[R] Differencing data by groups

[jim holtman]

What problem are you actually having with 'diff'? Now if you are using
'diff', you will get a vector that is shorter by one than the
original.  Now do you want to do do something like:

Xbar = Sum{c(Depth[1], diff(Depth))*temp}/Sum(c(Depth[1], diff(Depth))


On 10/12/07, Thomas Miller <miller at cbl.umces.edu> wrote:
> Colleagues,
>
> I am analyzing data collected during oceanographic cruises. We have
> conducted many cruises over the last decade.  On each cruise we visit ~50
> stations. At each station (termed EventNum)we lower an instrument that
> measures depth, temperature, salinity and oxygen every few seconds as it is
> lowered through the water.  Data from all EventNums on all cruises are
> stacked, generating a data file that might look like
>
>  CruiseID Year Season EventNum Region Depth Salinity    Temp  Oxygen
> 1    CF0101 2001 Spring      849  Lower 0.056  21.1753 15.5626 8.12344
> 2    CF0101 2001 Spring      849  Lower 0.103  21.3264 15.5511 8.11873
> 3    CF0101 2001 Spring      849  Lower 0.131  21.4780 15.5492 8.10568
> 4    CF0101 2001 Spring      849  Lower 0.169  21.5296 15.5492 8.10633
> ....
>
> 15000   CF0103 2001   Fall      929    Mid 14.449   17.692 22.901  7.743
> 15001   CF0103 2001   Fall      929    Mid 14.478   17.691 22.900  7.743
> 15002   CF0103 2001   Fall      929    Mid 14.527   17.692 22.899  7.743
> 15003   CF0103 2001   Fall      929    Mid 14.595   17.692 22.900  7.743
> 15004   CF0103 2001   Fall      929    Mid 14.654   17.694 22.902  7.742
> 15005   CF0103 2001   Fall      929    Mid 14.722   17.695 22.903  7.742
> 15006   CF0103 2001   Fall      929    Mid 14.790   17.695 22.903  7.742
> 15007   CF0103 2001   Fall      929    Mid 14.839   17.694 22.903  7.742
> 15008   CF0103 2001   Fall      929    Mid 14.887   17.695 22.902  7.742
> 15009   CF0103 2001   Fall      929    Mid 14.946   17.694 22.902  7.742
> 15010   CF0103 2001   Fall      929    Mid 14.985   17.693 22.902  7.743
> ......
>
> In total there are ~350k rows of data
>
> I can calculate simple EventNum averages using
>
> meanobs<-aggregate(ctd[,7:9],by=list(EventNum=EventNum),mean)
>
> But, I want to calculate a depth-weighted mean for salinity, temp and oxygen
> for each EventNum.  The depth-weighted mean is calculated as
>
> Xbar = Sum{depth_interval*temp}/Sum(depth_intervals)
>
> Where the depth interval is the differenced depths  (i.e., 0.103-0.056 for
> the second row of data).
>
> I cannot seem to get diff to work the way I want it to - advice appreciated
>
> Tom
>
> Thomas J. Miller, Ph.D.
> Professor
> Chesapeake Biological Laboratory
> University of Maryland Center for Environmental Science
> Solomons, MD 20688
> Tel: (410) 326-7276
> Fax: (410) 326-7318
> WWW: hjort.cbl.umces.edu
>
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem you are trying to solve?