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

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Emily Hokett
Emily Hokett 2018-10-11
回答： Guillaume 2018-10-15
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
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Guillaume
Guillaume 2018-10-11
编辑：Guillaume 2018-10-11
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
Emily Hokett 2018-10-15
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.

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回答（2 个）

gonzalo Mier
gonzalo Mier 2018-10-11
I am not sure if this is what you want:
matrix(:,33,2) = sort(matrix(:,33,2));
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Emily Hokett
Emily Hokett 2018-10-15
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?

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Guillaume
Guillaume 2018-10-15
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)));

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