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pythoniterator
Kunal Koppula
Kunal Koppula

Reputation: 1

Python: Editing list while iterating over it

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.

Upvotes: 0

Views: 143

Answers (7)

Colonder
Colonder

Reputation: 1576

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

Upvotes: 0

Sushant
Sushant

Reputation: 3669

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

Upvotes: 0

Jordy Van Landeghem
Jordy Van Landeghem

Reputation: 17

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)

Upvotes: 0

Rakesh
Rakesh

Reputation: 82765

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]

Upvotes: 2

Sheldore
Sheldore

Reputation: 39052

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

Upvotes: 0

Bill the Lizard
Bill the Lizard

Reputation: 405745

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.

Upvotes: 1

glibdud
glibdud

Reputation: 7840

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

Upvotes: 0

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