x <- letters[1:4]
x
# [1] "a" "b" "c" "d"
t(combn(x, 2))
# [,1] [,2]
# [1,] "a" "b"
# [2,] "a" "c"
# [3,] "a" "d"
# [4,] "b" "c"
# [5,] "b" "d"
# [6,] "c" "d"
How should I write the code if I also what inverse combinations with b-a, c-a...d-c. 12 combinations in total.
You can make use of expand.grid from base R to get all possible combinations of the vector (i.e. 16 combinations) and then use subset (or [.data.frame) so that the values in both columns are never equal in a row (resulting in the expected 12 combinations):
x <- letters[1:4]
subset(expand.grid(rep(list(x),2)), Var1 != Var2)
# Var1 Var2
#2 b a
#3 c a
#4 d a
#5 a b
#7 c b
#8 d b
#9 a c
#10 b c
#12 d c
#13 a d
#14 b d
#15 c d
An alternative with data.table's cross-join (CJ) function:
libraray(data.table)
CJ(x, x)[V1 != V2]
# V1 V2
# 1: a b
# 2: a c
# 3: a d
# 4: b a
# 5: b c
# 6: b d
# 7: c a
# 8: c b
# 9: c d
#10: d a
#11: d b
#12: d c