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

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.