Removing elements from list with condition (VB) - vb.net

Question

I have a list
Dim list As New List(Of Double)
I want to remove the last entries, if the differences are > 20.
My idea to check the last 30 entries:
Do While index >= list.Count - 30
If Math.Sqrt((list(index) - list(index + 1)) ^ 2) > 20 Then
list.RemoveAt(index)
Exit Do
End If
Loop
It does not lead to my solution. Can somebody help? Thank you very much.

Answer 1

Instead of using a List of doubles I would use a LinkedList of doubles to make use of the class methods and properties like AddFirst, Last, Last.Previous
So let's assume that you have a LinkedList declared like
Dim list As New LinkedList(Of Double)
And you have added elements to this list using
list.AddFirst(134.5678)
Now, you could remove from the end of the list with something like this
' You want to have a list of at least 30 elements
Do While list.Count > 30
' Last node and the previous one
Dim dLast = list.Last
Dim dLastPrev = list.Last.Previous
' Evaluate the two elements
If Math.Sqrt(dLastPrev.Value - dLast.Value) ^ 2 > 20 Then
' Remove the last and continue to evaluate the next pair
list.RemoveLast()
Else
' Stop if the condition is not met.
Exit Do
End If
Loop
Of course this could also be done using your current list type.
Also notice how I have swapped the two elements to verify. This is done to avoid any possibility of an IndexOutOfRangeException
Dim index as Integer = list.Count - 1
Do While index >= list.Count - 30
Dim prev = index - 1
If Math.Sqrt((list(prev) - list(index)) ^ 2) > 20 Then
list.RemoveAt(index)
else
Exit Do
End If
index = index - 1
Loop