[Numpy-discussion] fast method to to count a particular value in a large matrix
Sat Feb 4 14:35:08 CST 2012
On Saturday, February 4, 2012, Naresh Pai <firstname.lastname@example.org> wrote:
> I am somewhat new to Python (been coding with Matlab mostly). I am trying
> simplify (and expedite) a piece of code that is currently a bottleneck in
> I have a large array (7000 rows x 4500 columns) titled say, abc, and I am
> to find a fast method to count the number of instances of each unique
> it. All unique values are stored in a variable, say, unique_elem. My
> is as follows:
> import numpy as np
> #allocate space for storing element count
> elem_count = zeros((len(unique_elem),1))
> #loop through and count number of unique_elem
> for i in range(len(unique_elem)):
> elem_count[i]= np.sum(reduce(np.logical_or,(abc== x for x
> This loop is bottleneck because I have about 850 unique elements and it
> about 9-10 minutes. Can you suggest a faster way to do this?
> Thank you,
no.unique() can return indices and reverse indices. It would be trivial to
histogram the reverse indices using np.histogram().
Does that help?
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