Dear All, Let ab is the combination of matrices a and b. a <- matrix(c(1,2,3,4), nrow = 2, ncol = 2) b <- matrix(c(4,3,1,2), nrow = 2, ncol = 2) ab <- rbind(a, b) From matrix ab, could someone give some advice on the way to sort all elements in column 2, by the sequence of the elements in column 1 in ascending order? Thank you. Fir