On Sun, 8 Oct 2006, Thomas P. Colson wrote: > I'm trying to calculate a cumulative area distribution (graph) of drainage > areas. This is defined as P(A > A*). Simple in principle. I can do this in > excel, with "COUNTIF", which will count the number of cells in the row > "area" that have area A, then determine, for each cell in the row "area, how > many cells exceede that area, then dividing that number by the total number > of cells, which gives me the probability that drainage area A exceeds > drainage area A*.
Is this ecdf() of the vector or its suitable subset? If so, it runs very fast even for large data sets. For plotting, bear in mind that you are generating a lot of output, though: > t0 <- runif(100000) > system.time(t1 <- ecdf(t0)) [1] 0.222 0.022 0.248 0.000 0.000 > system.time(plot(t1, pch=".")) [1] 1.089 0.079 1.186 0.000 0.000 isn't at all bad! > > E.g, drainage area of 6 sq meters (One DEM grid cell) has a high probability > of exceedance(.99), while a drainage area of 100,000 square meters has a low > probability of exceedance (.001). > > I wish to plot this relationship, and we all know that excel is not the tool > of choice when working with hundreds of thousands of records. I'd like to > port the CAD into a few R functions that I've already developed for other > tests as well. > > So my challenge, in R, is to > (1)count the number of rows in column "Area" that have AREA(*), > > (2) determine, by row, how many rows have an area greater than the area > given in that one row > > (3) divide step 2 by number of rows (how can I do a row count and port that > to a variable, as I have to do this on 10 datasets?) > > Thanks for any advice you can offer to this endevour > > ______________________________________________ > R-help@stat.math.ethz.ch mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- Roger Bivand Economic Geography Section, Department of Economics, Norwegian School of Economics and Business Administration, Helleveien 30, N-5045 Bergen, Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43 e-mail: [EMAIL PROTECTED] ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.