dplyr's tally() function

dply-tally.R

library(dplyr)

# Set up an example data frame
stations = c("A", "B", "C")
species = c("M", "N", "O", "P", "Q")
baseline = c(5, 50, 500)

n <- 25
mydf <- data.frame(station=sample(stations, n, replace=TRUE), species=sample(species, n, replace=TRUE))
mydf$baseline <- baseline[match(mydf$station, stations)]
mydf[order(mydf$station, mydf$species),]

# Count by stations+species
mydf %>% group_by(station, species) %>% tally()

# Count by station+species, weighted by 1/baseline
mydf %>% group_by(station, species) %>% tally(wt=1/baseline)