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

*To*: mathgroup at smc.vnet.net*Subject*: [mg37222] RE: grouping and averaging {x,y} pairs of data*From*: "DrBob" <drbob at bigfoot.com>*Date*: Thu, 17 Oct 2002 00:08:51 -0400 (EDT)*Reply-to*: <drbob at bigfoot.com>*Sender*: owner-wri-mathgroup at wolfram.com

You'll get more efficient methods from others, but I think the following is instructive: data = {{3, 1}, {4, 3}, {3, 2}, {1, 10}, {4, 2}, {1, 6}, {5, 2}, {2, 5}, {7, 1}}; ClearAll[total, count] total[x_] := 0 count[x_] := 0 {total[#[[1]]] += #[[2]], count[#[[1]]]++} & /@ data; ?total ?count {#, total[#]/count[#]} & /@ Union[data[[All, 1]]] DrBob -----Original Message----- From: David E. Burmaster [mailto:deb at alceon.com] To: mathgroup at smc.vnet.net Subject: [mg37222] grouping and averaging {x,y} pairs of data Dear Fellows in MathGroup, 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?/ many thanks dave +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ David E. Burmaster, Ph.D. Alceon Corporation POBox 382069 (new Box number effective 1 Sep 2001) Harvard Square Station Cambridge, MA 02238-2069 (new ZIP code effective 1 Sep 2001) Voice 617-864-4300 Web http://www.Alceon.com Email deb at Alceon.com +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++