Question

Given a list of 0 to N sequential integers, where N is an even number, e.g. A = [0 , 1 , 2 , 3 , 4] I’m looking for a list comprehension, i.e. of type [a for a in A], for creating another list B = [0 , 1 , 1 , 2 , 3 , 3 , 4] that repeats the odd numbers in the input list, A.

Answer 1

Here’s a (somewhat ugly) solution with a list comprehension:

a = range(5)
list(itertools.chain(*[[x, x] if x % 2 == 1 else [x] for x in a]))
# => [0, 1, 1, 2, 3, 3, 4]

The list comprehension builds a list of sublists:

[[x, x] if x % 2 == 1 else [x] for x in a] # => [[0], [1, 1], [2], [3, 3], [4]]

The itertools.chain call is just one of the many ways to flatten a list in Python.