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

From: Marc Schwartz <marc_schwartz_at_comcast.net>
Date: Fri, 27 Jun 2008 09:17:09 -0500

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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