Re: Sum elements in a list with conditions

*To*: mathgroup at smc.vnet.net*Subject*: [mg69846] Re: [mg69828] Sum elements in a list with conditions*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Mon, 25 Sep 2006 03:53:16 -0400 (EDT)*Reply-to*: hanlonr at cox.net

data={{a1, b1},{a2,b2},{a2,b3}, {a2,b4},{a3, b5}, {a3, b6}}; The first two methods assume that your data is sorted/clustered by the first element (if not, sort first): {#[[1,1]],Plus@@#[[All,2]]}&/@ Split[data,#1[[1]]==#2[[1]]&] {{a1, b1}, {a2, b2 + b3 + b4}, {a3, b5 + b6}} data//.{s___,{x_,y_},{x_,z_},r___}:> {s,{x,y+z},r} {{a1, b1}, {a2, b2 + b3 + b4}, {a3, b5 + b6}} Module[{v=Union[data[[All,1]]]}, Table[{v[[n]], Plus@@Cases[data,x_?(#[[1]]==v[[n]]&):>x[[2]]]}, {n,Length[v]}]] {{a1, b1}, {a2, b2 + b3 + b4}, {a3, b5 + b6}} %==%%==%%% True Bob Hanlon ---- Guillermo Sanchez <guillermo.sanchez at hotmail.com> wrote: > Dear group > I have a pair of elements list. The second elements of the list should > be summed when the first element of the pairs are equals. Example > > Given > > {{a1, b1},{a2,b2},{a2,b3},{a2,b4},{a3, b5}, {a3, b6}} > > the output should be > > {{a1, b1},{a2, b2+b3+b4},{a3, b5+b6}} > > Thanks > > Guillermo >