How can I list the items deleted from DeleteDuplicatesBy?

For example:

DeleteDuplicatesBy[{{a, b}, {f, c}, {d, b}, {e, c}, {a, c}}, Part[#, 2] &]


{{a, b}, {f, c}}

However, is there any simple way to get the list {{d, b}, {e, c}, {a, c}} of deleted pairs?


Based on @Coolwater's fine answer to Replace element in array by checking condition in another list you can use GatherBy:

Catenate @ GatherBy[
][[All, 2;;]]

{{d, b}, {e, c}, {a, c}}


Here's another very similar version:

Catenate @ GroupBy[

{{d, b}, {e, c}, {a, c}}

  • $\begingroup$ Thanks a lot! This would be very useful to me. $\endgroup$ – wkong Jul 14 '17 at 1:17

Here is one way.

data = {{a, b}, {f, c}, {d, b}, {e, c}, {a, c}};
p1 = DeleteDuplicatesBy[data, Part[#, 2] &]

{{a, b}, {f, c}}

p2 = DeleteCases[data, Alternatives @@ p1]

{{d, b}, {e, c}, {a, c}}

Here is another.

p2 = Complement[data, p1]
  • 1
    $\begingroup$ Note that both of these solutions fail for data with two identical items. I assume that the goal is to get a list p2 s.t. Sort[p1~Join~p2]==Sort[data] $\endgroup$ – Lukas Lang Jul 13 '17 at 6:51
  • $\begingroup$ @Mathe172 I am aware of the your 1st assertion, but it not clear to me the 2nd applies. $\endgroup$ – m_goldberg Jul 13 '17 at 6:54
  • $\begingroup$ @m_goldberg Although there is some problem for two identical items, it is still workable in my case as there is no identical items in my data set. Thanks a lot! $\endgroup$ – wkong Jul 14 '17 at 1:18

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