MATLAB: index a cell array with cell array of arrays and return a cell array

Question

Say I have a cell array of (n X 1) vectors, A, and a cell array of vectors containing indices into A, called B. I wish to extract a cell array, C, such that C{i} = [A{B{i}}].
In other words, I have a cell array of arrays of indices, and I want to pull out the matrices corresponding to the concatenations of the vectors in A indexed by each of those arrays of indices.

for i = 1:length(B)
    %# B{i} is an array of indices, C{i} is a matrix
    C{i} = [ A{ B{i} } ];
end

The loop is equivalent to:

C = cellfun(@(x)[A{x}],B,'UniformOutput',false); %# implicit for loop w/ closure

Can I do that using an indexing expression alone? Or at least without the loop?
I think deal() might have to be involved but can't figure it out.

Answer 1

Here are two alternative solutions:

• Collect all the indices of B together with the function cell2mat, index the contents of A to make one large matrix, then divide that matrix up using the function mat2cell and the sizes of the index arrays in B:

  N = size(A{1});                        % Size of an array in A
  M = cellfun('prodofsize', B);          % Array of sizes of elements in B
  C = mat2cell([A{cell2mat(B)}], N, M);
  

• Here's a more compact version of your cellfun-based solution:

  C = cellfun(@(x) {[A{x}]}, B);
  

Ultimately, I would decide what solution to use based on speed and readability, which may actually turn out to be your for-loop-based solution.