[R] how to sum multiple data entries for the same sampling event?

I'm having trouble with some catch per unit effort data (CPUE, fisheries data). Some of the samples were retained and some unretained, and they are entered as 2 separate entries for the same sampling event (Date and time). I want to calculate the total CPUE (so sum the retained and unretained

Re: [R] how to sum multiple data entries for the same sampling event?

Perhaps something like # Untested library(plyr) ddply(DATA, Date, function(d) sum(d$lmb.cpue)) For example, on some fake data DATA - data.frame(class = rep(letters[1:5], each = 2), type = rep(c(good, bad), 5), value = rnorm(10)) ddply(DATA, class, function(d) sum(d$value)) If you want to send