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
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