Find index of first element with condition in numpy array in python

Question

I'm trying find the index of the first element bigger than a threshold, like this:

index = 0
while timeStamps[index] < self.stopCount and index < len(timeStamps):
    index += 1

Can this be done in a one-liner? I found:

index = next((x for x in timeStamps if x <= self.stopCount), 0)

I'm not sure what this expression does and it seems to return 0 always... Could somebody point out the error and explain the expression?

Why is it important to write it in a one-liner? One liners produce the same machine code and are more difficult to understand and debug — Bogdan Doicin, Sep 03 '19 at 10:35
Good point. I found it rather slow, but perhaps there is no faster way. It is merely for comparison. — Frank, Sep 03 '19 at 10:37

score 5 · Accepted Answer · answered Sep 03 '19 at 11:16

5

Another option is to use np.argmax (see this post for details). So your code would become something like

(timeStamps > self.stopCount).argmax()

the caveat is that if the condition is never satisfied the argmax will return 0.

answered Sep 03 '19 at 11:16

tomjn

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That is exactly what I want, including caveat. – Frank Sep 03 '19 at 13:37

score 0 · Answer 2 · answered Sep 03 '19 at 10:46

0

I would do it this way:

import numpy as np

threshold = 20

sample_array = np.array([10,11,12,13,21,200,1,2])

idx = np.array([np.where(sample_array > threshold)]).min()
print(idx)
#4

answered Sep 03 '19 at 10:46

Péter Leéh

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Wrapping in an extra list and an array seem like total overkill here – Mad Physicist Sep 03 '19 at 11:27

Dev Khadka · Answer 3 · 2019-09-03T12:17:57.537

0

this one liner will work

sample_array = np.array([10,11,12,13,21,200,1,2])

# oneliner
print(sum(np.cumsum(arr>threshold)==0))

np.cumsum(sample_array>threshold)==0) will have value 0 until element is bigger than threshold

edited Sep 03 '19 at 12:17

answered Sep 03 '19 at 11:05

Dev Khadka

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This is such a convoluted way, but it sure does work. – Mad Physicist Sep 03 '19 at 11:25

Find index of first element with condition in numpy array in python

3 Answers3