How to repeat a list n times and reverse its elements alternately?

My input is a list of numbers that are not necessarily sorted but for the example I made a simple one :

my_list = [0, 1, 2, 3, 4]

I want to repeat it n times horizontaly but when i is odd we need to reverse the list :

# [[0, 4, 0, 4, 0, 4],
#  [1, 3, 1, 3, 1, 3],
#  [2, 2, 2, 2, 2, 2],
#  [3, 1, 3, 1, 3, 1],
#  [4, 0, 4, 0, 4, 0]]

For that I made the code below but I’m not proud of it.

final = []

for i in range(6):
    if i%2 == 0:
        final.append(my_list)
    else:
        final.append(my_list[::-1])
        
final = list(zip(*final))

I heard that numpy and pandas are more suitable for this kind of operations.

Do you guys have any idea how to do it ? I’m open to any suggestion.

>Solution :

Using numpy‘s tile (or repeat) and indexing:

my_list = [0, 1, 2, 3, 4]
N = 6

# repeat the array
out = np.tile(np.array(my_list)[:, None], (1, N))
# or
# out = np.repeat(np.array(my_list)[:, None], N, axis=1)

# invert the odd columns
out[:, 1::2] = out[::-1, 1::2]

Or with a small hack that might be faster for large N, for a pair of columns in the ascending/descending order, then tile it to the closest upper even value, and slice to remove the last column if even:

N = 6
a = np.array(my_list)
out = np.tile(np.c_[a, a[::-1]], (1, (N+1)//2))[:, :N]

Output:

array([[0, 4, 0, 4, 0, 4],
       [1, 3, 1, 3, 1, 3],
       [2, 2, 2, 2, 2, 2],
       [3, 1, 3, 1, 3, 1],
       [4, 0, 4, 0, 4, 0]])

timings

for N = 10_000:

# first approach
44.9 µs ± 1.14 µs per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

# second approach
24.7 µs ± 1.86 µs per loop (mean ± std. dev. of 7 runs, 10,000 loops each)