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

[Marc Schwartz]

on 06/27/2008 09:17 AM Marc Schwartz wrote:
> 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

This can be shortened somewhat by flipping rows rather than columns:

 > as.vector(matrix(Vec, nrow = 2)[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