Quickly generate the cartesian product of a matrix

Question

Let\'s say I have a matrix x which contains 10 rows and 2 columns. I want to generate a new matrix M that contains each unique pair of rows from

Answer 1

Inspired from the other answers, here is a function implementing cartesian product of two matrices, in the case of two matrices, the full cartesian product, for only one argument, omitting one of each pair:

cartesian_prod <- function(M1, M2) {
if(missing(M2)) {  M2 <- M1
     ind  <- expand.grid(1:NROW(M1), 1:NROW(M2))
     ind <- ind[ind[,1] >= ind[,2],] } else {
                                          ind  <- expand.grid(1:NROW(M1), 1:NROW(M2))}
rbind(cbind(M1[ind[,1],], M2[ind[,2],]))

}