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.
