I got a 2 dimensional list:

[[5, 80, 2, 57, 5, 97], [2, 78, 2, 56, 6, 62], [5, 34, 3, 54, 6, 5, 2, 58, 5, 61, 5, 16]]

In which I need to change every second element to 0, starting from first one. So it should look like this:

[[0, 80, 0, 57, 0, 97], [0, 78, 0, 56, 0, 62], [0, 34, 0, 54, 0, 5, 0, 58, 0, 61, 0, 16]]

Algorithm I use:

for i in tempL: for j, item in enumerate(i):if i.index(item) % 2 == 0:print('change, index:'),print(i.index(item))i[j] = 0else:print('not change, index:'),print(i.index(item))

But what I get is this:

change, index: 0
not change, index: 1
change, index: 2
not change, index: 3
change, index: 4
not change, index: 5
change, index: 0
not change, index: 1
change, index: 2
not change, index: 3
change, index: 4
not change, index: 5
change, index: 0
not change, index: 1
change, index: 2
not change, index: 3
change, index: 4
not change, index: 5
change, index: 6
not change, index: 7
not change, index: 5
not change, index: 9
not change, index: 5
not change, index: 11
[[0, 80, 0, 57, 0, 97], [0, 78, 0, 56, 0, 62], [0, 34, 0, 54, 0, 5, 0, 58, 5, 61, 5, 16]]

Some elements are not changed, and it's because (I added index print to see that) it thinks that index of those elements are 7 and 9 for some reason. What can it be, because I am looking for a bug for so long still cannot find..

I double checked, there are not extra spaces or anything in the list.

Well, this task should be obvious. Use slice assignment! You need to assign an array of zeros, that are half length. To create one, simply multiply single element array with value:

for l in tempL: l[::2] = [0] * ((len(l)+1)/2)

Or use repeat from itertools (unfortunately this is twice slower for small array):

from itertools import repeatfor l in tempL: l[::2] = repeat(0,(len(l)+1)/2)