Pairwise combination

Hi there,

I am looking for a function that takes a vector as input and generates
all pair wise combination of the elements of the input vector.

For example, the input vector is c(1,2,3,4). The output vector is c(1
and 2, 1 and 3, 1 and 4, 2 and 3, 2 and 4, 3 and 4).

The
representation here is generic.

I can use a for loop to do it, but it gets slow when the number of
elements of the input vector is large.

Best,
Jia