[Numpy-discussion] Fwd: Fast histogram

Peter Creasey p.e.creasey.00@googlemail....
Thu Apr 17 12:45:57 CDT 2008


On 17/04/2008, Zachary Pincus wrote:
 >  But even if indices = array, one still needs to do something like:
 >  for index in indices: histogram[index] += 1
 >
 >  Which is slow in python and fast in C.
 >

 I haven't tried this, but if you want the sum in C you could do

 for x in unique(indices):
    histogram[x] = (indices==x).sum()

 Of course, this just replaces an O(N log N) algorithm by an O(N * M)
 (M is the number of bins), which is only going to be worth it for very
 small M.


 Peter


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