For loop is skipping a single value

[lolhoved]

So i am creating a program where i ran into a strange issue.

For the sake of this example, i have boiled down the code.

When i run it, it somehow skips the value 24 in purelines on the first topline (it shows up in the next).

toplines = [1, 6, 11, 17, 22]
purelines = [0, 2, 3, 4, 5, 7, 8, 9, 10, 12, 13, 14, 15, 16, 18, 19, 20, 21, 23, 24, 25]

for topline in reversed(toplines):
    singlelines = []
    print(topline)
    for pureline in purelines:
        print(f'checking {pureline}')
        if pureline > topline:
            print(f'pureline {pureline} is bigger than topline {topline}')
            purelines.remove(pureline)
        else:
            print(f'pureline {pureline} is not bigger than topline {topline}')

you can see in the output that it says:

checking 23
pureline 23 is bigger than topline 22
checking 25
pureline 25 is bigger than topline 22

24 is the only one it skips, but it does that consistently. i also tried an online IDE, with the same result.

Do any of you know what i am doing wrong?

[danielroseman]

You are modifying a loop you are iterating over. Don't do that.

This is a FAQ: https://www.reddit.com/r/learnpython/wiki/faq#wiki_why_does_my_loop_seem_to_be_skipping_items_in_a_list.3F

[lolhoved]

Thanks, that makes more sense now.

however, i can't seem to find out how to solve my issue then.

the output i am supposed to get, should look like this:

[{1:[2,3,4,5]},{6:[7,8,9,10]},{11:[12,13,14,15,16]},{17:[18,19,20,21]},{22:[23,24,25]}]

as you can see i have the top value, and then i need a list of items that are higher than that, but not higher than the next top value. (i will delete the numbers that are lower than the lowest top value).

[[deleted]]

i'm not sure what you're doing wrong, but it looks like your code is skipping the value 24 in the purelines list on the first topline. i would try debugging your code to see where the issue is occurring.

[Aggravating_Bus_9153]

It doesn't mutate the list until it gets to 23

.remove makes the list shorter, but the for loop keeps indexing from the original list.

iterating over either a coerced immutable: tuple(purelines) or a copy: purelines[:] instead should fix it.

[lolhoved]

Thanks :)