Question

I have this function for removing duplicates in arrays:

Function remove_duplicates(array)
    Dim dup_count As Integer = 0

    For Each element As String In array

        For Each inner_element As String In array

            If (element = inner_element) Then
                If (dup_count = 0) Then
                    dup_count = 1
                Else

                    array.Remove(inner_element)
                End If
            End If

        Next 'dies here

        dup_count = 0
    Next

    Return array
End Function

Into which I am feeding the following test arrayList:

(0) test1
(1) test2
(2) test3
(3) test2

Which I expect to return:

(0) test1
(1) test3
(2) test2

What is happening at the moment is that the inner loop reaches the array.Remove(inner_element) line for the first time in its second loop during the second loop of the main foreach, reaches the inner next line and then just stops executing. There are no warnings / notices, the debugging step though just ends as if there was no further logic.

Can anyone suggest a work around or why this is happening?

Answer 1

You cannot modify a list that you are currently iterating over.

Furthermore, there are better ways of removing from a list using the RemoveAll method:

 array.RemoveAll(Function (x) array.Count(Function (y) x = y) > 1)

This requires the Linq method Count (hence Import System.Linq).

EDIT

Rather than doing this manually, it’s of course better, more readable and more efficient to resort to pre-existing methods:

Dim result = array.Distinct()

Distinct internally constructs a HashSet which makes this method run in asymptotic time O(n) rather than O(n^2) (which is the runtime of the above method).