RE: Keeping order with Union

*To*: mathgroup at smc.vnet.net*Subject*: [mg32611] RE: [mg32590] Keeping order with Union*From*: "David Park" <djmp at earthlink.net>*Date*: Thu, 31 Jan 2002 01:45:42 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

Dave, If your list is composed of integers or exact expressions, then there is the very efficient and clever routine that Carl Woll put forth a year or so ago on MathGroup. RemoveRepeats[x_] := Block[{i}, i[n_] := (i[n] = Sequence[]; n); i /@ x] list = Array[Random[Integer, {0, 9}] &, 100] {1,9,0,7,6,4,3,1,7,6,2,3,0,1,7,7,0,1,0,9,1,1,5,4,5,0,7,6,7,9,2,4,8,3,6,2,1,2 ,\ 0,8,7,7,0,7,6,0,8,1,8,0,7,2,0,2,2,8,8,2,4,9,6,0,2,9,6,3,7,6,8,3,4,0,8,6,9,4, 7,\ 3,4,8,5,0,3,6,2,0,6,6,8,0,8,2,9,3,9,6,7,0,0,8} RemoveRepeats[list] {1, 9, 0, 7, 6, 4, 3, 2, 5, 8} Here is an alternative method, much slower, but it can be used on approximate expressions if you adjust the SameTest to something appropriate for the specific case. list2 = Transpose[{Range[Length[list]], list}]; list2 = Union[list2, SameTest -> (#1[[2]] == #2[[2]] & )]; Last /@ list2 When I ran on a list of 100000 integers, the Woll routine took 0.39 seconds, and the second routine took 6.42 seconds. David Park djmp at earthlink.net http://home.earthlink.net/~djmp/ > From: Dave Snead [mailto:dsnead6 at charter.net] To: mathgroup at smc.vnet.net > > I'd like a function that throws out duplicate entries in a list and keeps > the > original list order. (when it sees 2 equal entries it throws out the one > that is later in the list). I'd use Union, but it sometimes changes the > order of the elements in the list. What's the tersest way to do this? > Thanks in advance, > Dave Snead >