Efficient reordering of vector

I would like to reorder vector as following:

a = [1 2 3 4 5 6 7 8 9 10 11 12]

to

r = [1 5 9 2 6 10 3 7 11 4 8 12]

what would be efficient way of doing so?

Answer 1

[Walter Roberson]

reshape( reshape(a, 4, 3).', 1, [])

[Guillaume]

or more generically

reshape(reshape(a, [], stride).', 1, [])

[Walter Roberson]

In that line, "stride" would have to mean the number of elements in each sub-group. I tend to instead think of stride in terms of the difference between adjacent indices, and for that you would use

reshape(reshape(a,stride,[]).' 1, [])

[Guillaume]

That is also what I meant, but... brain did not engage properly.