Re: Selecting by first element of each list

• To: mathgroup at smc.vnet.net
• Subject: [mg48147] Re: Selecting by first element of each list
• From: astanoff at yahoo.fr (Valeri Astanoff)
• Date: Fri, 14 May 2004 20:59:02 -0400 (EDT)
• References: <c81i3m\$509\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```"camartin at snet.net" <camartin at snet.net> wrote in message news:<c81i3m\$509\$1 at smc.vnet.net>...
> 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
Here is my oneliner solution :
In[1]:=
myList={{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}};
In[2]:=
myList // Function[n,Select[#,First[#] == n&]] /@ Union[#[[All,1]]]&
Out[2]=
{{{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}}}
---
hth
va

```

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