Using index of 3d array from max option to access elements from another 3d array

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jonathan williams
评论： hamed amini 2017-4-16
Here is an example of code that executes what I'd like to do, but is very slow for large arrays and can likely be written much more simply and faster. I have an array A that I take the max over the rows, where A is a 3d array. I'd then like to use the indices from the max operator to index into another array that is the same size as B and collect those elements in an array where the size of the final array is [1,size(A,2),size(A,3)]. Any help would be greatly appreciated. Thank you!
A = rand(3,2,10);
[~,Aind] = max(A,[],1);
B = rand(3,2,10);
tmp3 = zeros(1,size(A,2),size(A,3));
for j = 1:size(A,2)
tmp1 = squeeze(Aind(:,j,:))+ 3*((0:1:10-1)');
tmp2 = squeeze(B(:,j,:));
tmp3(1,j,:) = tmp2(tmp1);
end

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Guillaume
Guillaume 2017-4-6
编辑：Guillaume 2017-4-6
I always find it difficult to think in 3D, but the following should do what you want. In any case, sub2ind (and ind2sub) is your friend.
[~, Aind] = max(A, [], 1);
tmp3 = B(sub2ind(size(B), Aind, ...
repmat(1:size(A, 2), 1, 1, size(A, 3)), ...
repmat(permute(1:size(A, 3), [1 3 2]), 1, size(A, 2), 1)))
A little squeeze beforehand would make it slightly more readable:
tmp3 = B(sub2ind(size(B), squeeze(Aind), ...
repmat((1:size(A, 2))', 1, size(A, 3)), ...
repmat(1:size(A, 3), size(A, 2), 1)))

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