Sorting and indexing multidimensional arrays

I would like to clarify if indexing is supported for multidimensional arrays.

For example, consider the code below.

According to the help page, the line B2 = A(I) a recover the sorted array.

However, from MATLAB Online R2020b the code snippet produces array full of ones as the result for B2.

A(:,:,1) = 1*ones(3);
A(:,:,2) = 2*ones(3);
A(:,:,3) = 3*ones(3);
A(:,:,4) = 4*ones(3);

[B,I] = sort(A,3,'descend');
B2 = A(I);

[Stephen23]

"According to the help page, the line B2 = A(I) a recover the sorted array."

What the sort documentation actually states is "if A is a vector, then B = A(I)" (emphasis added).

Are your data in a vector? (hint: no).

[Shing Bo Peh]

Thanks for the explanation, indeed A is not a vector. Your comment and Walter's answer have been helpful in extending the example to 3D arrays.

Answer 1

[Walter Roberson]

See https://www.mathworks.com/matlabcentral/answers/645463-help-required-for-sorting#answer_542268 -- I just happened to write up an explanation for someone of the same basic problem.

You would need to expand what I wrote there into the third dimension. You would want to change the repmat() that I show there into something more like

[ROWIDX, COLIDX] = ndgrid(1:size(A,1), 1:size(A,2));
B2 = sub2ind(size(A), ROWIDX, COLIDX, sortidx);

This is untested; you might need to use meshgrid() instead of ndgrid()

[Shing Bo Peh]

Thank you for the example.

The size of the inputs was slightly off in your comment but I found the below code to work after your explanation:

[ROWIDX,COLIDX,~]=ndgrid(1:size(A,1),1:size(A,2),1:size(A,3));

lix = sub2ind(size(A),ROWIDX,COLIDX,sortidx);

B2 = A(lix);

Many thanks for the help!

Edit: Change from meshgrid to ndgrid following Bruno's comment.

[Walter Roberson]

Thanks for the test case, Bruno!

[Shing Bo Peh]

Confirmed, thank you Bruno.

Answer 2

[Bruno Luong]

[B,I] = sort(A,3,'descend');

I is ndarray same size as A and B, values is 1:size(A,3) such that for any i,j

A(i,j,I(i,j,:)) 

is equal to

B(i,j,:) 

(Both are then sorted in descending order)