Re: Sum elements in a list with conditions

• To: mathgroup at smc.vnet.net
• Subject: [mg128427] Re: Sum elements in a list with conditions
• From: Daniel <dosadchy at its.jnj.com>
• Date: Fri, 19 Oct 2012 02:40:03 -0400 (EDT)
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• Delivered-to: l-mathgroup@wolfram.com
• Delivered-to: mathgroup-newout@smc.vnet.net
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```> 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}]
>

In[1]:= list1 = {{1, a}, {2, b}, {1, c}, {2, d}, {2, e}, {3, f}};

In[4]:= list2 = GatherBy[list1, First];

In[3]:= {#[[1, 1]], #[[All, 2]] // Total} & /@ list2

Out[3]= {{1, a + c}, {2, b + d + e}, {3, f}}

```

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