Get index value in python list comprehension (for in preceded with variable)

Question

This syntax is able to define new list above list (or array):

[v for v in ['a', 'b']]

But how to get the index of the list? This doesn't work

[k for k, v in ['a', 'b']]

Expected output

[0, 1]

Does this answer your question? [Accessing the index in 'for' loops?](https://stackoverflow.com/questions/522563/accessing-the-index-in-for-loops) — wjandrea, Jan 16 '21 at 22:12

Tugay · Accepted Answer · 2021-01-16T22:21:30.893

4

Probably you have meant indexes, as there are no keys in list. We can get them like this:

indexes = [index for index,element in enumerate(your_list)]

But this isn't really necessary, we can get length of the list and get indexes from it as indexes are from 0 to [length of list]-1:

length = len(your_list)
indexes = list(range(length))

edited Jan 16 '21 at 22:21

answered Jan 16 '21 at 22:13

Tugay

2

Sidenote, [`l` is a bad variable name since it looks like `1` and `I`](https://www.python.org/dev/peps/pep-0008/#names-to-avoid). I would use `n` instead, personally. – wjandrea Jan 16 '21 at 22:16

Albert G Lieu · Answer 2 · 2021-12-07T17:15:28.387

3

[idx for idx, val in enumerate(['a', 'b'])]

output

[0, 1]

edited Dec 07 '21 at 17:15

answered Jan 16 '21 at 22:13

Albert G Lieu

3

Please do not use `k, v`. There are no keys and values in a list, OP is already confused about it. Maybe use `index, element` instead. – DeepSpace Jan 16 '21 at 22:15
it's just index as there is no key in the list object – Albert G Lieu Jan 16 '21 at 22:15
I assume luky who asked the question is from another programming language field – Albert G Lieu Jan 16 '21 at 22:17

Shane Bishop · Answer 3 · 2021-01-16T22:36:36.673

1

You can use range():

range(len(['a', 'b']))

range() will return a sequence. If you need a list specifically, you can do

list(range(len(['a', 'b'])))

As mentioned in the comments, in python lists do not have keys.

edited Jan 16 '21 at 22:36

answered Jan 16 '21 at 22:14

Shane Bishop

1

[`range`](https://docs.python.org/3/library/functions.html#func-range) isn't a [generator](https://docs.python.org/3/glossary.html#term-generator) or even an [iterator](https://docs.python.org/3/glossary.html#term-iterator). It's actually a [sequence](https://docs.python.org/3/glossary.html#term-sequence). – wjandrea Jan 16 '21 at 22:22

score 1 · Answer 4 · answered Jan 16 '21 at 22:27

1

enumerated_list = [(index, value) for index, value in enumerate(['a', 'b'])]
print(enumerated_list)

output: [(0, 'a'), (1, 'b')]

answered Jan 16 '21 at 22:27

Dennis

1

No need to unpack `enumerate` just to pack it again. Just do `list(enumerate(['a', 'b']))` – DeepSpace Jan 16 '21 at 22:27

score 1 · Answer 5 · answered Jan 16 '21 at 22:31

1

I think you meant index. Remember that lists have no keys, those are dictionaries.

[index for index, value in enumerate(['a', 'b'])]

The enumerate function is basically range but it returns the index and the value.

5 Answers5