Count all values in a matrix greater than a value

Question

I have to count all the values in a matrix (2-d array) that are less than 200.

The code I wrote down for this is:

za=0    p31 = numpy.asarray(o31)    for i in range(o31.size[0]):        for j in range(o32.size[1]):            if p31[i,j]<200:                za=za+1    print za 

o31 is an image and I am converting it into a matrix and then finding the values.

Is there a simpler way to do this?

Answer 1

This is very straightforward with boolean arrays:

p31 = numpy.asarray(o31) za = (p31 < 200).sum() # p31<200 is a boolean array, so `sum` counts the number of True elements 

Answer 2

The numpy.where function is your friend. Because it's implemented to take full advantage of the array datatype, for large images you should notice a speed improvement over the pure python solution you provide.

Using numpy.where directly will yield a boolean mask indicating whether certain values match your conditions:

>>> data array([[1, 8],        [3, 4]]) >>> numpy.where( data > 3 ) (array([0, 1]), array([1, 1])) 

And the mask can be used to index the array directly to get the actual values:

>>> data[ numpy.where( data > 3 ) ] array([8, 4]) 

Exactly where you take it from there will depend on what form you'd like the results in.