a running sum, its more accurate to use the loop counter to multiply the increment, because of the imprecision of doubles. With odd numbers you might eventually run one cycle short or long. The loop counter should always be an integer. Calculation of Y is handed off to a function with the function string and the current value of X. The function substitutes the numerical value of X whereever X the symbol appears.

Debug.Print strfunc
strfunc = Replace(strfunc, “x”, x, , , vbTextCompare)
Debug.Print strfunc

the first time thru where X is at the starting value of, say, -6, the string and the modified string look like –

X^3 – 3*X^2 + 2*x + 2
-6^3 – 3*-6^2 + 2*-6 + 2

then the final scalar value is calculated and the value returned as Y.

dbl_result = Evaluate(strfunc)

This code was being run from Excel. Evaluate was a method of the top level Application.

For Visual Studio VB.Net we need a replacement for Evaluate.

A lot of people have asked this question. There are quite a few homebuilt parser class solutions. Another method is to link to another language and use its Eval method. A quick easy solution was found, credit to user kleinma

https://www.vbforums.com/showthread.php?412232-RESOLVED-02-03-Question-about-using-VBScript-(Eval)-in-VB-NET

As kleinma says, “Set a reference in your app to “Microsoft Script Control 1.0″ It is under the COM tab in references, not in the .NET tab”

A lot of testing will show if its a decent fix or just the first try.

    Public Xmin As Double
    Public Xmax As Double
    Public Xinc As Double
    Public Ylim As Double
    Public g_stop As Boolean
    Public g_err_counter As Integer


    Function calc_xy(strfunc As String) As Double()
        'returns an array of x and y doubles
        'Xmax, Xmin, Xinc, Ylim are all assumed to have values
        Dim x, y As Double
        Dim i, numpts As Integer
        Dim Pt() As Double

        g_err_counter = 0
        g_stop = False

        numpts = CInt((Xmax - Xmin) / Xinc) 'number of line segments 
        numpts = numpts + 1   'one more pt than line segment
        ReDim Pt(0 To numpts * 2 - 1) 'store x and y for one pt

        For i = 1 To numpts
            x = Xmin + ((i - 1) * Xinc)
            y = eval_func(strfunc, x)

            If g_stop Then  'if eval_func set g_stop then
                MsgBox("errorcounter at 3 - exiting")
                Exit For
            End If
            Pt(i * 2 - 2) = x
            Pt(i * 2 - 1) = y
        Next i

        Return Pt
    End Function

    Function eval_func(ByVal strfunc As String, x As Double) As Double
        'returns a double
        'on error returns Ylim but sets a flag to stop after 3 errors
        'which would indicate a bad formula
        'one or two error is ok because havent solved the mid-graph asymptote yet
        Dim result As Double
        strfunc = Replace(strfunc, "x", x.ToString, , , vbTextCompare)

        Dim SC As New MSScriptControl.ScriptControl
        SC.Language = "VBSCRIPT"
        Try
            result = Convert.ToDouble(SC.Eval(strfunc))
        Catch ex As Exception         
            MessageBox.Show(ex.GetType.ToString & " " & ex.Message)
            g_err_counter = g_err_counter + 1
            If g_err_counter > 2 Then
                g_stop = True
            End If
            result = Ylim
        End Try

        Return result
    End Function

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.