How to insert list into nested list in python?

Question

Right now I am practicing List
I am facing a problem of inserting nested list.
for instance; I have a nested list and i want to make another nested.

example

a = [[1, 'a', 2], [2, 'a', 3], [3, 'b', 4], [4, 'c', 5]]

output should be:

output = [['a'[1,2],[2,3]], ['b'[3],[4]], ['c'[4],[5]]]

Asked By: Motiur Rahman

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Source

Answer 1

Use a dictionary to collect all the sublists with the same string.

from collections import defaultdict

a = [[1, 'a', 2], [2, 'a', 3], [3, 'b', 4], [4, 'c', 5]]
output_dict = defaultdict(lambda: [[], []])

for num1, key, num2 in a:
    output_dict[key][0].append(num1)
    output_dict[key][1].append(num2)

output = [[key, *val] for key, val in output_dict.items()]

output is

[['a', [1, 2], [2, 3]], ['b', [3], [4]], ['c', [4], [5]]]

Answered By: Barmar

Answer 2

You could do this with an list, however, a dictionary is probably better in your use case.

a = [[1, 'a', 2], [2, 'a', 3], [3, 'b', 4], [4, 'c', 5]]
dct = {}
for arr in a:
    key = arr[1]
    del arr[1]
    if key not in dct:
        dct[key] = [arr]
    else:
        dct[key].append(arr)

print(dct)  # => {'a': [[1, 2], [2, 3]], 'b': [[3, 4]], 'c': [[4, 5]]}

If you need a list, then you can use list comprehension to turn the dictionary into a list

lst = [[key, *dct[key]] for key in dct]
print(lst)  # => [['a', [1, 2], [2, 3]], ['b', [3, 4]], ['c', [4, 5]]]

Answered By: Michael M.

How to insert list into nested list in python?

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