How to mask a list using boolean values from another list

Question

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.

Answer 1

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])

Output:

['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']

Answer 2

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']

print(list(np.array(y)[x]))

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]:
        temp.append(y[index])

print(temp)