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.

gnovice · Accepted Answer · 2017-12-28 15:32:38Z

6

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.

edited Dec 28, 2017 at 15:32

answered Mar 18, 2011 at 14:49

gnovice

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1 Comment

Andrew Janke Over a year ago

+1 Nice getting rid of 'UniformOutput','false'. What an eyesore.

zellus · Accepted Answer · 2011-03-18 09:16:32Z

0

Try the following expression:

C = A(cell2mat(B))

You may have a look at Loren's blog post about Cell Arrays and Their Contents

edited Mar 18, 2011 at 9:16

answered Mar 18, 2011 at 9:10

zellus

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1 Comment

reve_etrange Over a year ago

This expression makes a cell array with each element a single vector from A. I want to create a cell array with each element a matrix composed of vectors from A corresponding to the indices in an element of B.

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