Re: Sum elements in a list with conditions

• To: mathgroup at smc.vnet.net
• Subject: [mg128440] Re: Sum elements in a list with conditions
• From: Ray Koopman <koopman at sfu.ca>
• Date: Fri, 19 Oct 2012 02:44:24 -0400 (EDT)
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• Delivered-to: l-mathgroup@wolfram.com
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• References: <k5o8g2\$ic1\$1@smc.vnet.net>

```On Oct 17, 11:46 pm, Guillermo Sanchez <guillermo.sanc... at hotmail.com>
wrote:
> I have a list of sublist of pairs. I wish sum the second elements of
> the sublists when the first elements are equals
> Example:
> Int[]:={{1, a}, {2, b}, {1, c}, {2, d}, {2, e}, {3, f}}
> I hope
> Out[]:= {{1, a + c}, {2, b + c + e}, {3, f}}
>
> I can do it as  is it is shown below but could any body find and
> easier solution.
>
> list1 = {{1, a}, {2, b}, {1, c}, {2, d}, {2, e}, {3, f}};
> list2 = GatherBy[list1, First];
> {l1, l2} = Transpose[Plus @@@ GatherBy[list, First]];
> Transpose[{l1/Length /@ list2 , l2}]

{#[[1,1]],Tr@#[[All,2]]}& /@ list2

```

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