slicing multi-dimensionsal arrays

I am just trying to learn about manipulating multidimensional arrays in MATLAB. I feel like I am really not getting something...

Given these statements:

A = [1 2; 3 4];
B = [10 20; 30 40];
C = [42 3.14; 97 21];

I can build a 2x2x3 block like this:

ary3d(:,:,1) = A;
ary3d(:,:,2) = B;
ary3d(:,:,3) = C;

ary3d
ary3d(:,:,1) =
     1     2
     3     4
ary3d(:,:,2) =
    10    20
    30    40
ary3d(:,:,3) =
           42         3.14
           97           21

I want to collect each of the second rows into a 3x2 array. This can be accomplished like this:

squeeze(ary3d(2,:,:))'
ans =
     3     4
    30    40
    97    21

but it seems odd to me that the transpose operator would need to be involved.

Stranger yet is the fact that all three of these commands give the same answer:

cat(1, ary3d(2,:,:))
cat(2, ary3d(2,:,:))
cat(3, ary3d(2,:,:))

I would have expected one of these commands to give a 2x3x1 array, one to give a 6x1x1, and one to give a 1x2x3 array. In fact, all three give this answer:

cat(3, ary3d(2,:,:))
ans(:,:,1) =
     3     4
ans(:,:,2) =
    30    40
ans(:,:,3) =
    97    21

Can anybody shed any light here?

Is there some way (some relatively obvious way I ought to be seeing) to do this command without using the transpose operator?

squeeze(ary3d(2,:,:))'
ans =
     3     4
    30    40
    97    21

Thanks, Erik

[Oleg Komarov]

Could you please edit the EDU>> out to make the code copy-pastable?

Answer 1

[Oleg Komarov]

Your input:

A = cat(3, [1 2; 3 4], [10 20; 30 40], [42 3.14; 97 21]);

1. The squeeze removes all singleton dimensions:

squeeze(A(2,:,:))
ans =
     3    30    97
     4    40    21

because A(2,:,:) selects one specific row, you have a (1 by) 2 by 3 slice.

2. Not strange at all since you're concatenating one slice with nothing else!

B = A(2,:,:)

B(:,:,1) =
     3     4
B(:,:,2) =
    30    40
B(:,:,3) =
    97    21

B is one variable, not 3. Don't be fooled by the way it prints on the screen.

3. Not using the transpose:

permute(A(2,:,:), [3,2,1])

first comes the 3rd dim, then the 2nd and last the 1st.

It takes a little time to get used "visually". I suggest to draw things on paper.

[Erik]

Yes - I thought I was concatenating three different elements. I see what you mean now.

[Erik]

Thank you for your answer.

Answer 2

[Honglei Chen]

In your code, you only passed on data input to cat. You can try the following

 cat(1,ary3d(2,:,1),ary3d(2,:,2),ary3d(2,:,3))

[Erik]

Thank you - I think I see what is going on now.