4 ms·
But shouldn't it be [-0] to get the last element if [0] denotes the first element?
by KarlKode 6y ago
But shouldn't it be [-0] to get the last element if [0] denotes the first element?
- codeflo 6y agoI think the most mathematically natural way to interpret negative indices is to threat them as numbers modulo list length. Then 0 = length, and -1 = length - 1.
- bvrmn 6y agoC programmers can give you a couple of advises about "correct" modulo on negative numbers.
- nemetroid 6y agoNegative indices have an implied ”len(list)” in front of them. They can be seen as an application of the idea of half-open intervals (0 <= i < len(list)), so it makes sense that they’re not symmetrical.
- naniwaduni 6y agoIs this sensible, or is it an unprincipled ad-hoc justification for a feature that happens to be useful? Or another way: Is this more reasonable than, say, implicitly taking indices mod len(list)? How about implicitly flooring indices? If so, why?
- nemetroid 6y agoI'm not sure on what principles you'd make a principled justification. My most common use of negative indices is for slicing the end of the list, in which context the interpretation similar to len(list) makes sense. E.g., list[:-2] does what you'd expect (the same as list[2:], except from the other end). > implicitly taking indices mod len(list)? Isn't this doing that (plus some bounds checking)? -1 mod 4 evaluates to 3. > implicitly flooring indices? Not sure I understand what this means.
- bvrmn 6y ago[0] denotes an element with offset 0 from the start of the array. [-1] denotes an element with offset -1 from the start of the array.