Hi, i am trying to do a small job but it's bugging and i don't know what's happening. let's say i have four matrix, M01_a, M02_a, M01_b, M02_b that looks like this col1 col2 1 20 2 25 3 26 4 30 5 35 6 64 7 75 8 80 9 85 and a big matrix m:(21044*18) what i want to do is m1<- rbind(m[M01_a[,2],], m[M01_b[,2],]) m2 <- rbind(m[M02_a[,2],], m[M02_b[,2],]) the best will be to integrate this in a loop, so i just did this for (i in 1:2){ mb<- rbind(m[paste("M0",i,"_a[,2]", sep=""),], m[paste("M0",i,"_b[,2]", sep=""),]) } but this didn't work :-( i've been tring the whole afternoon , plz some help. thx, josephine