Question

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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).

Answer 1

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.