Removing rows from a Dictionary based on the next or last row

Question

This is an offshoot of this question. Consider data thus:

1/1/2000  10000
1/1/2001  10000
1/1/2002  10000
10/1/2003  11000
1/1/2004  11000
1/1/2005  11000
6/1/2006 10000
9/1/2006  12000

This data is being collected into a SortedDictionary with the dates as the keys. I would like to convert it into this format:

1/1/2000  10000
10/1/2003  11000
6/1/2006 10000
9/1/2006  12000

That is, I want the earliest dated item for any given unique value. Normally when I'm presented with this sort of problem, I would iterate backwards...

for i = items.count - 1 to 1 step -1
    if item(i) is like item(i-1) remove item(i)
next

But how do you do that with a SortedDictionary? Linq provides a reverse enumerator, and an index-ish thing, but Linq is only available on Windows. Is there an easy way to do this within basic VB.net I am unfamiliar with?

I solved this by making a List(Of Date), iterating backwards over that, then removing entries from the List. Then I iterate over the result and remove any key from the SortedDictionary. But that's seriously ugly and many more lines than I would like.

Answer 1

To remove rows from SortedDictionary by compare item N to N-1 in a dictionary and without use Linq, I propose the following solution :

1-Convert dictionary values into array.

2-Compare item N to N-1 in array.

3-Removing rows by index from dictionary.

'sd is your SortedDictionary
Dim sdArray(sd.Keys.Count - 1) As Integer
sd.Values.CopyTo(sdArray, 0)

For i = sd.Keys.Count - 1 To 1 Step -1
    If sdArray(i) = sdArray(i-1) then
        Dim index As Integer = 0
        For Each p As KeyValuePair(Of DateTime, integer) In sd
            if index = i then sd.Remove(p.Key) : Exit For
            index += 1
        Next
    End If
Next

If you want the earliest dated item for any given unique value use this code :

For i = sd.Keys.Count - 1 To 1 Step -1
    For j = i - 1 To 0 Step -1
        If sdArray(i) = sdArray(j) then
            Dim index As Integer = 0
            For Each p As KeyValuePair(Of DateTime, integer) In sd
                if index = i then sd.Remove(p.Key) : Exit For
                index += 1
            Next
            Exit For
        End If
    Next
Next