MATLAB: Speed up for loop, accumarray

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I would like to speed up my two for loop with accumarray (I have old version of MatLab and I've read on internet that accumarray will speed up my operation)
if true
 for i = 1:size(img, 1)
  for j = 1:size(img, 2)
      R = idx(i, j, 1);
      G = idx(i, j, 2);
      B = idx(i, j, 3);
      H(R,G,B) = H(R,G,B) + 1;
  end
end
end

Best Answer

This gives exactly the same output as your code:
H = accumarray(reshape(idx,[],3),1);
First I ran your code on some random arrays:
img = randi(6,5,4,3);
idx = img;
H   = zeros(6,6,6);
for i = 1:size(img, 1)
  for j = 1:size(img, 2)
      R = idx(i, j, 1);
      G = idx(i, j, 2);
      B = idx(i, j, 3);
      H(R,G,B) = H(R,G,B) + 1;
  end
end
Then compared the outputs:
>> Z = accumarray(reshape(idx,[],3),1);
>> isequal(Z,H)
ans = 1
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