Subtraction between members of a list of double - vb.net

Question

I'm creating a public function for 1D linear interpolation. I was asking if it could be possible to work with List of Double, instead of Double() because there arrays don't have fixed lenght, but it is parametric, depending on the number of rows inserted in a datagridview. I've tried to declare double() as list of double() but I'm receiving an error about subtraction between members - that seems to be not allowed. Here is the code:
Public Shared Function Interp1(ByVal xnative As Double(), ByVal ynative As Double(), ByVal xinterpol As Double()) As Double()
Dim yinterpol As Double()
For i = 1 To ynative.Length - 1
For j = 0 To xnative.Length - 1
If xinterpol(j) > xnative(i - 1) And xinterpol(j) < xnative(i) Then
yinterpol(j) = (ynative(i - 1) * (xinterpol(j) - xnative(i + 1)) / (xnative(i - 1) - xnative(i + 1))) - (ynative(i + 1) * (xinterpol(j) - xnative(i - 1)) / (xnative(i - 1) - xnative(i + 1)))
Else i += 1
End If
Next
If xinterpol(xinterpol.Length) > xnative(xnative.Length) Then
Exit For
End If
Next
End Function
Where am I doing wrong? I can't understand where the mistake can be. Other thing I was asking is, if with the provided code, yinterpol is the result of my function, because that is what I need to have as result. Thanks in advance
Edit: Here is the version of my code using List of Double():
Public Shared Function Interp1(ByVal xnative As List(Of Double)(), ByVal ynative As List(Of Double)(), ByVal xinterpol As List(Of Double)()) As List(Of Double)()
Dim yinterpol As New List(Of Double)()
For i = 1 To ynative.Length - 1
For j = 0 To xnative.Length - 1
If xinterpol(j) > xnative(i - 1) And xinterpol(j) < xnative(i) Then
yinterpol.add((ynative(i - 1) * (xinterpol(j) - xnative(i + 1)) / (xnative(i - 1) - xnative(i + 1))) - (ynative(i + 1) * (xinterpol(j) - xnative(i - 1)) / (xnative(i - 1) - xnative(i + 1))))
Else i += 1
End If
Next
If xinterpol(xinterpol.Length) > xnative(xnative.Length) Then
Exit For
End If
Next
End Function
I'm having back the error BC30452: operators "-", ">" and "<" are not defined for List of Double
Edit II: What I have to do with this function is to rebuild two list of double that I'm gonna type in the application through a Datagridview.
What I need to have is:
To rebuild the starting List, I have to implement a 1D linear interpolation function, in which, with the values in the column x-ending (from 0 to 106 in the example), I can obtain the y-ending. I've updated my code with your corrections but I think that my code is missing something to do what is my aim. I hope this is clearer now.

Answer 1

Your parameters for your function look like arrays of lists considering the trailing (). This can be used when declaring a New list to indicate you are calling the constructor, although in vb.net it is not necessary (unlike C#). You are asking calling code to provide and arrays of lists.
The error you are getting is because xinterpol(j) is the list present at index j in the array of list. Obviously you can't convert a List(Of Double) to a Double.
.Length is not available for List(Of T). You should use .Count
Your index j makes me a bit nervous. You are basing this on the .Count of xnative but applying it to xinterpol. What if xnative is longer than xinerpol? You are begging for an Index Out of Range exception.
I can't see why you want to possibly add a single item to yinterpol and then quit if the .Count of xinterpol is greater than the .Count of xnative
Public Shared Function Interp1(xnative As List(Of Double), ynative As List(Of Double), xinterpol As List(Of Double)) As List(Of Double)
Dim yinterpol As New List(Of Double)
For i = 1 To ynative.Count - 1
For j = 0 To xnative.Count - 1
If xinterpol(j) > xnative(i - 1) And xinterpol(j) < xnative(i) Then
yinterpol.Add((ynative(i - 1) * (xinterpol(j) - xnative(i + 1)) / (xnative(i - 1) - xnative(i + 1))) - (ynative(i + 1) * (xinterpol(j) - xnative(i - 1)) / (xnative(i - 1) - xnative(i + 1))))
Else i += 1
End If
Next
If xinterpol(xinterpol.Count) > xnative(xnative.Count) Then
Exit For
End If
Next
Return yinterpol
End Function

Answer 2

Re: The error you're receiving using arrays, you have a couple of issues here.
One is that you're not actually returning a value from the function.
Another is that you haven't initialized the yinterpol array with a size.
It's also good practice to declare the types of your indices rather than infer them (set Option Explicit on, Option Infer off and always set Option Strict on).
Try this:
Public Shared Function Interp1(ByVal xnative As Double(), ByVal ynative As Double(), ByVal xinterpol As Double()) As Double()
Dim yinterpol(9) As Double '<- Initialize array length
For i As Integer = 1 To ynative.Length - 1 '<- Declare type of i
For j As Integer = 0 To xnative.Length - 1 '<- Declare type of j
If xinterpol(j) > xnative(i - 1) AndAlso xinterpol(j) < xnative(i) Then
yinterpol(j) = (ynative(i - 1) * (xinterpol(j) - xnative(i + 1)) / (xnative(i - 1) - xnative(i + 1))) - (ynative(i + 1) * (xinterpol(j) - xnative(i - 1)) / (xnative(i - 1) - xnative(i + 1)))
Else
i += 1 '<- Don't modify outer loop index
End If
Next j
If xinterpol(xinterpol.Length) > xnative(xnative.Length) Then Exit For
Next i
Return yinterpol '<- Return a value
End Function
I prefer Lists over Arrays personally. Let me know if you want that version posted. Some sample data for the call would be helpful.
UPDATE: You shouldn't change a loop variable directly. So i += 1 within the "j" For is a bad idea. (In fact, I'm not even sure the changed value will persist in the outer "i" loop.)
List Version
You List example is using ARRAYs of List(of Double) on account of the trailing "()". Most of the other issues still apply, eg changing loop index, missing return value, etc.
Public Shared Function Interp1(ByVal xnative As List(Of Double), ByVal ynative As List(Of Double), ByVal xinterpol As List(Of Double)) As List(Of Double)
Dim yinterpol As New List(Of Double)
For i As Integer = 1 To ynative.Count - 1
For j As Integer = 0 To xnative.Count - 1
If xinterpol(j) > xnative(i - 1) AndAlso xinterpol(j) < xnative(i) Then
yinterpol.Add((ynative(i - 1) * (xinterpol(j) - xnative(i + 1)) / (xnative(i - 1) - xnative(i + 1))) - (ynative(i + 1) * (xinterpol(j) - xnative(i - 1)) / (xnative(i - 1) - xnative(i + 1))))
Else
i += 1
End If
Next j
If xinterpol(xinterpol.Count) > xnative(xnative.Count) Then Exit For
Next i
Return yinterpol
End Function