Counting occurrences in order

I am wondering if anyone knows of a toolbox function that can perform this task quickly. I am not familiar with the Stats toolbox, but I have a feeling something could be there. Or maybe even the Image Processing Toolbox?

Here is the basic problem, and a naive solution. I want to count the occurrences of each unique item in a vector in the order in which they appear.

N = 5e5;   % Typical _minimum_ size, max N = 5e7.
C = 50;    % Best case shown, worst case: C = 1.5;
A = randi(floor(N/C),1,N);  % Data looks like this.
% Now to produce our results.  Need to find B.
Au = unique(A);
H = histc(A,Au);
B = zeros(size(A));

for ii = 1:length(Au)
   B(A==Au(ii)) = 1:H(ii);
end

As a simple example for clarity, for N=10, C=3:

[A(:) B(:)]

ans = 

       3     1
       2     1
       3     2
       3     3
       3     4
       2     2
       3     5
       3     6
       3     7
       1     1

I have written code that reduces the run-time by a large factor with the worst case C and minimum N, but it is a bit convoluted. After doing so I thought there might already be something like this out there, perhaps in a toolbox, but I couldn't find it. Perhaps there is a way to make further improvements for larger N.

Thanks!

EDIT I have not found any other solution to the this, so I may delete the question....

[Sean de Wolski]

I would hazard a guess that removing unique() would be the biggest improvement. Though the one time cost might be worth it..

[Ashish Uthama]

This is interesting. Thinking along the lines of using some kind of hash table. Please dont delete!

Answer 1

[Image Analyst]

I'm not sure why you want that but anyway it seems kind of esoteric/specialized. I can't think of anything in the Image Processing Toolbox that would get you the same thing as your code.

[Matt Fig]

Thanks, IA. I thought there might be something either in the IPT or Stats. Perhaps not. Yes, it is very specialized ;-).

Answer 2

[Matt J]

Sounds very similar to this:

http://www.mathworks.com/matlabcentral/answers/49616-count-how-many-times-a-number-appears-in-a-matrix-and-assign-that-increasing-value-to-a-new-matrix

So I offer it as a solution.

[Matt Fig]

Haha, Matt J! That is very similar. Perhaps my question is not as specialized as I thought. Thanks for the link, I don't know how I missed that question.