Repeat elements in one list based on elements from another

Question

Given the following lists:

a = [0, 5, 1]
b = [1, 2, 1]

I\'d like to repeat each element of [a] by the number of its corresponding position

Answer 1

Use the zip() function with itertools.repeat() and itertools.chain.from_iterable():

try:
    # use iterator zip on Python 2 too
    from future_builtins import zip
except ImportError:
    pass
from itertools import repeat, chain

list(chain.from_iterable(repeat(value, count) for value, count in zip(a, b)))

(I added a Python 2 forward-compatible import for those that can't switch to Python 3 yet).

Demo:

>>> from itertools import repeat, chain
>>> a = [0, 5, 1]
>>> b = [1, 2, 1]
>>> list(chain.from_iterable(repeat(value, count) for value, count in zip(a, b)))
[0, 5, 5, 1]

An alternative approach would be to use a list comprehension; this is slower as repeating elements is done in bytecode instead of C:

[value for value, count in zip(a, b) for _ in range(count)]