[R] - counting factor occurrences within a group: tapply()

[jim holtman]

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.chidister at 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]]
>
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>



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

What is the problem that you are trying to solve?