如何在NumPy中执行此python列表理解？

Question

Let's say I have an array of values, r , which range anywhere from 0 to 1 . I want to remove all values that are some threshold value away from the median. Let's assume here that that threshold value is 0.5 , and len(r) = 3000 . Then to mask out all values outside of this range, I can do a simple list comprehension, which I like:

mask = np.array([ri < np.median(r)-0.5 or ri > np.median(r)+0.5 for ri in r])

And if I use a timer on it:

import time
import numpy as np

start = time.time()
r = np.random.random(3000)
m = np.median(r)
maxr,minr = m-0.5, m+0.5
mask = [ri<minr or ri>maxr for ri in r]
end = time.time()
print('Took %.4f seconds'%(end-start))

>>> Took 0.0010 seconds

Is there a faster way to do this list comprehension and make the mask using NumPy ?

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Edit:

I've tried several suggestions below, including:

• An element-wise or operator: (r<minv) | (r>maxv) (r<minv) | (r>maxv)

• A Numpy logical or: r[np.logical_or(r<minr, r>maxr)]

• A absolute difference boolean array: abs(mr) > 0.5

And here is the average time each one took after 300 runs through:

Python list comprehension: 0.6511 ms
Elementwise or: 0.0138 ms
Numpy logical or: 0.0241 ms
Absolute difference: 0.0248 ms

As you can see, the elementwise Or was always the fastest, by nearly a factor of two (don't know how that would scale with array elements). Who knew.

Answer 1

You can use numpy conditional selections to create new array, without those values.

start = time.time()
m = np.median(r)
maxr,minr = m-0.5, m+0.5
filtered_array = r[ (r < minr) | (r > maxr) ]
end = time.time()
print('Took %.4f seconds'%(end-start))

filtered_array is slice of r without masked values (all values that will be later removed by mask already removed in filtered_array ).

Update: used shorter syntax suggested by @ayhan.

Answer 2

一个班轮......

new_mask = abs(np.median(r) - r) > 0.5