How to insert first element into list of lists using list comprehension

Question

I have a list of lists and want to insert a value into the first position of each list. What is wrong with this code? Why does it return none data types at first, and then if I access the variable again, it shows up with data? I end up with the right answer here, but I have a much larger list of lists I am trying to this with, and it does not work. What is the right way to do this? Thanks.

lol = [[1,2,3],[1,2,3],[1,2,3]]
[lol[i].insert(0,0) for i in np.arange(0,3)]

The results:

lol = [[1,2,3],[1,2,3],[1,2,3]]
[lol[i].insert(0,0) for i in np.arange(0,3)]
Out[201]: [None, None, None]

lol
Out[202]: [[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]

Why do you want a list comprehension? Do you want new list objects or do you want to insert into the existing ones? What do you mean with "it does not work" when talking about the larger list? You also say you "end up with the right answer", so which is it? Do you get the right result or not? How large is your outer list and how large are your inner lists? — Kelly Bundy
– Kelly Bundy, Commented Sep 4, 2022 at 21:31

Andrej Kesely · Accepted Answer · 2022-09-04 20:53:13Z

3

list.insert inserts the value in-place and always returns None. To add new value into a list with list comprehension you can do:

lol = [[0, *subl] for subl in lol]
print(lol)

Prints:

[[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]

answered Sep 4, 2022 at 20:53

Andrej Kesely

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7 Comments

Kelly Bundy Over a year ago

Even much more inefficient than the already inefficient insert, though.

Andrej Kesely Over a year ago

@KellyBundy If you want to insert to a list at first position, it will be always inefficient (O(n))

Andrej Kesely Over a year ago

@KellyBundy The question is named "How to insert first element into list of lists using list comprehension" so I showed how it's done using list-comprehension.

Kelly Bundy Over a year ago

They also ask "What is the right way to do this?", though, and I see no indication that they want new lists (they say "end up with the right answer") so I don't think this is the right way. Oh well, they already made up their mind...

Andrej Kesely Over a year ago

@KellyBundy Please post a new answer here what you think is right way to do it. I'll upvote it :) It's always good to see alternatives.

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shashank2806 · Accepted Answer · 2022-09-04 21:03:55Z

3

This will also do the trick.

lol = [[0] + l for l in lol]

answered Sep 4, 2022 at 21:03

shashank2806

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