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Re: List complement operator

  • To: mathgroup at smc.vnet.net
  • Subject: [mg85057] Re: List complement operator
  • From: Roland Franzius <roland.franzius at uos.de>
  • Date: Sat, 26 Jan 2008 05:00:16 -0500 (EST)
  • Organization: Universitaet Hannover
  • References: <fn1ndi$97t$1@smc.vnet.net>

zac schrieb:
> Dear group,
> 
> I'm loooking for an elegant and efficient way to subtract a list (not
> a set) from an other one.
> 
> a = {1, 1, 1, 2, 3, 3, 4, 5, 5, 5, 2, 6, 7, 7};
> b = {1, 2, 2, 3, 4, 7, 8};
> 
> Complement[a,b] returns {5, 6}, as expected. However I would like to
> treat each entry as a discrete element, therefore my target result is:
> {1, 1, 3, 5, 5, 5, 6, 7}. Union can not be used, although Sort  is
> allowed (ie. whether the result is sorted or not is of no concern).
> Any idea?

sublists[a_List, b_List] :=
(Part[#,
      Complement[
       Range[Length[a]], Union@Flatten@((Position[a, #] &) /@ b)]
      ] &)[Sort[a]]

-- 

Roland Franzius


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