Python: Editing list while iterating over it

问题

There are some related questions on Stack but I wanted to be as clear as possible. I am using Python 3

If I have a list, N, and I use the following code:

N = [1,2,3,4,6,7,8]
for x in N:
    N[x] = N[x] * -1
return N

I am getting a Index out of range error. I understand you shouldn't be adding and deleting elements while iterating over a list, but I wanted a clear definition of why the above example won't work.To me, it seems like there shouldn't be a problem. In the first iteration, x should evaluate to 1. So if I want to edit N[1], I don't see why I wouldn't be able to.

As a side note, I know enumerate() is the proper way to do it.

回答1:

Use enumerate

Ex:

N = [1,2,3,4,6,7,8]
for x, value in enumerate(N):
    N[x] = value * -1
print(N)

or a list comprehension.

Ex:

N = [x * -1 for x in N]

回答2:

In for x in N:, x takes on each value in N, then you use it like an index. Lists in Python are 0-indexed, so you get an Index out of range error when you reach the end of the list and try to access N[8], which doesn't exist. You can use for x in range(len(N)): instead.

回答3:

The problem here is actually a little different: you're treating members of the list as if they were indices of the list. Since N[8] doesn't exist, that's where your error is coming from. I think what you meant to do was:

N = [1,2,3,4,6,7,8]
for x in range(len(N)):
    N[x] = N[x] * -1
return N

回答4:

In the first iteration, x=1 which means N[1] equals 2 and so your new N[1] becomes 2 * -1 = -2. Now when x is 8 since you are using for x in N, your code tries to access N[8] but since the length of N is 8, the indexing starts from 0 and goes up to 7. Hence there is no index 8 and hence you get an error Index out of range error

回答5:

Use good ol' lambda's or a specific callable in list comprehension:

lambda:

N = [1,2,3,4,6,7,8]
M = [(lambda x:-x)(element) for element in N]
print(M)

callable/function:

def negate(x):
    return -x

N = [1,2,3,4,6,7,8]
M = [negate(element) for element in N]
print(M)

回答6:

In your code here -

N = [1,2,3,4,6,7,8]
for x in N: 
# x will be 1, 2, 3, 4

And the way you are accessing is -

N[x]

But indices run from 0 to n-1, where n-1 is the last element. Also, in your list, you are missing a 5 so indices will change badly. If you had

N = [1,2,3,4,5,6,7,8]

your code would have worked fine. But since that's not the case, you'd have to use range like -

for i in range(len(N)):
    N[i] = N[i] * -1

回答7:

You are enumerating in a wrong way

N = [1,2,3,4,6,7,8]
for i, x in enumerate(N):
    N[i] = N[i] * -1
return N

来源：https://stackoverflow.com/questions/51633619/python-editing-list-while-iterating-over-it

标签

python

iterator