MATLAB: How to get indices of unique elements in matrix in separate arrays

dynamic arraymatrix array

1.I have a 1D array eg. A=[2,5,7,8,3,9…..]. Elements have been extracted from a 2d matrix eg
M=  [1  5   2  17
         2  2  8    9
         3  4  7    5
          ..............]
2. Now, I need to find the indices of each element of A in Matrix M. Like,
A[1]= [1,3; 2,1; 2,2]
A[2]= [1,2;3,4]
A[3]= [3,3]
A[4]= [3,4].... so and so...
What is the nest way to do?

Best Answer

I would prefer one of the above suggestions, but for completeness a dull (but unexpectedly faster) loop:
M = [1  5  2  17; ...
     2  2  8   9; ...
     3  4  7   5];
A = [2,5,7,8,3,9];
R = cell(1, max(A));
for iA = A
  [r, c] = find(M == iA);
  R{iA}  = [r, c];
end
A small speed comparison:
M = randi([1, 100], 100, 100);
A = randperm(100, 80);
tic;
for kk = 1:100
  [R,C] = arrayfun(@(n)find(M==n),A,'Uni',0);
  Z = cellfun(@(r,c)[r(:),c(:)],R,C,'Uni',0);
end
toc;
tic;
for kk = 1:100
  [tf, idx] = ismember(M, A);
  used_subset = idx(tf);
  linear_idx = (1:numel(idx)).';
  indices = accumarray( used_subset, linear_idx(tf), [], @(C) {C}, {});
  r = size(M,1);
  locations = cellfun(@(LI) [mod(LI-1, r)+1, floor((LI-1)/r)+1], ...
                      indices, 'Uniform', 0);
end
toc;
tic;
for kk = 1:100
  R = cell(1, max(A));
  for iA = A
    [r, c] = find(M == iA);
    R{iA}  = [r, c];
  end
end
toc;
tic;
for kk = 1:100
  locations = regionprops(M, 'PixelList');
  locations = {locations(A)};
end
toc;
 % Matlab R2016b/64, Win7, Core2Duo
 Elapsed time is 0.700541 seconds.   % arrayfun/cellfun
 Elapsed time is 0.789638 seconds.   % accumarray
 Elapsed time is 0.455393 seconds.   % dull loop/find
 ... Could someone please post the results with regionprops of the IPT?
Oh, the dull loop runs fastert than it looks like.
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