[R] Switching entries in vector in by groups of two
Marc Schwartz
marc_schwartz at comcast.net
Fri Jun 27 16:17:09 CEST 2008
on 06/27/2008 09:11 AM David Afshartous wrote:
> 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
How about this:
> as.vector(t(matrix(X, ncol = 2, byrow = TRUE)[, 2:1]))
[1] 10 8 3 6 1 20
Trying a longer vector as well:
> Vec <- sample(20)
> Vec
[1] 13 20 14 10 12 7 3 19 11 15 1 4 9 18 6 8 5 16 17 2
> as.vector(t(matrix(Vec, ncol = 2, byrow = TRUE)[, 2:1]))
[1] 20 13 10 14 7 12 19 3 15 11 4 1 18 9 8 6 16 5 2 17
HTH,
Marc Schwartz
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