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Mapping tag-value lists to a third

• To: mathgroup at smc.vnet.net
• Subject: [mg132442] Mapping tag-value lists to a third
• From: carlos.felippa%colorado.edu at gtempaccount.com
• Date: Mon, 17 Mar 2014 02:27:31 -0400 (EDT)
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• Delivered-to: l-mathgroup@wolfram.com
• Delivered-to: mathgroup-outx@smc.vnet.net
• Delivered-to: mathgroup-newsendx@smc.vnet.net

Hi,

I have 2 lists of equal length such as

tag={0, 1, 0, 0, 0, 1, 0, 1};  val={f1,f2,f3,f4,f5,f6,f7,f8}

and want to produce a third:

tagval={f2,f6,f8}

with length Count[tag,1] containing the entries of val marked by 1-tags. Should be
something efficient, since the source lists may be large (10^6 to 10^7).  Thanks.

• Follow-Ups:
  • Re: Mapping tag-value lists to a third
    • From: Carl K Woll <carlw@wolfram.com>
  • Re: Mapping tag-value lists to a third
    • From: Bob Hanlon <hanlonr357@gmail.com>
  • Re: Mapping tag-value lists to a third
    • From: Sseziwa Mukasa <mukasa@gmail.com>
  • Re: Mapping tag-value lists to a third
    • From: Francisco Gutierrez <fgutiers2002@yahoo.com>