Re: Summing list subsets

• To: mathgroup at smc.vnet.net
• Subject: [mg30754] Re: Summing list subsets
• From: "Orestis Vantzos" <atelesforos at hotmail.com>
• Date: Sun, 9 Sep 2001 03:26:56 -0400 (EDT)
• Organization: National Technical University of Athens, Greece
• References: <9ncfgn\$prb\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```In[13]:=
zzz[x_, y_, func_] := {#, func @@ Extract[y, Position[x, #]]} & /@ Union[x]

In[15]:=
zzz[{a, a, b, b, b, b, c, d, d, d, d, d}, {1, 1, 2, 2, 2, 2, 3, 4, 4, 4, 4,
4}, Plus]

Out[15]=
{{a, 2}, {b, 8}, {c, 3}, {d, 20}}

The elements don't have to be consecutive.
Orestis

"Mark Coleman" <mcoleman at bondspace.com> wrote in message
news:9ncfgn\$prb\$1 at smc.vnet.net...
> Greetings:
>
> Consider two lists:
>
> x = {a,a,b,b,b,b,c,d,d,d,d,d,} and  y = {1,1,2,2,2,2,3,4,4,4,4,4}
>
> I would like to have a function that returns the sum (or any other
function)
> of each unique element of x, given the corresponding value in y. That is,
> for a 'Sum', the result would be
>
> z={{a,2},{b,8},{c,3},{d,20}}
>
> This is similar in spirit to a common database aggregation problem.
>
> Any ideas?
>
> Thanks.
>
> -Mark
>
>

```

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