[RESOLVED] String manipulation

[J-Deezy]

I was hoping that the OP would be able to do some thinking for themselves but, as we're taking that responsibility away from them:

Private Function GetSubset(ByVal numbers As String, ByVal startIndex As Integer) As String
    Dim superset As String() = numbers.Split(New Char() {","c}, _
                                             StringSplitOptions.RemoveEmptyEntries)
    Dim subset As New List(Of String)
 
    For index = startIndex To superset.GetUpperBound(0) Step 3
        subset.Add(superset(index))
    Next
 
    Return String.Join(",", subset.ToArray())
End Function
 
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim str1 = "11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,"
    Dim str2 = "11,12,13,14,15,16,17,18,19,20,21,22,23,24,25"
 
    MessageBox.Show(Me.GetSubset(str1, 1 - 1))
    MessageBox.Show(Me.GetSubset(str2, 8 - 1))
End Sub

EDIT: Actually, now that I look at the examples provided in the first post, this code won;t do what they indicate is required, although it will do what's actually described in the words of the post.

I like your solution, I should have though of building a list(of string)

I didn't know about the Step in a for statement so cheers for that