The effects of the three different methods to remove an element from a list:
remove removes the first matching value, not a specific index:
>>> a = [0, 2, 3, 2]
>>> a.remove(2)
>>> a
[0, 3, 2]
del removes the item at a specific index:
>>> a = [9, 8, 7, 6]
>>> del a[1]
>>> a
[9, 7, 6]
and pop removes the item at a specific index and returns it.
>>> a = [4, 3, 5]
>>> a.pop(1)
3
>>> a
[4, 5]
Their error modes are different too:
>>> a = [4, 5, 6]
>>> a.remove(7)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: list.remove(x): x not in list
>>> del a[7]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list assignment index out of range
>>> a.pop(7)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: pop index out of range
Answer from Martijn Pieters on Stack OverflowCan someone explain me what the difference between pop and remove is that makes sense? Why would you use pop over remove or vice versa?
Thanks!
How to disable the annoying box that opens up on VS code, which explains the details of the function, in the middle of typing?
How to delete from a deque in constant time without "pointers"?
There's a technique which I call 'lazy popping' which can help here.
The idea is that you don't delete immediately from the queue. Rather, you leave deleted items in the queue, but mark them as deleted in another data structure -- usually a set. Whenever you have to pop an item to execute, keep popping until you reach an item that hasn't yet been deleted.
This gives you constant-time push, amortized constant-time pop (although you may pop multiple deleted items off the queue each time you pop an item to execute, each item only gets popped exactly once) , and constant-time deletion, which is better than what you can get by maintaining a list and deleting from start or middle.
In this case, you'd save the IDs of deleted items in the set. It looks like this (untested code):
import collections
class DeletableQueue:
def __init__(self):
self.deleted = set()
self.queue = collections.deque()
def push(self, item):
self.queue.append(item)
def pop(self):
# Precondition: there is at least one non-deleted item on the queue.
while id(q[0]) in deleted:
q[0].pop_left() # Discard an already-deleted item.
return q.pop_left() # Return the actual item to pop
def delete(self, item_to_delete):
self.deleted.add(id(item_to_delete)) More on reddit.com Difference between pop and remove
Hevy - Workout Tracker & Planner
The effects of the three different methods to remove an element from a list:
remove removes the first matching value, not a specific index:
>>> a = [0, 2, 3, 2]
>>> a.remove(2)
>>> a
[0, 3, 2]
del removes the item at a specific index:
>>> a = [9, 8, 7, 6]
>>> del a[1]
>>> a
[9, 7, 6]
and pop removes the item at a specific index and returns it.
>>> a = [4, 3, 5]
>>> a.pop(1)
3
>>> a
[4, 5]
Their error modes are different too:
>>> a = [4, 5, 6]
>>> a.remove(7)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: list.remove(x): x not in list
>>> del a[7]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list assignment index out of range
>>> a.pop(7)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: pop index out of range
Use del to remove an element by index, pop() to remove it by index if you need the returned value, and remove() to delete an element by value. The last requires searching the list, and raises ValueError if no such value occurs in the list.
When deleting index i from a list of n elements, the computational complexities of these methods are
del O(n - i)
pop O(n - i)
remove O(n)