Extracting a specific element from each cell within cell array

Question

I have a cell array A of size 10x10 (say). Each cell in turn contains a 5x20 matrix. I want to select (i,j) element from each

Answer 1

If memory is not an issue, you can concat all matrices along a third dim; and then indexing is very easy:

%// Example data
A = {[1 2;5 6], [3 4; 6 7]; [3 4; 6 7], [9 8; 5 6]};
ii = 2;
jj = 1;

%// Compute the result B
A2 = cat(3,A{:}); %// concat along third dim
B = reshape(A2(ii,jj,:),size(A{1})); %// index A2 and reshape

Answer 2

CELL2MAT gets all the data from a cell array that consists of numeric data only, into a numeric array. So, that helped us here. For your original problem, try this -

celldim_i = 10;
celldim_j = 10;

block_size_i = 5;
block_size_j = 20;

search_i = i; %// Edit to your i
search_j = j; %// Edit to your j

A_mat = cell2mat(A);
out = A_mat(search_i+block_size_i*(0:celldim_i-1),search_j+block_size_j*(0:celldim_j-1))

Answer 3

The easy to use cellfun one-liner would be:

ii = 2;
jj = 1;
A = {[1 2;5 6], [3 4; 6 7]; [3 4; 6 7], [9 8; 5 6]};

B = cell2mat( cellfun( @(x) x(ii,jj), A, 'uni', 0) )

gives:

B =

     5     6
     6     5

---

Advantage over Divakar's Solution: it works also for inconsistent matrix sizes in A.

And if you want to avoid also the outer loop, another fancy two-liner:

dim = [2 2];
[II, JJ] = meshgrid( 1:dim(1), 1:dim(2) );

C = cellfun( @(y) ...
    { cell2mat( cellfun( @(x) x( real(y), imag(y) ), A, 'uni', 0) ) },...
      num2cell( II(:)+1i*JJ(:) ))

gives:

>> celldisp(C)

C{1} =            % ii = 1 , jj = 1

     1     3
     3     9



C{2} =            % ii = 1 , jj = 2

     2     4
     4     8



C{3} =            % ii = 2 , jj = 1

     5     6
     6     5



C{4} =            % ii = 2 , jj = 2

     6     7
     7     6