Re: Find Position of many elements in a large list.

*To*: mathgroup at smc.vnet.net*Subject*: [mg127681] Re: Find Position of many elements in a large list.*From*: Peter Pein <petsie at dordos.net>*Date*: Wed, 15 Aug 2012 03:31:05 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net*References*: <k0d4bu$m8f$1@smc.vnet.net>

Am 14.08.2012 11:05, schrieb benp84 at gmail.com: > I have a sorted, 1-dimensional list X of 1,000,000 integers, and a sorted, 1-dimensional list Y of 10,000 integers. Most, but not all, of the elements of Y are also elements of X. I'd like to know the positions of the elements in X that are also in Y. What's the fastest way to compute this? > > I have an algorithm in mind but it requires lots of custom code and I'm wondering if there's a clever way to do it with built-in functions. Thanks. > Hi, the fastest way I was able to find is about four times faster than the naive approach Flatten[Position[x, Alternatives @@ Intersection[x, y], 1]] Dropping successively the irrelevant parts of the huge list x, searching becomes faster: Reap[ Fold[ Drop[#1, Sow[LengthWhile[#1, Function[x0, x0 <= #2]]]] &, x, Intersection[x, y] ] ][[2, 1]] // Accumulate I tried patterns and compilation; to no avail (see [1]). Cheers, Peter [1] https://dl.dropbox.com/u/3030567/Mathematica/getpositions.nb