[swift-evolution] [Pitch] 'Double modulo' operator

[Stephen Canon]

I’m not really sold on the `%%` spelling, but I think the operation itself is worth exposing.  This is the remainder of a “flooring” division (as opposed to the C-family “truncating” division[1]).  If we do provide it, we should also provide the accompanying divide operation.

– Steve

[1] there are several other ways to define division beyond these two: remainder is always positive, remainder is closest to zero, etc.  Truncating and flooring division are the most common by a wide margin, however.

> On May 21, 2016, at 4:22 PM, Adam Nemecek via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hello, 
> 
> I think that Swift could use the 'double modulo' operator which is for example in CoffeeScript (some discussion can be found here https://github.com/jashkenas/coffeescript/issues/1971 <https://github.com/jashkenas/coffeescript/issues/1971>).
> 
> This operator, unlike normal modulo, takes sign from the divisor, not the dividend e.g. -10 % 3 == -1, but -10 %% 3 == 2.
> 
> In practice, this operator is useful for 'cyclical' indexing. For example, it would be useful for calculating the real index into a collection when we are using an index outside of the range of valid indices and could be used to index into a collection using a negative index à la Python and Ruby (where [1,2,3,4][-1] == 4).
> 
> 
> The implementation would probably be something along these lines:
> 
> infix operator %% {
>   associativity left
>   precedence 150
> }
> 
> func %%<T: IntegerArithmeticType>(lhs:T, rhs:T) -> T {
>   return (lhs % rhs + rhs) % rhs
> }
> 
> If accepted, this could be later incorporated into a method or operator that works directly with collections using their count property. 
> Maybe the syntax could be something like [1,2,3,4] %% -1 == 4.
> 
> Ideas, suggestions?
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