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Sum elements in a list with conditions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg128422] Sum elements in a list with conditions
  • From: Guillermo Sanchez <guillermo.sanchez at hotmail.com>
  • Date: Thu, 18 Oct 2012 02:36:46 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • Delivered-to: l-mathgroup@wolfram.com
  • Delivered-to: mathgroup-newout@smc.vnet.net
  • Delivered-to: mathgroup-newsend@smc.vnet.net

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}]



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