Split a list into half by even and odd indexes?

Question

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

Problem

Given a list like this:

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

I expect to get this output:

list = [3, 2, 1, fire]

So what I want is to make a list of even elements of the former list.

What I tried

I tried using a for statement and tried to delete 2nd 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

This question (and it's answers) are both better and more generic than the question which it is marked a duplicate of. — Waylon Flinn, Apr 17 '15 at 13:43
And this answer to a third question is the best: http://stackoverflow.com/a/19153541/74291 — Waylon Flinn, Apr 17 '15 at 13:52

Diego Allen · Answer 1 · 2012-07-28T15:54:46.560

62

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']

edited Jul 28 '12 at 15:54

answered Jul 28 '12 at 15:48

Diego Allen

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milancurcic · Accepted Answer · 2016-12-19T15:32:07.710

47

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.

edited Dec 19 '16 at 15:32

answered Jul 28 '12 at 15:47

milancurcic

6,202
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1

thank you very much :) i didn't know this kind of thing existed! it really helped – H.Choi Jul 28 '12 at 15:57
1

@H.Choi: this is covered very early in every tutorial, including [the official one](http://docs.python.org/tutorial/). You're going to be missing out on a lot of cool stuff if you don't read one.. – DSM Jul 28 '12 at 16:03
17

Why `[0:][::2]` and `[1:][::2]` instead of `[::2]` and `[1::2]`? This will copy the list unnecessarily. – senderle Jul 28 '12 at 16:30
4

-1 with this syntax. It is unnecessary and non-ideomatic. Use the idiomatic Python slice `[::2]` for even and `[1::2]` for odd. – dawg Jul 28 '12 at 16:53
I think the answer below is easier to understand. Combining two concepts in one answer confused me. – Ray Salemi May 24 '19 at 18:20

score 5 · Answer 3 · answered Jul 28 '12 at 15:51

5

You can just slice the list: For odd : a[1::2] For even : a[::2]

answered Jul 28 '12 at 15:51

Arsh Singh

2,038
15
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Split a list into half by even and odd indexes?

Problem

What I tried

3 Answers3

Linked