'Iterating through list of lists of lists

I am trying to iterate through a 3-D list in python(not numpy but I am willing to convert to a numpy array if this makes it easier) in such a way that from a list like this:

a = [[[0, 0], [3, 0]], [[1, 0], [4, 0]], [[2, 0], [6, 0]] ]

I can get the output

[0,0]
[1,0]
[2,0]
[3,0]
[4,0]
[6,0]

I can't figure out how to make it iterate like this...

My code:

a = [[[0, 0], [0, 0]], [[1, 0], [0, 0]], [[2, 0], [0, 0]] ]
for i in range(len(a)):
    for z in range(len(a[i])):
        print(a[i][z])

I've tried different things but can't seem to get this output.

pythonlistloopsfor-loopiteration


Solution 1:[1]

I think you want to print the nth sub-sublists consecutively from each sublist. You could unpack and zip a to get an iterable of tuples, then print each pair in them:

for tpl in zip(*a):
    for pair in tpl:
        print(pair)

Output:

[0, 0]
[1, 0]
[2, 0]
[3, 0]
[4, 0]
[6, 0]

Solution 2:[2]

Try:

print(*[i for v in zip(*a) for i in v], sep="\n")

Prints:

[0, 0]
[1, 0]
[2, 0]
[3, 0]
[4, 0]
[6, 0]

Solution 3:[3]

In case you want to just traverse the elements column wise, you can convert the list to numpy array, then stack the columns and print:

import numpy as np
a = [[[0, 0], [3, 0]], [[1, 0], [4, 0]], [[2, 0], [6, 0]] ]
arr = np.stack(np.array(a), axis=1)

for p in arr:
    for x in p:
        print(x)

The solution below is based on the previous assumption of printing each item in the sublist based on sorted order: You can use itertools to flatten the 3d to 2d list and then sort the 2d list based on first element in each sublist. Then print the sorted list.

import itertools
a = [[[0, 0], [3, 0]],[[1, 0], [4, 0]],[[2, 0], [6, 0]]]
flat_list = list(itertools.chain(*a))
sorted_list = sorted(flat_list, key=lambda x: x[0])

for i in sorted_list:
    print(i)

Solution 4:[4]

This is the correct way of iterating like you're trying to do:

a = [[[0, 0], [0, 0]], [[1, 0], [0, 0]], [[2, 0], [0, 0]] ]
for i in range(len(a)):
    for z in range(len(a[i])):
        print((a[i])[z])

If it is simpler for you, instead of iterating on the length of the lists with an index, try iterating on the elements of the lists:

a = [[[0, 0], [0, 0]], [[1, 0], [0, 0]], [[2, 0], [0, 0]] ]
for a1 in a:
    for a2 in a1:
        print(a2)

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1
Solution 2 Andrej Kesely
Solution 3 Dharman
Solution 4 lemon

Related Questions