MATLAB: Arrange indices

Question

hi, if i have this vector

r=[2 4 18 5 6 15 3 4 16]; i need arrange index 3,6,9 with ascending, and each both previous index are arranged according for that as:

r=[5 6 15 3 4 16 2 4 18]; i.e just 3,6,…..are arranged with ascending and the index befor it are arranged according to third , sixth,….. i would to say ,this is just example i have too long vector

Answer 1

[ii,ii] = sort(r(3:3:end));
out = reshape(r(bsxfun(@plus,ii*3,(-2:0)')),1,[]);

OR:

out = reshape(sortrows(reshape(r,3,[])',3)',1,[]);