' FB 1.05.0 Win64

Function formatRange (a() As Integer) As String
  Dim lb As Integer = LBound(a)
  Dim ub As Integer = UBound(a)
  If ub = - 1 Then Return ""
  If lb = ub Then Return Str(a(lb))
  Dim rangeCount As Integer = 1
  Dim range As String = Str(a(lb))
  For i As Integer = lb + 1 To ub
    If a(i) = a(i - 1) + 1 Then
      rangeCount += 1
    ElseIf rangeCount = 1 Then
      range += "," + Str(a(i))
    ElseIf rangeCount = 2 Then
      rangeCount = 1
      range += "," + Str(a(i-1)) + "," + Str(a(i))
    Else
      rangeCount = 1
      range += "-" + Str(a(i-1)) + "," + Str(a(i))
    End If
  Next
  If rangeCount = 2 Then
    range += "," + Str(a(ub))
  ElseIf rangeCount > 2 Then
    range += "-" + Str(a(ub))
  End If
  Return range
End Function

Dim a(1 To 20) As Integer = {-6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20}
Print formatRange(a())
Print

Dim b(1 To 33) As Integer => _
{ _
   0,  1,  2,  4,  6,  7,  8, 11, 12, 14, _
  15, 16, 17, 18, 19, 20, 21, 22, 23, 24, _
  25, 27, 28, 29, 30, 31, 32, 33, 35, 36, _
  37, 38, 39 _
}

Print formatRange(b())
Print
Print "Press any key to continue"
Sleep