Indexing into a multidemesional array using a multidimensional array

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

Answer 1

[Guillaume]

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))

[Kevin McAughey]

This seems to work perfectly, now I'll just have to look through it so that I can understand why!

[Guillaume]

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.

Answer 2

[Jan]

Try this:

output = X(sub2ind(size(X), Y(:), 1:size(x,2), 1:size(x,3))

[Kevin McAughey]

When I tried this it threw the following error:

Error using sub2ind (line 51)
The subscript vectors must all be of the same size.