Jim-
That did the trick- thanks so much for taking the time to help me out.
Sincerely,
Ian Chidister
On Wed, Jul 29, 2009 at 11:57 AM, 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
>
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