change odd elements in list to negative

Question

I am trying to change odd elements in a list of int to negative using slicing

l[1::2] = -1 * l[1::2]

However, I encounter the following error:

ValueError: attempt to assign sequence of size 0 to extended slice of size 2

Asked By: user20250841

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Answer 1

Best way would be to do it vectorized with numpy. Without numpy, I don’t see a way to do it without creating a new loop. Here you have two examples of how to do it. With and without numpy:

import numpy as np

# vectorized
a = np.arange(10)
a[1::2] *= -1
print(a)
# >>> [ 0  -1 2  -3 4  -5 6  -7 8  -9]

a = [i for i in range(10)]
a = [i if i%2==0 else i*-1 for i in a] # need to loop through list again..
print(a)
# >>> [0, -1, 2, -3, 4, -5, 6, -7, 8, -9]

Edited to fix odd elements to negative, not pos.

Answered By: Steinn Hauser Magnusson

Answer 2

If you wish to uses slices:

>>> a = [1,4,7,8,2]
>>> a[1::2] = [-x for x in a[1::2]]
>>> a
[1, -4, 7, -8, 2]

When assigning to an extended slice like a[1::2], the list/slice on the right hand side must be the same length as the left hand side. In the above, we’ve mapped to a[1::2] with a list comprehension, so the length is unchanged. If we consider -1 * a[1::2] this yields [] which is of a different length.

Answered By: Chris

Answer 3

You can fix it like this:

mylist[1::2] = [-i for i in mylist[1::2]]

Answered By: Gudarzi

change odd elements in list to negative

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