Hi All-
Thanks for your quick responses. I was looking for unique instances, so
Jim's and Daniel's suggestions got the job done. Using "length" alone
didn't discriminate between multiple occurrences of the same species and
multiple species.
I do have one followup question- my full data set (not the example data) has
a number of NAs in the SppID column, and [r] is currently counting the NAs
as species occurrences. Using Jim's code, I tried:
>tapply(SppID, PlotID, function(Trees, na.rm=T) length(unique(Trees,
na.rm=T)))
and alternately:
>tapply(SppID, PlotID, function(Trees) length(unique(Trees)), na.rm=T)
which doesn't seem to convince [r] to ignore the NAs. What am I doing
wrong?
Thanks,
Ian
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