python at while loop IndexError: list index out of range

Question

def solution(progresses, speeds):
    answer = []
    while len(progresses) != 0:
        count = 0
        for i in range(len(progresses)):
            if progresses[i] < 100:
                progresses[i] += speeds[i]
        while (progresses[0] >= 100) and (len(progresses) != 0):
            print("pop: ", progresses.pop(0))
            speeds.pop(0)
            count += 1
        if count != 0:
            answer.append(count)

    return answer


solution([93, 30, 55], [1, 30, 5])

my python terminal show me an error like this.

    while (progresses[0] >= 100) and (len(progresses) != 0):
IndexError: list index out of range

i dont know why this error comes up..... help me

Don't modify lists while you're iterating over them. You're poping everything off the list, eventually there's nothing left and then you have an index error at 0 because the list is empty. — Layne B
– Layne B, Commented Apr 6, 2022 at 13:53

ucczs · Accepted Answer · 2022-04-07 06:04:13Z

1

As mentioned in other comments you should avoid modifying the list which is used in the while or for loop checks.

If you want to understand why your code is failing. The problem is this line:

while (progresses[0] >= 100) and (len(progresses) != 0):

You are accessing element zero (progresses[0]) but the list is empty. If you swap the two checks, it works fine (because if the first check already returns false the second check is not performed at all).

while (len(progresses) != 0 and progresses[0] >= 100):

edited Apr 7, 2022 at 6:04

answered Apr 6, 2022 at 14:15

ucczs

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Comments

Aron Atilla Hegedus · Accepted Answer · 2022-04-06 14:10:01Z

1

Agree with the comment. If you want, you can do import copy and copy_of_speeds = copy.deepcopy(speeds), which will make a copy of the list that isn't tied to it, which you could loop over instead.

Your code also has the bug in the code, where

while progresses[0] >= 100:
            progresses.pop(0)

could potentially have progresses pop all the way to an empty list, and then progresses[0] hits that index error!. So if you do

while len(progresses) > 0 && progresses[0] >= 100 instead, which won't hit that issue.

answered Apr 6, 2022 at 14:10

Aron Atilla Hegedus

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1 Comment

Aron Atilla Hegedus Over a year ago

In the future, why not try printing information in your code as the code is running, which will mean that you can find out what's going on

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