Re: Summing list subsets

• To: mathgroup at smc.vnet.net
• Subject: [mg30742] Re: Summing list subsets
• From: bghiggins at ucdavis.edu (Brian Higgins)
• Date: Sun, 9 Sep 2001 03:26:35 -0400 (EDT)
• References: <9ncfgn\$prb\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Mark,

Try this,

x = {a, a, b, b, b, b, c, d, d, d, d, d} ;
y = {1, 1, 2, 2, 2, 2, 3, 4, 4, 4, 4, 4};

In[3]:=Transpose[{Union[x], Map[Apply[Plus, #] &, Split[y]]}]

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

Brian

"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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