Modify a list while iterating

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You are not modifying the list, so to speak. You are simply modifying the elements in the list. I don’t believe this is a problem.

To answer your second question, both ways are indeed allowed (as you know, since you ran the code), but it would depend on the situation. Are the contents mutable or immutable?

For example, if you want to add one to every element in a list of integers, this would not work:

>>> x = [1, 2, 3, 4, 5]
>>> for i in x:
...     i += 1
... 
>>> x
[1, 2, 3, 4, 5]

Indeed, ints are immutable objects. Instead, you’d need to iterate over the indices and change the element at each index, like this:

>>> for i in range(len(x)):
...     x[i] += 1
...
>>> x
[2, 3, 4, 5, 6]

If your items are mutable, then the first method (of directly iterating over the elements rather than the indices) is more efficient without a doubt, because the extra step of indexing is an overhead that can be avoided since those elements are mutable.

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