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Quick question about the built in python list object. Say you have a list with the numbers 0 - 99. You are writing a program that takes the last item in the list and uses it for some other purpose. Is it more efficient to use list[-1] than to use list[99]? In other words, does python iterate through the whole list in either case?
Thanks for your help.
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The index() method has linear runtime complexity in the number of list elements. For n elements, the runtime complexity is O(n) because in the worst-case you need to iterate over each element in the list to find that the element does not appear in it.
Python is a zero-indexed language (things start counting from zero), and is also left inclusive, right exclusive you are when specifying a range of values. This applies to objects like lists and Series , where the first element has a position (index) of 0.
Two common operations are indexing and assigning to an index position. In Python lists, values are assigned to and retrieved from specific, known memory locations. No matter how large the list is, index lookup and assignment take a constant amount of time and are thus O ( 1 ) O(1) O(1).
One of the main advantages of using sets in Python is that they are highly optimized for membership tests. For example, sets do membership tests a lot more efficiently than lists.
Python does not iterate through lists to find a particular index. Lists are arrays (of pointers to elements) in contiguous memory and so locating the desired element is always a simple multiplication and addition. If anything, list[-1] will be slightly slower because Python needs to add the negative index to the length to get the real index. (I doubt it is noticeably slower, however, because all that's done in C anyway.)
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