Yeah, you are right i want to post an short example what i want to do .. and in the meantime i solved the problem ... but here is: i have something like this dataframe: c1<-c(1,2,3,2,2,3,1,2,2,2) c2<-c(5,6,7,7,5,7,5,7,6,6) c3<-rnorm(10) x<-cbind(c1,c2,c3) > x c1 c2 c3 [1,] 1 5 0.08279036 [2,] 2 6 0.59135988 [3,] 3 7 1.45520468 [4,] 2 7 -1.70094640 [5,] 2 5 0.13065228 [6,] 3 7 -1.12080980 [7,] 1 5 0.42779354 [8,] 2 7 -1.53111972 [9,] 2 6 0.29299987 [10,] 2 6 -0.01602095
#whith aggregate i receive this: >aggregate(x[,3],list(x[,1],x[,2]),mean) Group.1 Group.2 x 1 1 5 0.2552920 2 2 5 0.1306523 3 2 6 0.2894463 4 2 7 -1.6160331 5 3 7 0.1671974 and the problem was that i was grouping by 2 columns, so i couldn't copy the result to x. the solution was i made another column with paste(x[,1],x[,2],sep="_") and then i used the solution from this link: http://tolstoy.newcastle.edu.au/R/help/06/07/30184.html so i solved my problem Ivan, many thanks for your support and quik responses! :) -- View this message in context: http://r.789695.n4.nabble.com/group-by-in-data-frame-tp3324240p3324608.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.