Find Position of many elements in a large list.

*To*: mathgroup at smc.vnet.net*Subject*: [mg127670] Find Position of many elements in a large list.*From*: benp84 at gmail.com*Date*: Tue, 14 Aug 2012 04:22:21 -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

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.

**Follow-Ups**:**Re: Find Position of many elements in a large list.***From:*Bob Hanlon <hanlonr357@gmail.com>

**Re: Find Position of many elements in a large list.***From:*Tomas Garza <tgarza10@msn.com>

**Re: Find Position of many elements in a large list.***From:*Matthias Bode <lvsaba@hotmail.com>