迭代时减少列表索引超出范围错误列表

Question

I think I'm getting an error because the range of the list is reduced during iteration of the loop. I have 95 nested lists within a large list mega4 , and I'm trying to delete some strings with if and else. The list ufields consist of 18 strings.

>>> for i in range(len(mega4)):
...     for j in range(len(mega4[i])):
...         for f in ufields:
...             if (f in mega4[i][j]) is False:
...                 mega4[i].remove(mega4[i][j])
...             else:
...                 pass
... 
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
IndexError: list index out of range

The fourth line is basically asking if each element within mega4[i] has each strings within ufields , how can I remove the mega4[i][j] in this case?

Edit:

I followed comments below and tried building a new list but this one does not seem to work

>>> mega5 = []
>>> for i in range(len(mega4)):
...     for f in ufields:
...         mega5.append([x for x in mega4[i] if f in x is True])

len(mega5) is larger than len(mega4) whereas it should be the same.

Answer 1

1. Depending on how ufields is defined, you could eliminate the innermost loop by using if mega4[i][j] in ufields .

2. Instead of modifying mega4 within this loop, you could build up a list of what elements you want to eliminate, and then do the actual elimination afterwards (looping over your list of candidates instead of mega4 itself).

Answer 2

Simpler way:

result = [[sub for sub in item if sub] for item in mega4]

Your way wasn't working because you were editing list while iterating over it.