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 x - that is, a new matrix with 55 rows and 4 columns.

E.g.,

x <- matrix (nrow=10, ncol=2, 1:20)

M <- data.frame(matrix(ncol=4, nrow=55))
k <- 1
for (i in 1:nrow(x))
for (j in i:nrow(x))
{
    M[k,] <- unlist(cbind (x[i,], x[j,]))
    k <- k + 1
}

So, x is:

      [,1] [,2]
 [1,]    1   11
 [2,]    2   12
 [3,]    3   13
 [4,]    4   14
 [5,]    5   15
 [6,]    6   16
 [7,]    7   17
 [8,]    8   18
 [9,]    9   19
[10,]   10   20

And then M has 4 columns, the first two are one row from x and the next 2 are another row from x:

> head(M,10)
   X1 X2 X3 X4
1   1 11  1 11
2   1 11  2 12
3   1 11  3 13
4   1 11  4 14
5   1 11  5 15
6   1 11  6 16
7   1 11  7 17
8   1 11  8 18
9   1 11  9 19
10  1 11 10 20

Is there either a faster or simpler (or both) way of doing this in R?

Answer 1

The expand.grid() function useful for this:

R> GG <- expand.grid(1:10,1:10)
R> GG <- GG[GG[,1]>=GG[,2],]     # trim it to your 55 pairs
R> dim(GG)
[1] 55  2
R> head(GG)
  Var1 Var2
1    1    1
2    2    1
3    3    1
4    4    1
5    5    1
6    6    1
R> 

Now you have the 'n*(n+1)/2' subsets and you can simple index your original matrix.