Get every third element from a list

Question

For example if I have a list like the following

lst = [1, 10, 100, 2, 20, 200, 3, 30, 300];

How could I get back [1, 2, 3]? Basically it would be starting from the first number, and every third number.

Is the fact that the element is a single digit the only condition you have? — Lix, Dec 08 '13 at 19:09
@AshwiniChaudhary - all of the information was in the original version. The text speaking about "a single digit" was referring to the possible regex that would match `1,2,3` - but not the condition of "every third element". — Lix, Dec 08 '13 at 19:18
@Lix I just removed the `regex` tag, I am not sure what are you referring to. All the [other changes](http://stackoverflow.com/posts/20457638/revisions) were made by OP himself. — Ashwini Chaudhary, Dec 08 '13 at 19:20
@AshwiniChaudhary - I'm talking about your (now deleted) comment. The OP's edit didn't render your answer invalid - it didn't change the underlying question. — Lix, Dec 08 '13 at 19:22

score 9 · Accepted Answer · edited May 23 '17 at 12:15

9

The step argument using slice notation.

>>> lst[::3]
[1, 2, 3]

Update:

Take a look at this post: Python's slice notation, it will explain further in detail.

edited May 23 '17 at 12:15

Community

answered Dec 08 '13 at 19:07

hwnd

Do you have a psychic crystal ball debugger that we are not aware of? – Lix Dec 08 '13 at 19:10
Haha, I wish, I just looked at the list, he said 1, 2, 3 and assumed I suppose. – hwnd Dec 08 '13 at 19:11
Well, you're the only one that got it :P +1 – Lix Dec 08 '13 at 19:12

score 0 · Answer 2 · answered Dec 08 '13 at 19:20

0

if don;t exactly understand your question , you could use the built in sorted function to sort it and then get the first 3 elements

s = sorted(lst)[:3]

if you want 1 or 2 or 3 or whatever its there in the list

[x for x in lst if x in range(1,4)]

Hope this helps

answered Dec 08 '13 at 19:20

karthik27

This is not what the OP is asking for. The question states that the output should be every third element starting from (and including) the first element. `"starting from the first number, and every third number"` – Lix Dec 08 '13 at 19:26

score 0 · Answer 3 · answered Dec 08 '13 at 19:32

0

lst = [1, 10, 100, 2, 20, 200, 3, 30, 300]

print [lst[i] for i in xrange(0, len(lst), 3)]

answered Dec 08 '13 at 19:32

smac89

score -1 · Answer 4 · answered Dec 08 '13 at 19:07

-1

for item in lst:
    if item in [1,2,3]:
        new_lst.append(item)

or shorter using list comprehension:

new_lst=[item for item in lst if item in [1,2,3]]

answered Dec 08 '13 at 19:07

yemu

score -1 · Answer 5 · answered Dec 08 '13 at 19:08

-1

Simple list comprehension:

[x for x in lst if x < 10 and x >= 0]

answered Dec 08 '13 at 19:08

mchfrnc

5 Answers5