> rowsum(value, paste(factor1, factor2, factor3)) That is dangerous in general, and always inefficient. Imagine factor1 is c("a", "a b") and factor2 is ("b c", "c"). Use interaction with drop = T.
Hadley -- Assistant Professor / Dobelman Family Junior Chair Department of Statistics / Rice University http://had.co.nz/ ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.