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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)
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• Delivered-to: l-mathgroup@wolfram.com
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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>