VB co-ordinate calculation with NCalc

Question

I need this to loop from -n to n but this only produces one co-ordinate so far. Does anyone know any easy quick fixes so I can have values from -n to n (step 1) subbed into the y equation id input?

I'm also not sure if I'm using the expression section correctly as I've never tried NCalc before today.

Dim y As String
Dim n As Integer

n = txtX.Text
y = txtY.Text
For x As Integer = -n To n Step 1

    Dim exp As Expression = New Expression(y)
    exp.Parameters("n") = n
    Label1.Text = exp.Evaluate

    Chart1.Series("plot1").Points.AddXY(x, y)
Next

@AndrewMorton i still get the error `System.ArgumentException: 'Parameter was not defined Parameter name: x'` :( maybe its the n within the `exp.Parameters("n") = n` somehow causing it to not loop as its only using one value? and im not too sure how to fix the y but that isnt a big problem for the time being — Ralsie Geaff, Feb 04 '21 at 10:30

score 0 · Accepted Answer · answered Feb 04 '21 at 16:03

I looked at NCalc and managed to come up with the following.

On an empty form, I placed two textboxes, "tbEquation" to hold the equation, and "tbXextents" to hold the maximum x-value for evaluating the function; and a button "bnDrawGraph" to tell it to draw the graph.

Private Sub bnDrawGraph_Click(sender As Object, e As EventArgs) Handles bnDrawGraph.Click

    Chart1.Legends.Clear()
    Chart1.Series.Clear()
    Dim xy As New Series
    xy.ChartType = SeriesChartType.Line

    Dim expr = New NCalc.Expression(tbEquation.Text)
    Dim xMax = Integer.Parse(tbXextents.Text)

    For x = -xMax To xMax
        expr.Parameters("n") = x
        Dim y = Convert.ToDouble(expr.Evaluate())
        xy.Points.AddXY(x, y)
    Next

    Chart1.Series.Add(xy)

End Sub

You only need to create the expression once, before using it in the loop.
It seems that the Evaluate() method returns an Object rather than any particular type of variable, so I used Convert.ToDouble to get the value as a numeric type suitable for using in a chart series.

Sample output:

VB co-ordinate calculation with NCalc

1 Answers1