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

[Tim Vermeulen]

+1, I’ve never actually wanted a negative return value from %. Array subscripting and getting an enum case from a raw integer value both expect non-negative integers, so in these cases it only makes sense to deal with positive values.

> Hello,
> 
> I think that Swift could use the 'double modulo' operator which is for example in CoffeeScript (some discussion can be found herehttps://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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>