I want to iterate through a list of lists but not in the typical way such that I go through each element in the list of lists then move to the next list.
How does the logic work if I wanted to print the elements in the order 1, 4, 2, 5, 3, 6, which is the list of lists element then the lists and repeat, instead of 1, 2, 3, 4, 5, 6?
ls = [[1, 2, 3], [4, 5, 6]]
Assuming all the lists are the same length:
list_length = len(ls[0])
for i in range(list_length):
for list in ls:
print(list[i])
If you want short code
ls = [[1, 2, 3], [4, 5, 6]]
result = [v for tt in list(zip(*tuple([tuple(x) for x in ls]))) for v in tt]
print(result)
Output
[1, 4, 2, 5, 3, 6]
As suggested by @iz_, there's a shorter way for the same output
ls = [[1, 2, 3], [4, 5, 6]]
result = [v for tt in zip(*ls) for v in tt]
print(result)
[v for tt in zip(*ls) for v in tt]?Transform your list of lists into a transposed flat list (technically, a tuple, but it makes no difference). zip transposes the list, sum flattens it.
indexes = sum(zip(*ls), tuple())
# (1, 4, 2, 5, 3, 6)
If you still want a list (and perhaps you do not), call list(indexes).
There's already a library function defined for this : itertools.zip_longest():
import itertools
zipped_list = itertools.zip_longest(*ls, fillvalue='-')
This is more generic since it will automatically create items even if you have more than 2 sublists.
On top of that, it also takes care of sublists with different lengths
import numpy as np
ls = [[1, 2, 3], [4, 5, 6], [7,8,9], [10,11,12]]
print(ls)
res1 = [ls[r][c] for c in range(len(ls[0])) for r in range(len(ls)) ]
print(res1)
print('-' * 30)
# Now using numpy which is generally faster
np_ls = np.array(ls)
print(np_ls)
res = [j for i in range(len(np_ls[0])) for j in np_ls[...,i]]
print(res)
