MATLAB: Rearranging a matrix in an efficient way

Question

Hello, I would like to know what is the best way to extract rows from a matrix but in a specific order. I’ll explain: My program is generating HUGE matrices that always have their number of rows divisible by 3. The number of columns is always 3. I need to extract rows from these matrices in the following way: Let’s say I have a matrix 9×3:

1.1 1.2 1.3

2.1 2.2 2.3

3.1 3.2 3.3

4.1 4.2 4.3

5.1 5.2 5.3

6.1 6.2 6.3

7.1 7.2 7.3

8.1 8.2 8.3

9.1 9.2 9.3

I would like to end up with:

1.1 1.2 1.3

4.1 4.2 4.3

7.1 7.2 7.3

2.1 2.2 2.3

5.1 5.2 5.3

8.1 8.2 8.3

3.1 3.2 3.3

6.1 6.2 6.3

9.1 9.2 9.3

So basically it is taking these rows:

1:N/3:end

2:N/3:end

3:N/3:end

etc…….

Doing it with a ‘for’ loop is trivial but very time consuming. Is there an automatic way to do this? Thank you.

Answer 1

one way

out = data(reshape(1:size(data,1),3,[])',:); %%%EDIT

second way

out = reshape(permute(reshape(data,3,[],3),[3 2 1]),3,[])';

ADD after Yair's answer

out = A(reshape(1:size(A,1),size(A,1)/3,[])',:);

ADD 2

[~,ii] = sort(rem(0:size(data,1)-1,size(data,1)/3)+1);
out = data(ii,:);