How to mask a list using boolean values from another list


I have a list like this:

x = [True, False, True, False]

and a list like this:

y = [a, b, c, d]

I would like to mask x over y to get this output:

output = [a, c]

I know how to do this using while/for loops, but I’m ideally looking for an elegant one-line of code using list comprehension.

Asked By: NBC



You can use zip and a list comprehension to perform a filter operation on y based on corresponding truth values in x:

x = [True, False, True, False]
y = ["a", "b", "c", "d"]

print([b for a, b in zip(x, y) if a])


['a', 'c']

itertools.compress also does this:

>>> from itertools import compress
>>> x = [True, False, True, False]
>>> y = ["a", "b", "c", "d"]
>>> list(compress(y, x))
['a', 'c']
Answered By: ggorlen

I think the easiest way is to use numpy:

import numpy as np
>>> x = [True, False, True, False]
>>> y = ['a', 'b', 'c', 'd']
>>> np.array(y)[x]
array(['a', 'c'], dtype='<U1')

Without numpy, You could also enumerate in a list comprehension:

>>> [i for idx, i in enumerate(y) if x[idx]]
['a', 'c']
Answered By: sacuL

This is rather simple to solve. Consider writing properly your list:

x = [True, False, True, False]
y = [a, b, c, d]  # assuming that a, b, c and d are some kind of object
output = []
for i, k in enumerate(x):
    if k:
Answered By: eapetcho

There are several ways to do this.

The simplest way would be to zip the two lists together and use a list comprehension to keep the items you want.

x = [True, False, True, False]
y = ['a', 'b', 'c', 'd']

print([item for keep, item in zip(x, y) if keep])

You can also convert the y array to a numpy array and use the x array to mask the numpy array.

import numpy as np

x = [True, False, True, False]
y = ['a', 'b', 'c', 'd']


Finally, you can create an empty list, iterate through the x and y arrays using their indexes, and append elements in y to the empty list if the corresponding element in x is True.

x = [True, False, True, False]
y = ['a', 'b', 'c', 'd']

temp = []

for index in range(len(y)):
    if x[index]:

Answered By: tdurnford

If already have you some function that returns True/False for each element y, use filter(), example:

list(filter(lambda x: x < 'c' , y))


my_iter = iter([True, False, True, False])
list(filter(lambda x: next(my_iter), y))
Answered By: Jeka Golovachev
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