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