Best way to sort a 3-d matrix by one column?

Hi, I'd like to sort a 3 d matrix by the values in one column. I have a 204 x 33 x 9 matrix, and I'd like to sort the rows by the 33rd column of the second dimension. Thanks, Emily

[Guillaume]

Should each page be sorted separately or should it just be considered flattened into 2d? A short example of input/desired output would be useful.

[Emily Hokett]

Each page would sorted separately by the 33rd column so that it's still a 3-d matrix, just the rows would be sorted by the values in the 33rd column.

Answer 1

[gonzalo Mier]

I am not sure if this is what you want:

matrix(:,33,2) = sort(matrix(:,33,2));

[Emily Hokett]

Thanks for the response. I'm new to Matlab, so forgive me if I explain this awkwardly. I have a 3D matrix with 204 rows, 33 columns, and 9 pages. I want to sort the matrix by the 33rd column. Let me know if this makes sense. Thanks

[gonzalo Mier]

Can you put an example of what you want?

[Emily Hokett]

Okay, I just want to now if it's possible to somehow use sortrows for a 3-d matrix. If I have 204 rows, 33 columns, and 9 pages, could I sort the 204 rows by the values in the 33rd column and do this for all 9 pages?

Answer 2

[Guillaume]

You can't use sortrows on a 3D matrix. You have two options:

• Use a loop and use sortrows on the pages:

for page = 1:size(yourmatrix, 3)
   yourmatrix(:, :, page) = sortrows(yourmatrix(:, :, page), 33);
end

• Use sort and some sub2ind magic to sort everything at once. I'm not sure it'll be faster than the loop:

[~, roworder] = sort(yourmatrix(:, 33, :));
sortedmatrix = yourmatrix(sub2ind(size(yourmatrix), ...
                                  repmat(roworder, 1, size(yourmatrix, 2), 1), ... 
                                  repmat(1:size(yourmatrix, 2), size(yourmatrix, 1), 1, size(yourmatrix, 3)), ...
                                  repmat(permute(1:size(yourmatrix, 3), [1 3 2]), size(yourmatrix, 1), size(yourmatrix, 2), 1)));