Summing data frame columns on identical data
library(plyr) # Function to sum y by A-B combinations for a generic data frame dsum <- function(d) ddply(d, .(A, B), summarise, sumY = sum(y))
See count in plyr 1.4 for a much much faster way of doing this. Hadley
Assistant Professor / Dobelman Family Junior Chair Department of Statistics / Rice University http://had.co.nz/