Indexing into a multidemesional array using a multidimensional array

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Kevin McAughey
Kevin McAughey 2016-11-18
评论： Guillaume 2016-11-18
This question seems similar to a previously answered question, but I'm not sure if the answer there applies to my problem.
I have an array, X, which has dimensions k by m by n and I have another array, Y, with indices for the first dimension k, which has dimensions m by n. I want to create an output matrix of m by n of each value of X in the k dimension where the index in that k dimension corresponds to the value in Y for each value of m and n.
I know that is a bit confusing, but in essence I want to achieve the following code via clever application of indexing, rather than using nested for loops.
m = size(X,2);
n = size(X,3);
for m_count = 1:length(m)
for n_count = 1:length(n)
output(m_count,n_count) = X(Y(m_count,n_count),m_count,n_count)
end
end

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采纳的回答

Guillaume
Guillaume 2016-11-18
I think it should rather be:
[rows, columns] = ndgrid(1:size(Y, 1), 1:size(Y, 2));
output = X(sub2ind(size(X), Y, rows, columns))
  2 个评论
Kevin McAughey
Kevin McAughey 2016-11-18
This seems to work perfectly, now I'll just have to look through it so that I can understand why!
Guillaume
Guillaume 2016-11-18
rows and columns are just two matrices corresponding to the row and column indices of the corresponding elements of Y.
sub2ind converts these three coordinate matrices into a single linear index which can be used to index X.

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更多回答（1 个）

Jan
Jan 2016-11-18
编辑：Jan 2016-11-18
Try this:
output = X(sub2ind(size(X), Y(:), 1:size(x,2), 1:size(x,3))

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