Delete all values in list outside of specified range python

Question

I have a list (let's call it L1) with plenty of decimal values. How do I remove all values outside of some specified range while keeping all values within the range?
For instance, let's say I define my range as [-1, 1] and

L1 = [-2, 0.1, 0.75, 4]

I would want my output to be a new list, i.e.

L2 = [0.1, 0.75]

I've heard there was a way to do this with numpy (though I can't find the SO question for the life of me), but I was wondering if there was another way, just using the built-in functions (of course if numpy is better for this sort of thing, then that's fine too).

MSeifert · Accepted Answer · 2017-07-11 15:44:19Z

13

You can do it using boolean indexing with NumPy. For large lists/arrays this can be much faster than a list comprehension or filter approach:

>>> import numpy as np
>>> L1 = [-2, 0.1, 0.75, 4] 
>>> A1 = np.array(L1)           # convert it to an array
>>> A1[(A1 >= -1) & (A1 <= 1)]  # remove all values that are not in the range [-1, 1]
array([ 0.1 ,  0.75])

answered Jul 11, 2017 at 15:44

MSeifert

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1 Comment

user2699 Over a year ago

The right answer. List comprehensions will be much slower

Błotosmętek · Accepted Answer · 2017-07-11 15:15:32Z

4

Just use list comprehension:

L2 = [ x for x in L1 if -1 <= x <= 1 ]

answered Jul 11, 2017 at 15:15

Błotosmętek

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Comments

Oleksii Filonenko · Accepted Answer · 2017-07-11 15:16:29Z

0

Use filter.

L2 = list(filter(lambda x: -1 <= x <= 1, L1))

answered Jul 11, 2017 at 15:16

Oleksii Filonenko

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Sason Torosean · Accepted Answer · 2017-07-11 15:17:50Z

0

Here is another.

L2 = [ii for ii in L1 if (ii <= 1 and ii >= -1)]

answered Jul 11, 2017 at 15:17

Sason Torosean

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Tom Wyllie · Accepted Answer · 2017-07-11 15:23:19Z

0

Seems like the perfect job for filter. (only in python 2!)

L1 = [-2, 0.1, 0.75, 4]
filtered_L1 = list(filter(lambda x: -1. <= x <= 1., L1))
print(filtered_L1)
# [0.1, 0.75]

Python 3 you are better off with a list comprehension;

L1 = [-2, 0.1, 0.75, 4]
L1 = [x for x in L1 if -1. <= x <= 1.]
print(L1)
# [0.1, 0.75]

edited Jul 11, 2017 at 15:23

answered Jul 11, 2017 at 15:16

Tom Wyllie

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2 Comments

Błotosmętek Over a year ago

Not in Python3 (see @BrightOne's answer).

Tom Wyllie Over a year ago

Wow thanks @Błotosmętek, didn't know that. Python 2.7 virtualenv got me again :(

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