Thanks Peter. We did manage to solve the problem using the approach bellow.
Hab <- cbind(seq(1,21),habitat) hab <- matrix(NA,nrow=3,ncol=7) for (i in 1:3) {hab[i,] <- Hab[habitat==i,1]} The problem was that the hab matrix was not a multiple of habitat. -- View this message in context: http://r.789695.n4.nabble.com/replacement-has-length-zero-tp4635700p4636156.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ 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.