This is probably what you want; you need to count the number of unique instances:
> tapply(Trees$SppID, Trees$PlotID, function(x) length(unique(x))) BU3F10 BU3F11 BU3F12 1 2 4 > On Wed, Jul 29, 2009 at 12:57 PM, Ian Chidister<ian.chidis...@gmail.com> wrote: > Dear List, > > I'm an [R] novice starting analysis of an ecological dataset containing the > basal areas of different tree species in a number of research plots. > Example data follow: > >> Trees<-data.frame(SppID=as.factor(c(rep('QUEELL',2), rep('QUEALB',3), > 'CORAME', 'ACENEG', 'TILAME')), BA=c(907.9, 1104.4, 113.0, 143.1, 452.3, > 638.7, 791.7, 804.3), PlotID=as.factor(c('BU3F10', rep('BU3F11',2), > rep('BU3F12',5)))) >> Trees > SppID BA PlotID > 1 QUEELL 907.9 BU3F10 > 2 QUEELL 1104.4 BU3F11 > 3 QUEALB 113.0 BU3F11 > 4 QUEALB 143.1 BU3F12 > 5 QUEALB 452.3 BU3F12 > 6 CORAME 638.7 BU3F12 > 7 ACENEG 791.7 BU3F12 > 8 TILAME 804.3 BU3F12 > > Fields are (in order): Tree Species Code, Basal Area, and Plot Code. > > I've been successful in computing summary statistics by species or plot > groups using tapply(): > >> tapply(BA, PlotID, sum) > BU3F10 BU3F11 BU3F12 > 907.9 1217.4 2830.1 > > *My Question* I'd like to perform a similar function that tells me how many > species are in each plot, I thought this would be possible using something > like: > >> tapply(SppID, PlotID, nlevels) > BU3F10 BU3F11 BU3F12 > 5 5 5 > > however, this outputs the total number of levels for the factor SppID rather > than the number of species in each plot category which would look like: > > BU3F10 BU3F11 BU3F12 > 1 2 4 > > I understand, from reading the archive, that this occurs because R does not > subset factor levels, but I'm wondering if there's a simple way around this. > > > Thanks for your help, > > Ian Chidister > > Environment and Resources > The Nelson Institute for Environmental Studies > University of Wisconsin-Madison, USA > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org 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. > -- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem that you are trying to solve? ______________________________________________ R-help@r-project.org 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.