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Noob Alert! Wrong result using loop and if statemnent - Printable Version

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Noob Alert! Wrong result using loop and if statemnent - GJG - Dec-18-2020

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?

RE: Noob Alert! Wrong result using loop and if statemnent - bowlofred - Dec-18-2020

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)


RE: Noob Alert! Wrong result using loop and if statemnent - GJG - Dec-18-2020

Great! thanks bowlofred!

RE: Noob Alert! Wrong result using loop and if statemnent - deanhystad - Dec-18-2020

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.

RE: Noob Alert! Wrong result using loop and if statemnent - GJG - Dec-19-2020

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 :)

RE: Noob Alert! Wrong result using loop and if statemnent - buran - Dec-19-2020

(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

RE: Noob Alert! Wrong result using loop and if statemnent - GJG - Dec-19-2020

So basically there's no point for it..?

RE: Noob Alert! Wrong result using loop and if statemnent - buran - Dec-19-2020

(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.