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Show duplicates in Mathematica

Asked 2009-10-27T14:11:10.863
13

In Mathematica I have a list:

x = {1,2,3,3,4,5,5,6}

How will I make a list with the duplicates? Like:

{3,5}

I have been looking at Lists as Sets, if there is something like Except[] for lists, so I could do:

unique = Union[x]
duplicates = MyExcept[x,unique]

(Of course, if the x would have more than two duplicates - say, {1,2,2,2,3,4,4}, there the output would be {2,2,4}, but additional Union[] would solve this.)

But there wasn't anything like that (if I did understand all the functions there well).

So, how to do that?

list duplicates wolfram-mathematica
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2 Answers

4

Using a solution like dreeves, but only returning a single instance of each duplicated element, is a bit on the tricky side. One way of doing it is as follows:

collectDups1[l_] :=
  Module[{i, j},
    i[n_] := (i[n] := j[n]; Unevaluated@Sequence[]);
    j[n_] := (j[n] = Unevaluated@Sequence[]; n);
    i /@ l];

This doesn't precisely match the output produced by Will Robertson's (IMO superior) solution, because elements will appear in the returned list in the order that it can be determined that they're duplicates. I'm not sure if it really can be done in a single pass, all the ways I can think of involve, in effect, at least two passes, although one might only be over the duplicated elements.

answered 2009-10-27T20:01:46.570
1

Given a list A,
get the non-duplicate values in B
B = DeleteDuplicates[A]
get the duplicate values in C
C = Complement[A,B]
get the non-duplicate values from the duplicate list in D
D = DeleteDuplicates[C]

So for your example:
A = 1, 2, 2, 2, 3, 4, 4
B = 1, 2, 3, 4
C = 2, 2, 4
D = 2, 4

so your answer would be DeleteDuplicates[Complement[x,DeleteDuplicates[x]]] where x is your list. I don't know mathematica, so the syntax may or may not be perfect here. Just going by the docs on the page you linked to.

answered 2009-10-27T14:39:54.020

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