Because it's not backward-incompatible with the behaviour of positive
indices.

Le jeu. 26 nov. 2020 à 05:56, Mathew M. Noel via Python-ideas <
python-ideas@python.org> a écrit :

>
> Why not use list_name[-n%N] whenever you need to use negative indices and
> raise an index out of bounds exception with negative indices like other
> programming languages?
>
>
>
>
> ------------------------------
> *From:* Calvin Spealman <cspea...@redhat.com>
> *Sent:* Thursday, November 26, 2020 1:00 AM
> *To:* Mathew M. Noel
> *Cc:* python-ideas@python.org; m...@pradyunsg.me
> *Subject:* Re: [Python-ideas] Getting rid of FOR loops and simplifying
> cicular conviolutions with Circular Indexing
>
>
> days_of_the_week[14 % 7]
>
> There ya go!
>
>
> On Wed, Nov 25, 2020 at 12:51 PM Mathew M. Noel via Python-ideas <
> python-ideas@python.org> wrote:
>
>> If circular indexing is used then instead of using a double FOR loop to
>> go through a list M times we can iterate from 0 to M*N (where N is the
>> length of the list) !!!
>>
>>
>> Almost all Machine Learning (ML) algorithms iterate for some predefined
>> epochs over a large data-set. So a double FOR loop is extremely common in
>> ML. Using circular indexing gets rid of this extra FOR loop. If we have to
>> iterate 2 times you can iterate using range(-n,n) but in most cases you
>> need to iterate over 10 or more epochs in ML.
>>
>>
>> Most scientific applications of Python involve an outer FOR loop which
>> progressively refines an approximation with an inner FOR loop by going
>> through a list of items. So circular indexing is useful. In the following I
>> discuss increasingly compelling reasons for adopting a circular indexing
>> scheme in Python.
>>
>>
>> Python uses an index of -1 to index the last element in a list. Since -1
>> occurs before 0 we might think of the elements of the linear list are being
>> bent into a circle making the last element occur before the 0th element.
>> Consider a list with n elements: it would be perfectly reasonable to
>> address the element 0 of the list using an index of n since n occurs after
>> n-1 (if we assume that the list is bent into a circle). This feature can
>> prove to be extremely useful. Consider the following example:
>>
>>
>> days_of_the_week = 
>> ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"]
>>
>> It would be nice if
>>
>> days_of_the_week[0]
>>
>> is the same as
>>
>> days_of_the_week[7]
>>
>> is the same as
>>
>> days_of_the_week[14] etc
>>
>> In other words use modular indexing. In other words if the index is outside 
>> the range 0 to n-1, we simply take the remainder when the index is divided 
>> by n as the index.
>> Because of the close relationship between finite length sequences and 
>> periodic sequences this feature might simplify scientific computing(circular 
>> convolution etc).
>>
>> If circular indexing is used then we don't need the arbitrary rule that
>> -1 is the index of the last element. Since -1 is the same as n-1
>> automatically in modular arithmetic.
>>
>>
>> A trivial objection:  "why not use list_name[i%n] whenever we need this
>> feature?" By the same token we could do away with negative indices and use
>> -1%n for example when we need to index with -1!
>>
>> Its unclear why that people have an irrational preference for indices
>> that lie to the left of 0 while strongly rejecting the idea of indices that
>> lie to the right of n-1!
>>
>> Python does not raise a "index out of bound" exception for negative
>> indices like other programming languages. If this negative indexing is a
>> "feature" (although it allows some fatal errors to slip) then indices above
>> n-1 can also be considered a feature!
>>
>> Are there any deep mathematical reasons for adopting  circular
>> convention?
>> Circular convolution is a most important operation in a wide variety of
>> scientific disciplines since the Discrete Fourier Transform (DFT) of the
>> circular convolution of two signals is the product of the transforms.
>> Because of the universal applicability of Fourier ideas in science and the
>> close mathematical relationship between finite length and periodic
>> sequences circular indexing is extensively used in signal processing and
>> mathematics.
>>
>> We can extend the idea of circular indexing to multidimensional arrays. A
>> 2D array can be folded into a cylinder for indexing. Further this cylinder
>> can be folded into a toroid to reduce a triple FOR loop to a single FOR
>> loop. A deep mathematical justification for cylindrical indexing of 2D and
>> in general nD arrays is offered by the fact that n-dimensional DFT reduces
>> n-dimensional circular convolution to element-wise multiplication.
>>
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>
>
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Antoine Rozo
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