list.delete() affecting for-loop in python

Question

I have just discovered a very strange behaviour of list.delete() in my program. I am currently doing the second problem of the Project Euler, in which you have to get the sum of all even fibonacci numbers until 4 million. I created all of them in a list adn then wanted to iterate through them in order to find out all not even numbers and remove them. Here is the code I have:

print(fibonacci)

for zahl in fibonacci:
    print(zahl, end=' ')
    if zahl%2 != 0:
        print('not even')
        fibonacci.remove(zahl)
    else:
        print('even')

Which gives:

[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025, 121393, 196418, 317811, 514229, 832040, 1346269, 2178309, 3524578]
1 not even
2 even
3 not even
8 even
13 not even
34 even
55 not even
144 even
233 not even
610 even
987 not even
2584 even
4181 not even
10946 even
17711 not even
46368 even
75025 not even
196418 even
317811 not even
832040 even
1346269 not even
3524578 even
7465174

Strangely some numbers are missing!

5 and 377 for example (and some more). Now here it comes. If I comment out the fibonacci.delete(zahl) line, they are printed there, declared correctly as not even.

Where is this behavoiour coming from. Could it come from me affecting the list I'm iterating through?

I an not sure about that, but I'd really like to know. I googled for this phenomenon , but didn't find anything that was solveing my problem or explaining where it comes from.

I' like to know two things

Is the behaviour coming from me affecting the list I'm iterating through?
How can I correct that?

Could it come from me affecting the list I'm iterating through? YES — mehrdadep, Jan 21 '18 at 09:37
This is __not a duplicate__ since I want to understand the reasun as well why mya code didn't work and I want to know why only some numbers didn't appear, and (just remarked it) all of them are not even! — monamona, Jan 21 '18 at 09:46

ritchie46 · Accepted Answer · 2018-01-21T09:45:45.083

2

This is because of you removing elements from the list you're iterating over indeed. If you comment out the .remove part, you'll see that all numbers are printed. If the underlying iteration function (__iter__ and __next__), accesses the list by index and you are removing elements of the list, the indexes change.

A function that is perfectly suited for your problem is filter. This function filters the values from a list that match the condition. In your case you'll want all even numbers. You can do this:

print(list(filter(lambda x: x % 2 == 0, fibonacci)))

edited Jan 21 '18 at 09:45

answered Jan 21 '18 at 09:40

ritchie46

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That was it! I used it, and the answer was correct! Thank you very much for your suggestion! – monamona Jan 21 '18 at 09:44
But why are some numbers missing when I did it my way? – monamona Jan 21 '18 at 09:46
1

See, my edited answer. The elements aren't printed because the indexes change due to your deletion. – ritchie46 Jan 21 '18 at 09:48
I would have already but you have to wait like 5 minutes! :) – monamona Jan 21 '18 at 09:48

list.delete() affecting for-loop in python

1 Answers1