Extract fixed length arrays from each row of a matrix and build new matrix made of them

I want to extract fixed length arrays from each row of a matrix and build a new matrix made of them. For example, I want to make matrix B from matrix A as follows.

 A = magic(4);
 B = [a(1,1:3); a(2,2:4); a(3,1:3); a(4,2:4)];

It's complicated so I expected matlab to move like this

 b = a(:,[1:3;2:4;1:3;2:4]);

As you know, it doesn't work. But I feel like there's a simple grammar to do this. So if you know that, I'd like you to share that with me. Thanks.

Answer 1

[Walter Roberson]

There is no simple grammar for this. It is usually easiest and clearest to do it with a loop.

It is also possible to do this by using carefully using sub2ind, or to do the calculations equivalent to sub2ind.

   idx = [1:3;2:4;1:3;2:4];
   B = A( bsxfun(@plus, (1:size(A,1)).', (idx-1) * size(A,1)) );

However, I do not recommend this code. It might be efficient but it is also obscure. You will probably find yourself having to reinvent the proper combination of operations every time.

[Yusuke Kadowaki]

Thank you for sharing it. Okay I'll do it with a loop.

Answer 2

[Andrei Bobrov]

for MATLAB >= R2016b

[m,n] = size(A);
B = A(m*((0:n-2) + rem((1:n)-1,2)') + (1:m)');

[Yusuke Kadowaki]

How could you come up with that? So impressive and beautiful. However what I meant is doing this with arbitrary chosen index like the Walter's answer. Sorry that I confused you. I really appreciate your answer. I learned a lot from it.

[Andrei Bobrov]

fixed :)