Listing what has been deleted by DeleteDuplicatesBy

Question

How can I list the items deleted from DeleteDuplicatesBy?

For example:

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

gives

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

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

At least closely related: How to get list of duplicates when using DeleteDuplicates? — Kuba, Jul 13, 2017 at 6:50

Carl Woll · Accepted Answer · 2017-09-09 00:17:57Z

3

Catenate @ GatherBy[
    {{a,b},{f,c},{d,b},{e,c},{a,c}},
    Last
][[All, 2;;]]

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

update

Here's another very similar version:

Catenate @ GroupBy[
    {{a,b},{f,c},{d,b},{e,c},{a,c}},
    Last,
    Rest
]

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

answered Jul 13, 2017 at 6:52

Carl Woll

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$\begingroup$ Thanks a lot! This would be very useful to me. $\endgroup$
– wkong
Jul 14, 2017 at 1:17

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m_goldberg · Accepted Answer · 2017-07-13 06:42:41Z

4

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]

answered Jul 13, 2017 at 6:42

m_goldberg

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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, 2017 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, 2017 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, 2017 at 1:18

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2 Answers 2