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Consider this example:

Complement[{a, y, c, d, e}, {a, c}, {d}]
(*{e, y}*)

However, I was expecting the result to be:

(*{y,e}*)

Why did Complement[] reorder the resulting list?

Any idea how to get the desired results (list with elements in their original order), using Complement[]?

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  • $\begingroup$ Related, possibly duplicate? mathematica.stackexchange.com/q/1290/862 $\endgroup$
    – Simon Woods
    Sep 10, 2014 at 20:04
  • $\begingroup$ may be not related but instead easily found in the documentation. I was look for explanations why Complement sorts the results (the documentation says it is stored internally). $\endgroup$
    – Basheer Algohi
    Sep 10, 2014 at 20:12
  • $\begingroup$ @Simon Why not vote to close? $\endgroup$
    – Mr.Wizard
    Sep 11, 2014 at 4:53
  • $\begingroup$ Also related: (18100) $\endgroup$
    – Mr.Wizard
    Sep 11, 2014 at 5:01
  • $\begingroup$ @Mr.Wizard, it was late and I was too lazy to properly check if both questions were asking the same thing, so I just posted the link in case anyone else was looking for it. As there are still no close votes I guess the community consensus is that it's not a duplicate. $\endgroup$
    – Simon Woods
    Sep 11, 2014 at 9:20

3 Answers 3

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It uses sorting internally (as documented, actually). For unsorted, could do as below.

unsortedComplement[l1_, l2_] := Reap[Module[
    {remove},
    Map[(remove[#] = True) &, l2];
    Map[If[TrueQ[remove[#]], Null, Sow[#]] &, l1];
    Clear[remove];
    ]][[2, 1]]

unsortedComplement[{1, 3, 2, 8, 5}, {3, 6}]

(* Out[78]= {1, 2, 8, 5} *)

Extending to more lists is straightforward.enter code here

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  • $\begingroup$ Thanks. there are so many other ways to get the desired results. I was just wondering if there is any way to tell Complement to unsort the result. perhaps I should have read the document carefully. thanks for the answer $\endgroup$
    – Basheer Algohi
    Sep 10, 2014 at 20:08
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This should get you the desired result:

 Select[{a, y, c, d, e}, MemberQ[Union[{a, c}, {d}], #] == False &]

{y, e}

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If you want to stick with Complement[]

l = {a, y, c, d, e};;
l[[Sort[Complement[l, {a, c}, {d}] /. Thread[l -> Range@Length@l]]]]
(*{y, e}*)

or

SortBy[Complement[l, {a, c}, {d}], Position[l, #] &]
(*{y, e}*)
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