Split a list into half by even and odd indexes? [duplicate]

Question

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Python program to split a list into two lists with alternating elements

I have a list like this:

list1 = [blah, 3, haha, 2, pointer, 1, poop, fire] 

The output I want is:

list = [3, 2, 1, fire] 

So what I want is to make a list of even elements of the former list. I tried using a for statement and tried to delete 2nth element while appending them to the list, but it didn't work out:

count = 0 for a in list1:  list2.append(a)  if count % 2 = = 1:   list2.pop(count)  print list2 

Any suggestions?

Answer 1

You can use list slicing. The following snippet will do.

list1 = ['blah', 3, 'haha', 2, 'pointer', 1, 'poop', 'fire'] listOdd = list1[1::2] # Elements from list1 starting from 1 iterating by 2 listEven = list1[::2] # Elements from list1 starting from 0 iterating by 2 print listOdd print listEven 

Output

[3, 2, 1, 'fire'] ['blah', 'haha', 'pointer', 'poop'] 

Answer 2

This should give you what you need - sampling a list at regular intervals from an offset 0 or 1:

>>> a = ['blah', 3,'haha', 2, 'pointer', 1, 'poop', 'fire'] >>> a[0:][::2] # even ['blah', 'haha', 'pointer', 'poop'] >>> a[1:][::2] # odd [3, 2, 1, 'fire'] 

Note that in the examples above, the first slice operation (a[1:]) demonstrates the selection of all elements from desired start index, whereas the second slice operation (a[::2]) demonstrates how to select every other item in the list.

A more idiomatic and efficient slice operation combines the two into one, namely a[::2] (0 can be omitted) and a[1::2], which avoids the unnecessary list copy and should be used in production code, as others have pointed out in the comments.