MATLAB: Efficient number occurence count

histcountshistogramMATLABvectorization

I want to efficiently count the number of occurences of numbers between 1-numel(num) in a Matrix. I came up with two options for that:
sz = [3000 2000];
mx = prod(sz);
num = randi([1 mx],sz);
tic;
% First option
counts = zeros(numel(num),1);
for i = 1:numel(num)
    counts(num(i)) = counts(num(i)) + 1;
end
toc
tic;
% Second option
uni = unique(num);
uni = reshape(uni,[],1);
hc  = histcounts(num,[uni;uni(end)]);
toc
Execution times are:
Elapsed time is 0.098540 seconds.
Elapsed time is 0.342214 seconds.
So option 1 is clearly faster. However the for loop bugs me. Is there any possibility to vectorize this?

Best Answer

In this situation, accumarray will be faster than histcounts, but still not as fast as the for-loop,
tic;
hc=accumarray(num(:),1,[mx,1]).';
toc
Elapsed time is 0.172890 seconds. %for-loop

Elapsed time is 0.236013 seconds. %accumarray

unless the values are pre-sorted,
num=sort(num(:));
tic;
hc=accumarray(num(:),1,[mx,1]).';
toc
Elapsed time is 0.168976 seconds. %for-loop
Elapsed time is 0.075965 seconds. %accumarray
I think this is simply one of those situations where Matlab's for-loop optimization has caught up to vectorized code.
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