What is the big-o notation for the `len()` function in Python?

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Cost of len() function

Does len() iterate over the objects in a list and then return their count? Thus giving it a O(n).

Or....

Does a python list keep a count of any objects that are appended to it and removed from it and then simply return this "count" when len() is called? Thus giving it O(1).

it's `O(1)` this is what you need : http://wiki.python.org/moin/TimeComplexity — Ashwini Chaudhary, Sep 09 '12 at 20:02

score 16 · Accepted Answer · answered Sep 09 '12 at 20:00

16

A Python list knows its own length; len takes O(1) time. Lists are actually arrays, not linked lists as in Lisp, where length takes linear time.

answered Sep 09 '12 at 20:00

Fred Foo

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"proof" http://wiki.python.org/moin/TimeComplexity – mgilson Sep 09 '12 at 20:00

score 11 · Answer 2 · answered Sep 09 '12 at 20:00

11

For all built-in objects that define __len__(), it will be O(1). If you implement __len__() for your own objects, it might be anything.

answered Sep 09 '12 at 20:00

Sven Marnach

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What is the big-o notation for the `len()` function in Python?

2 Answers2