[R] Switching entries in vector in by groups of two

[David Afshartous]

All,

I have a long vector that contains an even number of entries. I'd like to
switch the 1st and 2nd entry, the 3rd and 4th, and so on, without writing a
loop.

This code works:

X = c(8, 10, 6, 3, 20, 1)
index = c(2,1,4,3,6,5)
X[index]

But for a long list is there a way to generate the index?  I can get the
parts to the index as:

index.odd = seq(1,length(X), by  = 2)
index.even = index.odd + 1

Is there a simple way to interweave them to produce the desired index?  Or
is there a better way?

Cheers,
David