Re: List Operations

*To*: mathgroup at smc.vnet.net*Subject*: [mg65713] Re: List Operations*From*: "Scout" <Scout at nodomain.com>*Date*: Sun, 16 Apr 2006 01:45:19 -0400 (EDT)*References*: <e1nn6s$lmm$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

"LectorZ" <lectorz at mail.ru> news:e1nn6s$lmm$1 at smc.vnet.net... > Hi guys, > > My question: > > mylist={{1,a, b},{1,a, b},{1,a, > b},{2,b,z},{2,b,z},{2,b,z},{2,b,z},....,{n,x,y},{n,x,y},{n,x,y},...{n,x,y}} > > The sublists are of different length. > > I need to calculate the product between the 2nd and 3rd element of > every sublist (e.g. a*b) and then add them up according to the 1st > element: sum of all products where the 1st element is 1, 2, ...n. > > The result should be a list like that: > {{1,a*b+a*b+a*b},{2,b*z+b*z+b*z+b*z}, ...,{n, x*y+x*y+x*y+...+x*y}} > > Thank you for your help. > > LZ > This might help you: In[1]:= Plus@@ # /. {n_, x_}->{n / Length[#], x} & /@ Split[mylist /. {n_, a_, b_}->{n, a*b}, First[#1]==First[#2]&] Cheers, ~Scout~