[Numpy-discussion] fast method to to count a particular value in a large matrix

Jerome Kieffer Jerome.Kieffer@esrf...
Sat Feb 4 14:42:48 CST 2012


On Sat, 4 Feb 2012 14:35:08 -0600
Benjamin Root <ben.root@ou.edu> wrote:

> 
> no.unique() can return indices and reverse indices.  It would be trivial to
> histogram the reverse indices using np.histogram().

Even np.histogram(abc,unique_elem) or something like this. 
Works if unique_elem is ordered.

np.histogram(abc,list(unique_elem)+[unique_elem[-1]+1])[0].reshape(-1,1)
is 40x faster and gives the same result.

-- 
Jérôme Kieffer
Data analysis unit - ESRF


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