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Selecting by first element of each list

• To: mathgroup at smc.vnet.net
• Subject: [mg48130] Selecting by first element of each list
• From: "camartin at snet.net" <camartin at snet.net>
• Date: Fri, 14 May 2004 00:12:28 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

Hi,

I'm trying to learn to handle a flat file kind of database using 
Mathematica. An example is the list of lists below:

{{2,3,4,5,77},{2,4,5,66,77},{3,4,7,8,90,6},{3,5,6,7,8,0},{3,45,67,77,12}}

It's actually more complicated than this but good enough for 
illustrative purposes. The first element in each list is actually an 
identifier, like an account number. I want to create a list of lists 
with each account, that is,

{{{2,3,4,5,77},{2,4,5,66,77}}, 
{{3,4,7,8,90,6},{3,5,6,7,8,0},{3,45,67,77,12}}}.

When I use Select with an anonymous function such as

Select[list1,#1[[1]] = = #2[[1]]&]

I get an error because it stops (of course) after the first two lists. I 
get the right grouping for the first list but it doesn't finish. I don't 
understand how to use the anonymous function to go through my list (it's 
got several thousand entries) and select and group by the first element 
in each group. I've been through the archive but nothing there quite helps.

I would appreciate some direction.

Thanks

Cliff

• Follow-Ups:
  • Re: Selecting by first element of each list
    • From: Tomas Garza <tgarza01@prodigy.net.mx>