Re: grouping and averaging {x,y} pairs of data

*To*: mathgroup at smc.vnet.net*Subject*: [mg37229] Re: grouping and averaging {x,y} pairs of data*From*: BobHanlon at aol.com*Date*: Thu, 17 Oct 2002 00:09:17 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

In a message dated 10/16/2002 4:30:28 PM, deb at alceon.com writes: >I have a list of 17,000+ {x,y} pairs of data > > each x value is a positive integer from 1 to 100+ > > each y value is a positive real number > >As a *short* example, let's consider: > > data = {{3,1},{4,3},{3,2},{1,10},{4,2},{1,6},{5,2},{2,5},{7,1}} > >I want to group the data by the x value and report the arithmetic average >of the y values in each group. > >For the example, i want to report: > > output = {{1,8},{2,5},{3,1.5},{4,2.5},{5,2},{6,0},{7,1}} > >In this example, x=6 does not occur so i report the average y[6] = 0. > >Can anyone suggest a way to do this efficiently?/ > The basic approach is (Plus @@ #)/Length[#] & /@ Split[Sort[data], #1[[1]] == #2[[1]] &] {{1, 8}, {2, 5}, {3, 3/2}, {4, 5/2}, {5, 2}, {7, 1}} To fill in the gaps: dataAvg[data_] := Module[{val = First /@ data, xData}, xData = Join[data, {#, 0} & /@ Complement[Range[Max[val]], val]]; (Plus @@ #)/Length[#] & /@ Split[Sort[xData], #1[[1]] == #2[[1]] &]]; dataAvg[data] {{1, 8}, {2, 5}, {3, 3/2}, {4, 5/2}, {5, 2}, {6, 0}, {7, 1}} Bob Hanlon