Re: Sum elements in a list with conditions

*To*: mathgroup at smc.vnet.net*Subject*: [mg69840] Re: [mg69828] Sum elements in a list with conditions*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>*Date*: Mon, 25 Sep 2006 03:52:58 -0400 (EDT)

On 24 Sep 2006, at 12:45, Guillermo Sanchez 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 > {{a1, b1}, {a2, b2}, {a2, b3}, {a2, b4}, {a3, b5}, {a3, b6}} //. {a___, {x_, y_}, b___, {x_, z_}, c___} :> {a, {x, y + z}, b, c} {{a1, b1}, {a2, b2 + b3 + b4}, {a3, b5 + b6}} Andrzej Kozlowski Tokyo, Japan