Re: Sum elements in a list with conditions

*To*: mathgroup at smc.vnet.net*Subject*: [mg128439] Re: Sum elements in a list with conditions*From*: Fred Simons <f.h.simons at tue.nl>*Date*: Fri, 19 Oct 2012 02:44:04 -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*References*: <20121018063647.0023568A4@smc.vnet.net>

After having used GatherBy, for each of the resulting list elements you want to have the first element of the first column and the sum of the second column: In[10]:= list1 = {{1,a},{2,b},{1,c},{2,d},{2,e},{3,f}}; In[11]:= {#[[1,1]], Total[#[[All,2]]]}& /@ GatherBy[list1, First] Out[11]= {{1,a+c},{2,b+d+e},{3,f}} Fred Simons Eindhoven University of Technology Op 18-10-2012 8:36, Guillermo Sanchez schreef: > 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}] > > >

**References**:**Sum elements in a list with conditions***From:*Guillermo Sanchez <guillermo.sanchez@hotmail.com>