How to iterate over the first n elements of a list?

Question

Say I've got a list and I want to iterate over the first n of them. What's the best way to write this in Python?

Mike Graham · Accepted Answer · 2010-04-22T03:53:28.527

122

The normal way would be slicing:

for item in your_list[:n]: 
    ...

edited Apr 22 '10 at 03:53

answered Apr 22 '10 at 03:45

Mike Graham

6

Note that this creates a copy of the first `n` elements of the list, which may be slow and memory intensive for large lists. `itertools.islice` is far more efficient (and also works with any iterable). – BallpointBen Apr 17 '18 at 22:10

score 41 · Answer 2 · edited Jan 26 '21 at 10:25

41

I'd probably use itertools.islice (<- follow the link for the docs), which has the benefits of:

Usage:

import itertools

n = 2
mylist = [1, 2, 3, 4]
for item in itertools.islice(mylist, n):
    print(item)

outputs:

1
2

One downside is that if you wanted a non-zero start, it has to iterate up to that point one by one: https://stackoverflow.com/a/5131550/895245

Tested in Python 3.8.6.

edited Jan 26 '21 at 10:25

answered Apr 22 '10 at 03:41

Michał Marczyk

2

Note that when you have a list, it's usually simpler just to use slicing (unless you have to worry about memory usage issues or something like that). If this wasn't the *first* chunk but if it was some later chunk, normal slicing can be faster as well as nicer-looking. – Mike Graham Apr 22 '10 at 03:51
Fair enough. Plus regular slicing is more concise, which the OP apparently cares about... – Michał Marczyk Apr 22 '10 at 04:13

score 13 · Answer 3 · edited Oct 02 '10 at 00:33

13

You can just slice the list:

>>> l = [1, 2, 3, 4, 5]
>>> n = 3
>>> l[:n]
[1, 2, 3]

and then iterate on the slice as with any iterable.

edited Oct 02 '10 at 00:33

Mike Graham

answered Apr 22 '10 at 03:46

ezod

score 2 · Answer 4 · answered Apr 22 '10 at 03:40

2

Python lists are O(1) random access, so just:

for i in xrange(n):
    print list[i]

answered Apr 22 '10 at 03:40

Michael Mrozek

8

Tinkering with indices is usually something worth striving to avoid. – Mike Graham Apr 22 '10 at 03:52
1

Yeah, slicing is better; I didn't think of it – Michael Mrozek Apr 22 '10 at 04:10

4 Answers4