Generate all possible n choose 2 pairs from a vector in R, efficient and fast [duplicate]

Answer 1

As pointed out by @Arun, you can use combn

> t(combn(x, 2))
     [,1] [,2]
[1,]    1    2
[2,]    1    3
[3,]    1    4
[4,]    2    3
[5,]    2    4
[6,]    3    4