R: Count unique values by category

Question

I have data in R that looks like this:

 Cnty   Yr   Plt       Spp  DBH Ht Age
 1  185 1999 20001 Bitternut  8.0 54  47
 2  185 1999 20001 Bitternut  7.2 55  50
 3   31 1999 20001    Pignut  7.4 71  60
 4   31 1999 20001    Pignut 11.4 85 114
 5  189 1999 20001        WO 14.5 80  82
 6  189 1999 20001        WO 12.1 72  79

I would like to know the quantity of unique species (Spp) in each county (Cnty). "unique(dfname$Spp)" gives me a total count of unique species in the data frame, but I would like it by county.

Any help is appreciated! Sorry for the weird formatting, this is my first ever question on SO.

Thanks.

Answer 1

with(mydf, tapply(Spp, list(Cnty, Yr), 
     FUN = function(x) length(unique(x))))

unique query is not working with large data set i mean data more than 1000k row.