Flattening unknown nesting level of list using looping in Python

Question

I need to solve the below problem using looping.

Statement

Your input is a list of lists with unknown nesting level. Could be like:

[
    [1, 2],
    [
        3,
        [4, 5],
    ],
    6,
    7,
]

Your challenge is to reshape it into a single list like that:

[1, 2, 3, 4, 5, 6, 7]

My code is :

import json

data = json.loads(input())
#WRITE YOUR CODE HERE 
list_data = list(data) 
flat_list = [item for items in list_data for item in items] 
print(flat_list)

TypeError: 'int' object is not iterable

Dont do this with list comprehension, it wont work. You will need recursion (or iteration with a stack) — Tom McLean, Oct 07 '22 at 20:09
@DanielHao Aside from not being canonical, that one only covers a single level of list nesting. — Karl Knechtel, Oct 12 '22 at 22:36

I'mahdi · Answer 1 · 2022-10-07T20:32:44.930

1

Try recursive approach:

lst = [[1, 2],[3,[4, 5],],6,7]

def flat_list(lst, res):
    for l in lst:
        if isinstance(l, list):
            flat_list(l, res)
        else:
            res.append(l)
            
result = []
flat_list(lst, result)
print(result)

Output:

[1, 2, 3, 4, 5, 6, 7]

Update: (use looping without recursive approach)

lst = [[1, 2],[3,[4, 5],],6,7]

res = []
for l in lst:
    tmp = [l]
    while tmp != []:
        for i in tmp:
            if isinstance(i, list):
                for j in i:
                    tmp.append(j)
            else:
                res.append(i)
            tmp = tmp[1:]
print(res)
# [1, 2, 3, 4, 5, 6, 7]

edited Oct 07 '22 at 20:32

answered Oct 07 '22 at 20:11

I'mahdi

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The problem is that I shouldn't use functions and recursion – Albina Hakobyan Oct 07 '22 at 20:14
@AlbinaHakobyan, so how do you want to solve this? use library? – I'mahdi Oct 07 '22 at 20:15
The problem is in the looping section so I guess with looping – Albina Hakobyan Oct 07 '22 at 20:17
@AlbinaHakobyan, see the update. Is this helping you? – I'mahdi Oct 07 '22 at 20:33

score 0 · Accepted Answer · answered Oct 07 '22 at 20:36

Here is a stack-based approach:

lst = [[1, 2], [3, [4, 5], ], 6, 7]

stack = lst[::-1]
res = []

while stack:
    item = stack.pop()
    if isinstance(item, list):
        for x in item[::-1]:
            stack.append(x)
    else:
        res.append(item)

print(res)

Flattening unknown nesting level of list using looping in Python

2 Answers2