CODEWITHSUNDEEP
pythonpython-3.xlist
cxnrachel
cxnrachel

Reputation: 23

How do I remove the first element in a list within a list?

I would like to know if there's any way to remove an index in a list within a list. For example, given [[1,2,3],[4,5,6],[7,8,9]], I would like to remove the the first element of each list so that it becomes [[2,3],[5,6],[8,9]]. I can do a for loop to slowly remove them but I was wondering if there's a more efficient manner to do so?

Upvotes: 1

Views: 1460

Answers (3)

jmd_dk
jmd_dk

Reputation: 13120

For small lists as in your example, doing something like list.pop(0) is OK:

nested = [[1,2,3], [4,5,6], [7,8,9]]
for inner in nested:
    inner.pop(0)  # Remove first element of each inner list

However, as lists are basically implemented as arrays (of pointers), removing the first element means that all other elements has to be shifted back by one, resulting in a lot of (pointer) copying if your lists are large. Also, using inner[1:] rather than inner.pop(0) does not solve the problem, as inner[1:] creates a new list rather than returning a view.

One way out is to make the redundant elements appear as the last element instead of the first, so that you can do inner.pop() instead, which removes the last element. No further shifting/copying is then required.

Another solution is to switch out your data structure from a list to a deque ("double-ended queue"). This has a popleft() method, which is equivalent to pop(0) but fast, as deques support fast popping from both ends:

import collections
nested = [[1,2,3], [4,5,6], [7,8,9]]
nested = [collections.deque(inner) for inner in nested]  # list of deques
for inner in nested:
    inner.popleft()  # Remove first element of each inner list, fast!

Upvotes: 0

Khalenn
Khalenn

Reputation: 121

I would say you have two options.

You can use pop():

for item in list:
    item.pop(0)

Or recreate a new list :

list2 = [item[1:] for item in list]

Upvotes: 0

Hagbart
Hagbart

Reputation: 105

You can do it with iterating over the list, and then call the pop() function on the inner list. This will remove an element at the specified index.

test_list = [[1,2,3],[4,5,6],[7,8,9]]

for i in test_list:
    i.pop(0)

print(test_list)

output: [[2, 3], [5, 6], [8, 9]]

Upvotes: 1

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