Hi there! I'm trying to leave out a specific integer in a list and remove every other number
nr = 9
my_list = [0,5,6,7,6,9,9,1,9]
for i in my_list:
if i is not nr:
my_list.remove(i)
print(my_list)However I get [5, 7, 9, 9, 9] instead of [9,9,9]
What am I doing wrong?
If you modify an object while iterating over it, it can cause problems.
Either iterate over a copy of it, or copy the data you do need and don't copy the rest. Examples:
Iterate over a copy:
nr = 9
my_list = [0,5,6,7,6,9,9,1,9]
for i in my_list[:]:
if i is not nr:
my_list.remove(i)
print(my_list)Pull out the data you want, and overwrite the old reference
nr = 9
my_list = [0,5,6,7,6,9,9,1,9]
my_list = [x for x in my_list if x == nr]
print(my_list)
As bowlofred mentions, the two examples not only solve the problem in different ways, but produce different results. The difference may be an unimportant side-effect, or it might break your code.
nr = 9
numbers = [0,5,6,7,6,9,9,1,9]
# This createa a new list
nines = [x for x in numbers if x == nr]
print('New List', nines, numbers)
# This code modifies the list
nines = numbers
for x in nines[:]:
if x != nr:
nines.remove(x)
print('Modifies', nines, numbers)Output:
New List [9, 9, 9] [0, 5, 6, 7, 6, 9, 9, 1, 9]
Modifies [9, 9, 9] [9, 9, 9]
Notice that the list comprehension does not change the original list. The remove.() for loop looks clunky, but it does modify the original list. All references to the list will see the changes.
Thanks deanhystad!
By the way, in the block that modifies a list, you added:
nines = numbers
for x in nines[:]:
rather than going straight to
for x in numbers[:]
Is there any particular reason for it?
Sorry for all the noobish questions :)
(Dec-19-2020, 08:31 AM)GJG Wrote: [ -> ]Is there any particular reason for it?
I guess he did it in order to preserve the original list and be able to show the original unmodified list to you afterwards.
Ops, that was stupid comment on my side. It shows that
nines and
numbers are actually the same [mutable] object and when modify mutable objects you need to be careful.
nines = numbers does not creaty a copy of
numbers, it just binds name
nines to same object as
numbers
So basically there's no point for it..?
(Dec-19-2020, 04:55 PM)GJG Wrote: [ -> ]So basically there's no point for it..?
yes
nr = 9
numbers = [0,5,6,7,6,9,9,1,9]
for x in numbers[:]:
if x != nr:
numbers.remove(x)
print(numbers)Output:
[9, 9, 9]
by the way - you can try it yourself. don't hesitate to experiment and play with the code.