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

Question

Imagine I have a vector x and i'd like to create a matrix with all the possible n choose 2 combinations of the elements of x.

More in detail, let us say x is,

x = c(1,2,3,4)

Then, all the possible (4 choose 2) = 6,

X = as.matrix(data.frame(col1 = c(1,1,1,2,2,3), col2 = c(2,3,4,3,4,4)))

Is there a function in R to do that?

More solutions: https://stackoverflow.com/questions/26828301/faster-version-of-combn — akraf, Mar 07 '18 at 14:15

score 26 · Accepted Answer · answered Nov 15 '13 at 15:10

26

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

answered Nov 15 '13 at 15:10

Jilber Urbina

1 Answers1