slicing multi-dimensionsal arrays

54 次查看（过去 30 天）
Erik
Erik 2012-4-12
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
  1 个评论
Oleg Komarov
Oleg Komarov 2012-4-12
Could you please edit the EDU>> out to make the code copy-pastable?

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Oleg Komarov
Oleg Komarov 2012-4-12
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.
  2 个评论
Erik
Erik 2012-4-12
Yes - I thought I was concatenating three different elements. I see what you mean now.
Erik
Erik 2012-4-12
Thank you for your answer.

请先登录，再进行评论。

更多回答（1 个）

Honglei Chen
Honglei Chen 2012-4-12
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))

类别

在 Help Center 和 File Exchange 中查找有关 Creating and Concatenating Matrices 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by