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

Naresh Pai npai@uark....
Sat Feb 4 14:01:24 CST 2012


I am somewhat new to Python (been coding with Matlab mostly). I am trying
to
simplify (and expedite) a piece of code that is currently a bottleneck in a
larger
code.

I have a large array (7000 rows x 4500 columns) titled say, abc, and I am
trying
to find a fast method to count the number of instances of each unique value
within
it. All unique values are stored in a variable, say, unique_elem. My
current code
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
in [unique_elem[i]])))

This loop is bottleneck because I have about 850 unique elements and it
takes
about 9-10 minutes. Can you suggest a faster way to do this?

Thank you,
Naresh
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