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Re: Selecting by first element of each list

• To: mathgroup at smc.vnet.net
• Subject: [mg48151] Re: [mg48130] Selecting by first element of each list
• From: Tomas Garza <tgarza01 at prodigy.net.mx>
• Date: Fri, 14 May 2004 20:59:07 -0400 (EDT)
• References: <200405140412.AAA04773@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

You may use Split with a test. This is your list, which you wish to have
grouped by the first element.

In[1]:=
theList = {{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}};

First, use a Sort so that all sublists with equal first element are adjacent
(in your example they already are):

In[2]:=
sortedList = Sort[theList]
Out[2]=
{{2, 3, 4, 5, 77}, {2, 4, 5, 66, 77},
  {3, 45, 67, 77, 12}, {3, 4, 7, 8, 90, 6},
  {3, 5, 6, 7, 8, 0}}

Then use Split on sortedList with the test requiring that two adjacent
sublists with the same first element remain in the same set.

In[3]:=
Split[sortedList, #1[[1]] == #2[[1]] & ]
Out[3]=
{{{2, 3, 4, 5, 77}, {2, 4, 5, 66, 77}},
  {{3, 45, 67, 77, 12}, {3, 4, 7, 8, 90, 6},
   {3, 5, 6, 7, 8, 0}}}

You would benefit from looking at Split in the Help browser. You'll find
there a couple of very illustrative and useful examples.

Tomas Garza
Mexico City
----- Original Message ----- 
From: <camartin at snet.net>
To: mathgroup at smc.vnet.net
Subject: [mg48151] [mg48130] Selecting by first element of each list


> 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
>
>
>

• References:
  • Selecting by first element of each list
    • From: "camartin@snet.net" <camartin@snet.net>