Function with generic type

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
}