Function with generic type - swift

Question

I have a function that can calculate the sum of numbers in array with condition like so:
func sumOfArrayWithCondition(array: [Int], filter: (element: Int) -> Bool) -> Int {
var result = 0
for i in 0..<array.count where filter(element: array[i]) {
result += array[i]
}
return result
}
Now I want it to work with Int, Float, Double type. I have tried, but didn't work.
protocol Addable {
func +(lhs: Self, rhs: Self) -> Self
}
extension Int: Addable {}
extension Double: Addable {}
extension Float: Addable {}
func sumOfArrayWithCondition<T: Addable>(array: [T], filter: (element: T) -> Bool) -> T {
var result = 0
for i in 0..<array.count where filter(element: array[i]) {
result += array[i] // <-------- Error here
}
return result // <-------- Error here
}
But it says:
Binary operator '+=' cannot be applied to operands of type 'Int' and 'T'
So how to do it.
Any helps would be appreciated. Thanks.

Answer 1

First issue is that the compiler is inferring the type Int for the var result because you don't declare a type and initialize it with 0. But you need result to be of type T.
First, in order to initialize result as an instance of type T with the value 0, you need to specify that Addable is also IntegerLiteralConvertible, which is already true for Int, Double and Float. Then you can declare result as type T and go from there.
As Rob pointed out, you also need to add the += function to your protocol if you want to be able to use it.
So the final code that achieves what you are looking for is:
protocol Addable : IntegerLiteralConvertible {
func +(lhs: Self, rhs: Self) -> Self
func +=(inout lhs: Self, rhs: Self)
}
extension Int: Addable {}
extension Double: Addable {}
extension Float: Addable {}
func sumOfArrayWithCondition<T: Addable>(array: [T], filter: (element: T) -> Bool) -> T {
var result:T = 0
for i in 0..<array.count where filter(element: array[i]) {
result += array[i]
}
return result
}

Answer 2

As Rob said, result is an Int. But there's no need to create that method at all. You're wanting to call that like so, based on your method signature:
let sum = sumOfArrayWithCondition(myArray, filter: myFilter)
instead, all you have to do is use existing methods provided by swift:
let sum = myArray.filter(myFilter).reduce(0) { $0 + $1 }