List of multiple elements

*To*: mathgroup at smc.vnet.net*Subject*: [mg112040] List of multiple elements*From*: "Dr. Wolfgang Hintze" <weh at snafu.de>*Date*: Thu, 26 Aug 2010 06:48:34 -0400 (EDT)

Given the list a={1,1,2,1,2}; we can reduce multiple instances of elements using b=Union[a] {1,2} The question is now how to compute the list of multiple elements. In our example this would be m={1,1,2}. A possible solution is m[x_]:= Flatten[Take[#, {1, Length[#] - 1}] & /@ Select[Split[x], Length[#] > 1 &]] m[a] {1,1,2} I'm sure there is a much more elegant solution. Can you suggest one? Remark m[a] can be viewed as a kind of difference: m[a] = a "MINUS" Union[a] Inverting gives the "SUM": a = m[a] "PLUS" Union[a] = Join[m[a],Union[a]] Regards, Wolfgang