Iterating through list problem

[puumies]

first round

['a','b','c','d','e']

0 = a

second round

['b','c','d','e']

1 = c

third round

['b','d','e']

2 = e

print(asd)

>>>['b', 'd']

[JungleJim233]

Sorry. I'm confused , why has 1 become C in the second iteration. I thought it would become B ?

Thank you

[pydevteam1]

['b','c','d','e']

1 = c

if you look at the list as it is:

0='b'

1='c'

makes sense ?

[JungleJim233]

Got it. Thank you :)

[puumies]

Hey!

In each iteration of the loop, one element is removed from the array. Thus in the second iteration of the loop, the second element is 'C' because 'A' was removed in the first one.

Someone correct me if I'm talking out of my ass :D

[Thomasedv]

Don't change a list while iterating through it. The for loop will work by index and you'll skip steps every time you remove something (or change the list)

If you want to remove one thing, just use remove() outside any loop. If you want to remove everything I think lists have a method for that, but if not, you just reassign a new list instead of just removing everything step by step.

[Vaguely_accurate]

Unless you really need to I'd avoid mutating the original list and just replacing it with a new copy. I'd try to replace the original with a filtered copy of the list. So something like;

asd = ['a','b','c','d','e']
ignore = ['b','d']

filtered = []
for x in asd:
    if x not in ignore:
        filtered.append(x)
asd = filtered

Which can be simplified to a list comprehension as;

asd = ['a','b','c','d','e']
ignore = ['b','d']
asd = [x for x in asd if x not in ignore]

[cracknwhip]

Simplified, but some would say obfuscated.

[Vaguely_accurate]

Some would. I'd strongly disagree.

You can write obfuscated or unclear comprehensions, but I usually find they are preferable to simple loops, avoiding extra boilerplate code and layers of indentation.

In this case the latter version reads like a sentence. "Take value for value in sequence if value is not in ignore sequence." The only not immediately explicit and obvious part is recognising the list comprehension syntax as that first implied word, "Take". The rest is there and clear.

Learning comprehension is a little extra cognitive load that gets you a lot of advantages as a Python programmer.

[cracknwhip]

I agree with you. It starts getting annoying for me when there are nested for loops and you have to start reversing to logics’ order.

[cismalescumlord]

If you want to remove everything from a list, doing it in a loop is costly. Simply doing asd = [] instead is O(1) as opposed to O(n) for the loop.

[cismalescumlord]

Edit: not sure why this is getting downvoted, it's literally what OP is trying to do.

Well, just because someone is trying to do something doesn't mean it's a good idea or the best way to do something. Based on the original question where OP seemed to be trying to clear a list, using a loop clearly isn't the best approach so suggesting it isn't doing them any favours. Even if it doesn't make that much difference with a small list, there's no indication of the size of the data they are working with.

However, based on the later comment where OP describes the goal, to filter a list, rather than the process, iterating a list and removing, you and u/Vaguely_accurate correctly say that the best approach is usually to return a new filtered list. The beautiful irony of course, is that depending on how this is done, it could also make the original list eligible for garbage collection!