RE: Sum elements in a list with conditions

*To*: mathgroup at smc.vnet.net*Subject*: [mg69835] RE: [mg69828] Sum elements in a list with conditions*From*: "David Park" <djmp at earthlink.net>*Date*: Mon, 25 Sep 2006 03:52:46 -0400 (EDT)

Guillermo, testlist = {{a1, b1}, {a2, b3}, {a2, b4}, {a3, b5}, {a3, b6}, {a2, b2}}; Print["The starting list"] testlist Print["Sort the list"] Sort[testlist] Print["Split the list on equality in the first element"] Split[%%, First[#1] === First[#2] &] Print["Sum the second elements in each group using the following function: ", \ {Part[#, 1, 1], Last@Total[#]} &] {Part[#, 1, 1], Last@Total[#]} & /@ %% David Park djmp at earthlink.net http://home.earthlink.net/~djmp/ From: Guillermo Sanchez [mailto:guillermo.sanchez at hotmail.com] To: mathgroup at smc.vnet.net 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