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

*To*: mathgroup at smc.vnet.net*Subject*: [mg37252] Re: grouping and averaging {x,y} pairs of data*From*: "Carl K. Woll" <carlw at u.washington.edu>*Date*: Fri, 18 Oct 2002 05:17:54 -0400 (EDT)*Organization*: University of Washington*References*: <aokc56$ah2$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hi David, One typical idea for your application is to sort the data and then use Split. For example, ({#1[[1,1]], Plus @@ #1[[All,2]]/Length[#1]} & ) /@ Split[Sort[data], #1[[1]] == #2[[1]] & ]; The only difference between the above function and your desired result is that when there is no data for a particular integer, the average for that integer does not appear in the answer. Carl Woll Physics Dept U of Washington "David E. Burmaster" <deb at alceon.com> wrote in message news:aokc56$ah2$1 at smc.vnet.net... > 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 > +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > > >