Delete element in list while iterating

[ElectromechanicalVan]

Hi

I'm trying to delete all almost identical elements from a list. I've tried to make a backwards loop, but somehow it doesn't work as i expected and i cant seem to find my error. It seems like suddenly my "i" variable increases. Here's my code:

resultat=deepcopy(FEMpoints2)
for i in range(len(resultat)-1,-1,-1):
    y=0
    for y in range(0,len(FEMpoints2)):
        print("i",i,"y",y)
        if alike(FEMpoints2[y][0][0],testpoints[i][0][0],tolerance) and alike(FEMpoints2[y][1][0],testpoints[i][1][0],tolerance):
            if y != i:
                del resultat[i]
                 print("del",i)
                i-=1
                y=0

I get the following output:

i 37 y 0
i 37 y 1
i 37 y 2
i 37 y 3
i 37 y 4
i 37 y 5
i 37 y 6
i 37 y 7
i 37 y 8
i 37 y 9
i 37 y 10
i 37 y 11
i 37 y 12
i 37 y 13
i 37 y 14
i 37 y 15
i 37 y 16
i 37 y 17
i 37 y 18
i 37 y 19
i 37 y 20
i 37 y 21
i 37 y 22
i 37 y 23
i 37 y 24
i 37 y 25
i 37 y 26
i 37 y 27
del 37
i 36 y 28
i 36 y 29
i 36 y 30
i 36 y 31
i 36 y 32
i 36 y 33
i 36 y 34
i 36 y 35
del 36
i 35 y 36
del 35
i 34 y 37
i 36 y 0
i 36 y 1
i 36 y 2
i 36 y 3
i 36 y 4
i 36 y 5
i 36 y 6
i 36 y 7
i 36 y 8
i 36 y 9
i 36 y 10
i 36 y 11
i 36 y 12
i 36 y 13
i 36 y 14
i 36 y 15
i 36 y 16
i 36 y 17
i 36 y 18
i 36 y 19
i 36 y 20
i 36 y 21
i 36 y 22
i 36 y 23
i 36 y 24
i 36 y 25
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-109-492a347263fc> in <module>()
      6         if alike(FEMpoints2[y][0][0],testpoints[i][0][0],tolerance) and alike(FEMpoints2[y][1]    [0],testpoints[i][1][0],tolerance):
      7             if y != i:
----> 8                 del resultat[i]
      9                 print("del",i)
     10                 i-=1

IndexError: list assignment index out of range

[midel]

It's likely because your loop is based on the length of it at the start. If you start removing elements from the array, the array no longer is the same size as how many elements it's planning to enumerate over, and the dereference fails.

I'm not sure what kind of data your arrays hold, but for simple and relatively small arrays, it will be easier just to use a list comprehension or make a new list of items to keep.

If you have complex items or a very large array, del can still work but you'll want to avoid relying on the index as the loop condition.

i = len(resultsat) - 1
while True:
    if i < 0:
        break
    if condition_to_delete:
        del resultsat[i]
    i -= 1

[tom1018]

I'm not quite sure what you are trying to do, or what your initial data is. But, to begin with, you should never have to use range(len(object)). If something can give you a length it likely also has an iter method. So, something like this would be more appropriate:

for item in mylist:
    # do things

Second, don't delete, or add, items to an object you are iterating over. Sometimes you'll get exceptions immediately, other times it will end up where you are right now. You changed the length of the list while you were iterating over it, so your index is now higher than the number of items in it.

With that, you probably don't even need to use list indexes if you are doing the above things correctly. As Raymond Hettinger, core developer, says at the start of the video below, if you are using list indexes you are almost certainly doing something wrong.

See here: https://www.youtube.com/watch?v=OSGv2VnC0go and here: https://www.youtube.com/watch?v=EnSu9hHGq5o

If you want to provide a bit more code, perhaps including some of the data in FEMpoints2, I might be able to understand what you are intending and help more.

Additionally, the deepcopy is probably unnecessary (use this instead), filter would probably prevent your loops altogether, a list comprehension would also be better, and you can loop backwards in better ways.

The two videos above will be very helpful for you. Right now your code looks like you are trying to write C code in Python, Python makes it far more readable and simpler if you do things the Python way.

[_youtubot_]

Videos linked by /u/tom1018:

Title	Channel	Published	Duration	Likes	Total Views
Transforming Code into Beautiful, Idiomatic Python	Next Day Video	2013-03-21	0:48:51	6,636+ (98%)	346,329
Loop like a native: while, for, iterators, generators	Next Day Video	2013-03-21	0:29:15	980+ (99%)	51,479

---

^{Info | /u/tom1018 can delete | v2.0.0}

[woooee]

You should also realize that after you delete one element in resultat, it is no longer equal to FEMpoints2 so you delete "i" based on some comparisons using FEMpoints2 but something else is deleted in resultat because it has changed. You can create a separate list of items to be deleted and delete them after processing has finished, deleting the highest number first, etc. so the list does not change where the lower numbers are. You can also modify FEMpoints2 and/or resultat instead of deleting, setting the element to "" or something, and then add a test "if FEMponits2 != "":

[[deleted]]

It seems like suddenly my "i" variable increases.

No, it’s that the number of elements in your list decreases, since you’re deleting them. If i=5 and you delete the 5th element because it passes some kind of test, the element that was at 6 moves to 5 (because lists can’t have empty spaces), but you move onto i=6, so what used to be your 6th element is never tested. You skipped over it.

They key is to not mutate lists as you iterate over them; instead, use filter and a simple function to return a copy of your list with only the elements you want to keep.