Change multiple items in a list

Question

I want to change ,multiple values in a list, for example, every multiple of 2. Using slicing. my logic is:

list = [0] * 10
list[::2] = 1

However, I get an error message: " must assign iterable to extended slice" Can someone explain the error and also the correct logic to preform something like this? Thanks.

The slicing you did will return a list, and you can't just set that list equal to 1. — SuperStew, Nov 27 '17 at 19:59
@SuperStew no, that *does not return a list at all*. That is slice-based assignment. — juanpa.arrivillaga, Nov 27 '17 at 19:59
@SuperStew that doesn't follow at all. the expression `my_list[] = ` **never creates an intermediate list**. Instead, it passes `` as an argument to `my_list.__setitem__`. It is simply incorrect to state that a `list` is returned. — juanpa.arrivillaga, Nov 27 '17 at 21:44

score 2 · Answer 1 · edited Nov 27 '17 at 20:05

2

When you assign to a slice of a list, you need the assignment to be a list of the same length as the slice. For your example, assign a list of 5 ones:

l = [0] * 10
l[::2] = [1] *5

It isn't obvious why in this example, but if you think about it, you were doing:

l[3:6] = 2

Obviously that doesn't make sense. You are trying to assign an int to a list, which won't work. l[::2] is just another way to slice a list, so you must assign a list to it.

In the future, don't name your lists "list" because doing so overrides the builtin list() function.

edited Nov 27 '17 at 20:05

Joe Iddon

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answered Nov 27 '17 at 19:59

Sam Craig

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*l[::2] is just another way to slice a list, so you must assign a list to it.* Well, not really. When this is on the left hand of an assignment statement it's hooking into `list.__setitem__`. That could be implemented to accept a scalar. – wim Nov 27 '17 at 20:04
@wim it obviously isn't implemented that way though. We are working with the Python built ins, not some library which you could change. – Sam Craig Nov 27 '17 at 20:05
3

It's still incorrect statement regardless. You can for example assign the string 'hello' here. – wim Nov 27 '17 at 20:07

score 2 · Answer 2 · answered Nov 27 '17 at 20:05

my_list[::2] has 10//2 (=5) elements, so the right part of the assignment should have 10//2 elements as well:

>>> my_list = [0] * 10
>>> my_list[::2] = [1]*(10//2)
>>> my_list
[1, 0, 1, 0, 1, 0, 1, 0, 1, 0]

Or you could use numpy with broadcasting:

>>> import numpy as np
>>> a = np.zeros(10)
>>> a
array([ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.])
>>> a[::2] = 1
>>> a
array([ 1.,  0.,  1.,  0.,  1.,  0.,  1.,  0.,  1.,  0.])

Change multiple items in a list

2 Answers2