Python delete all elements from a list that are on an odd index

Question

I’m trying to delete all elements from a Python list that are on an odd index of the list. In my case, the result from len(elements) is 40, so I assume after deleting all odd indexes, the total length should be 20. I tried the following code:

indx = 0
for element in elements:
   if indx % 2 == 1:
      elements.remove(element)
   indx += 1

After this, the result of len(elements) is 27, shouldn’t it be 20 since I deleted half of the elements?

Asked By: David Jimenez

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Source

Answer 1

It may be easier to just recreate the list with the indexes you want, e.g., by using a slice:

elements = elements[0::2]

Answered By: Mureinik

Answer 2

Because when you delete an element, it will mess up with the for loop. You can check with this piece of code:

indx = 0
elements = list(range(40))
for element in elements:
    print(element, indx)
    if indx % 2 == 1:
        elements.remove(element)
    indx += 1

print(len(elements))

and it prints:

As you can see, because you are iteratively removing elements, for element in elements do not loop all 40 items.

In this case, it is better to use elements = elements[0::2] like the other answer.

Answered By: Minh-Long Luu

Answer 3

You can create a new list for only the even indexes

new_elements = []
for i in range(len(elements)):
    if i % 2 == 0:
        new_elements.append(elements[i])
elements = new_elements

else you can make a list compare

elements = [elements[i] for i in range(len(elements)) if i % 2 == 0]

Answered By: vinoth raj

Python delete all elements from a list that are on an odd index

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